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8/9/2019 Info Winter 317 Manual http://slidepdf.com/reader/full/info-winter-317-manual 1/92 Chemistry 317 Winter Quarter 2011 Inorganic Chemistry Laboratory A. General Information Introduction to Chemistry 317 p. 2 Logistics and Schedule 2 Class Schedule 3 Assignments 3 Course Grading 4 Attendance/Punctuality Policy 4 Readings 5 Lab Notebook 5 Laboratory Safety 6 Guidelines for Writing Lab Reports 11 Techniques 15 B. Experiments 1. Chromous Acetate 29 2. The Chelate Effect 37 3. Phosphorous Acid 47 4. The Lewis Acid-Base Adduct BF 3 NH 3 57 5. Arene Molybdenum Tricarbonyl Chemistry 65 6. Linkage Isomers of Nitro-Pentammine-Cobalt(III) 73 7. Preparation of a Doped Phosphor, ZnS 86 Instructor: Dr. Jasmine Bryant, BAG 412 e-mail: [email protected] Lab Technician: Bill Cusworth, BAG 333C, 543-1207 e-mail: [email protected] Discussion Section/Lab Lecture: (first meeting) AA & AB sections: discussion hour Tuesdays 8:30-9:20 AM in Bagley 108 BA & BB sections: discussion hour Tuesdays 9:30-10:20 AM in Bagley 108

Transcript of Info Winter 317 Manual

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Chemistry 317Winter Quarter 2011

Inorganic Chemistry LaboratoryA. General Information

Introduction to Chemistry 317 p. 2Logistics and Schedule 2Class Schedule 3Assignments 3Course Grading 4Attendance/Punctuality Policy 4

Readings 5Lab Notebook 5Laboratory Safety 6Guidelines for Writing Lab Reports 11Techniques 15

B. Experiments1. Chromous Acetate 292. The Chelate Effect 373. Phosphorous Acid 474. The Lewis Acid-Base Adduct BF

3• NH

3 57

5. Arene Molybdenum Tricarbonyl Chemistry 656. Linkage Isomers of Nitro-Pentammine-Cobalt(III) 737. Preparation of a Doped Phosphor, ZnS 86

Instructor: Dr. Jasmine Bryant, BAG 412e-mail: [email protected]

Lab Technician: Bill Cusworth, BAG 333C, 543-1207e-mail: [email protected]

Discussion Section/Lab Lecture: (first meeting)AA & AB sections: discussion hour Tuesdays 8:30-9:20 AM in Bagley 108BA & BB sections: discussion hour Tuesdays 9:30-10:20 AM in Bagley 108

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Laboratory Sections and Teaching Assistants:

AA & BA sections: TTh 12:30-4:20, Bagley 191AA : Vlad Vlaskin ([email protected])BA : Tyler Stevens ([email protected])

AB & BB sections: WF 12:30-4:20, Bagley 191AB : Jessica Wittman ([email protected])BB : Thomas Porter ([email protected])

Introduction to Chemistry 317Welcome to Chemistry 317, Inorganic Chemistry Laboratory. Our goals are to make this

a stimulating, challenging and useful experience. You will be introduced to new techniques andnew kinds of chemicals and chemical reactivity. The class will tie together material you havehad in lecture courses, and will ask you to design and improve experiments. There are some lab periods for which no instructions are provided; you must choose what you want to do and inventa procedure to do it. We will help you, but we want you to bring your insights, enthusiasms,questions, and skills to the course. Some of the material will be familiar, while other parts of theclass will be quite new. Some of the experiments work like a charm, others we are still perfecting - and we hope you will help us make them better. We are eager to hear yoursuggestions and comments.

Logistics and ScheduleChemistry 317 consists of two laboratory periods and one “discussion” hour per week.

The experiments are designed for students to work in pairs, with a maximum of 20 students (10 pairs) in the laboratory at one time. In each lab period, half of the students will work on oneexperiment and the other half will work on another. Those in the A section will be doingexperiments from the first column on the schedule (next page), and must come to the Tuesday8:30 a.m. discussion hour. Those in the B section will work on the second column and mustcome Tuesday at 9:30 a.m. In this way the discussions will be related to the experiments you aredoing.

The discussion hours will include some lecturing, to provide background andunderstanding of the experiments. But primarily these hours will be forums for discussion of thelab just completed, for instance how to analyze your spectra or numerical data. Please bring

your data, your questions, and your opinions! Most students have found these sessions quitehelpful. Attendance will be taken and isrequired .

The schedule for the class is given on the next page.

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Chem 317 Class Schedule:Lab

periodSection AA, AB Due*† Section BA, BB Due*† Date

1 Check in Check in Jan 4/52 Cr Acetate MSDS Chelate Effect MSDS Jan 6/73 Cr Acetate Chelate Effect Jan 11/124 Chelate Effect Chromous Acetate Jan 13/145 Chelate Effect Chromous(TS) Chromous Acetate Chelate(TP) Jan 18/196 Phosphorous Acid ZnS phosphor Jan 20/217 Phosphorous Acid Chelate(TP) (Arene)Mo(CO)3 Chromous(TS) Jan 25/268 BF3•NH3 Chelate Effect II ZnS(TS) Jan 27/289 (Arene)Mo(CO)3 Phos Acid(TS) Linkage Isomers Arene (VV) Feb 1/210 Chelate Effect II BF3•NH3(JW) Linkage Isomers Chelate II(TP) Feb 3/411 Linkage Isomers Arene(VV) Linkage Isomers Feb 8/912 Linkage Isomers Chelate II(TP) Linkage Isomers Feb 10/1113 Linkage Isomers (Arene)Mo(CO)3 II Feb 15/1614 Linkage Isomers (Arene)Mo(CO)3 II Linkage(JW) Feb 17/1815 (Arene)Mo(CO)3 II (Arene)Mo(CO)3 II Feb 22/2316 (Arene)Mo(CO)3 II Linkage(JW) Linkage Isomers II Feb 24/2517 (Arene)Mo(CO)3 II Phosphorous Acid Arene II(VV) Mar 1/218 Linkage Isomers II Phosphorous Acid Linkage II(JW) Mar 3/419 ZnS phosphor Arene II(VV) BF3•NH3 Mar 8/920 Check out Linkage II(JW) Check out Phos Acid(TS) Mar 10/11

ZnS(TS) BF3•NH3(JW) Mar 15/16* - For AA and BA sections, assignments listed as due Tuesdays can be turned in by 12:30 pmon the following day (Wednesday). This will allow students to use the information presented atthe Tuesday morning discussion hour.Warning : A lot of work piles up at the end of the quarter in this class.†- The initials of the TA who is grading each assignment are indicated. Please contactthat TAwith questions about the report both before it is due, and after it has been graded.

Even with only five pairs of students working on a given experiment, there willoccasionally be times when you will have to wait to use a piece of equipment. Try to findsomething else that needs to be done while you’re waiting. Making efficient use of your time isa critical laboratory skill (and a skill you will be graded on). Within each experiment, you andyour lab partner will often be doing different things. You should try to follow what she or he is

doing, as the final lab write-up will require data from both of you.Assignments

Each lab write-up in the lab manual ends with a description of the required assignmentfor the lab. All assignments are due one week after completion of the experiment , as shownon the schedule. Assignments must be typed, double spaced. Tables, graphs, equations, andmolecular structures should be rendered digitally. For molecular structures, ChemDraw issuggested (this is available on the study center computers). A program called ChemSketch is

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• Lab reports must be turned in according to the schedule. Work not turned in by the appropriatedate and time will be assigned a grade of zero.

Readings

It is imperative that you carefully read the lab descriptions before entering the lab. Evenmore than reading, you must think through what you will be doing. This is critical for safeworking in the lab, and to manage your time efficiently. The TAs may take various steps toinsure that the reading is carefully done. In addition to the readings, you will also need to lookup an MSDS of a compound that you will be using and bring it with you to lab.

The lab descriptions contain occasional references to the “original literature,” thescientific articles which originally reported the chemistry. These are given in the standardAmerican Chemical Society (ACS) reference format: authors, Journal Title year , volume # , page. These and other articles are available in a folder in the 317 lab, along with variousreference books. We will also try to have books on reserve. These extra readings are optional, but may be quite useful and interesting. In the arene-molybdenum-tricarbonyl experiment, forinstance, many students have found these papers very useful when they design their own procedures. The extra readings provide background to help you understand your observationsand better interpret your data ( both are critical to good lab reports).

Lab NotebookYou must keep a good notebook in this laboratory (and inall scientific labs). Use a

bound book that pages cannot be removed from. Your notebook is your diary of what you did,and it should be written as you are working . Do not make notes on scratch paper and transcribethem into your notebook. The book should include numerical data (weights, volumes, voltages,etc.), procedures (A was added to B dropwise over 20 minutes using an addition funnel), andobservations (it turned green after half the A was added). The most important features of a goodlab notebook are clarity and completeness. You should never remove a page or plan to go backand fill in something later. If necessary, you can cross something out or recopy something forclarity, just indicate why and make sure the original is still legible. The notebook isnot a handy piece of scratch paper. It should enable you to reconstruct what you did, including good and badaspects of the procedures. A TA will look at your lab book periodically and may collect it atsome point.

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LABORATORY SAFETYIn this laboratory, as in any laboratory, there are a number of hazards. Learning how to

deal with hazardous situations safely is an important part of what you will learn in this class. Ifchemistry majors cannot handle hazardous situations involving chemicals, then who in society

can? It’s usually chemistry majors who write the rules for safe handling of chemicals. Safety isan important focus of this class and we want you to think about safety as you read this labmanual and, especially, as you work in the lab. Chemistry 317 has endured some scary incidentsin the past. Fortunately, no one was hurt and we will talk more in our discussion sections aboutwhat went wrong and how these incidents could have been prevented.

The most important safety rule is to THINK! Good common sense will get you throughmost situations. If there is anything that is unfamiliar or doesn’t seem right, stop what you aredoing and ask. There is at least one TA for every ten students in the lab, so there should always be someone nearby to assist or explain. You will be using a number of expensive pieces ofequipment during the lab, so it is important that you understand how this equipment works.Don’t just plow ahead if anything looks wrong. No one will be criticized for asking. It is,

however, critical that you arrive prepared for the laboratory, having worked out the procedures inyour own mind so you know what you’re going to do.There are a few safety rules we will strictly enforce. Safety goggles and a lab coat must

be worn at all times in the lab. Eating in the lab is forbidden. Shoes must be worn at all times(no sandals or open toed shoes), and no shorts or short skirts.

An increasingly important part of safety and safe handling of chemicals is their disposal.The disposal of the solutions and products in each lab experiment is either described in the labwrite-up or your TA will explain the procedures. Note that there are waste bottles for eachexperiment, as well as for acids and bases. Only chemicals go in these bottles. Syringe needlesgo in the sharps bin, and glass products (including glass pipettes) go in the glass waste. Place all

waste in the appropriate container. If you aren’t sure where it goes, ask your TA. Leave all“smelly” items in the hood with appropriate labels. When in doubt, put your waste in a bottleand label it to indicate the contents. No potentially hazardous waste should be disposed of downthe drain or allowed to evaporate into the fume hood. [Environmental Health and Safety evenviews Coca-Cola™ as “potentially hazardous waste”!]What follows is a detailed description of the safety rules for this class.Safety goggles must be worn in the laboratory at all times. Any failure by a student toobserve this state health may result in removal of that student from the laboratory. Eyes are toovaluable to risk. Students will not be allowed to work in the laboratory without approvedstandard laboratory goggles. Standard laboratory goggles that meet all state regulations may be

purchased from the bookstore or the stockroom. Safety glasses, etc. are not acceptable. If youalready have goggles, the stockroom personnel must first approve them before you can beginworking. Because of health regulations, goggles cannot be borrowed from the stockroom.Dress appropriately for the laboratory. In order to work with chemicals safely, bare skinshould be kept to a minimum. This means socks, full shoes (no open toes), and long pants. Inaddition, it is now a requirement to wear a lab coat in the lab at all times. Lab coats can be purchased at Health Science Stores. FYI: cotton clothing (including denim) is particularlysusceptible to being eaten by acid solutions. The laboratory is not a good place to wear your

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favorite clothes. Do not wear clothing so loose or bulky that it hampers your work (this is a goodway to break expensive glassware). Long hair should be tied back. Failure to dressappropriately will result in a loss of available lab points, and you will be sent out of lab toacquire the correct clothing. If you do not return in time to complete your work, the absence will be unexcused.

Do not eat, drink, or smoke in the laboratory. Do not even bring these materials into thelaboratory.

Wash hands often when working in lab, and always thoroughly before leaving. Do not tasteany chemicals. Do not put your hands, pens, or pencils in your mouth while working in the lab.Keep coats, backpacks and other non-essential materials away from areas where people areworking. Lockers are available in the hallway.Hall lockers: You may bring a lock from home and claim an empty hall locker for use duringthe quarter. These are ideal for storing coats, backpacks, and other bulky items. Also, a locker isthe best place to store your goggles. Lockers must be emptied by the end of the quarter - between quarters the locks will be cut off and the locker contents thrown away.Cell phones and headphones may not be used in the lab.

Never attempt any unauthorized or unassigned experiments. Follow the experimental procedures explicitly, checking and double-checking the identity of all reagents before you usethem. There are potentially hazardous combinations of chemicals present in the laboratory. Ifyou have an idea for further investigation, discuss it with your instructor.Learn the location and operation of the safety showers, emergency eyewashes and fireextinguishers in the laboratory . In the case of spills onto a person or clothing, the immediateaction should be water and lots of it. Do not hesitate to yell for help. Use the safety showersand/or eyewashes and don't worry about the resulting mess. Don't use the safety showers for

non-emergencies since they are designed to deliver about 50 gallons of water before shutting off.Report accidents to your instructor. All instructors have been certified to administer first aid. Ifyou are not familiar with the operation of the fire extinguishers ask your instructor to explain itto you. The fire extinguishers should only be used for real emergencies since the chemicals theycontain can cause considerable damage. In any emergency that requires the assistance of the firedepartment, aid car or police, send someone to the stockroom for assistance.Become familiar with all of the exits from the laboratory. A repeating siren and flashing ofthe FIRE indicator is the building evacuation signal. If this alarm goes off while you are in thelab, turn off any open flames, grab your valuables, and leave the building as quickly as possible.Don't sit in the hallways. Sitting in the hallways disrupts the natural flow of foot traffic and canconstitute a safety hazard. If you need to work on your lab report outside of the laboratory room, please use the Chemistry Study Center (BAG 330).

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WORKING WITH EQUIPMENT AND GLASSWAREDo not leave a Bunsen burner or other heated apparatus unattended . The person workingnext to you may not know what is involved with your setup and may be working with aflammable material. Turn off open flames if you must leave your area. Make sure the gas tapsare completely off whenever the Bunsen burner is not lit.Do not pick up hot objects with your bare hands. Be sure all apparatus is cool before pickingit up with your fingers.Chipped glassware and glass apparatus from your drawer may be traded for undamageditems at the stockroom. We can fire-polish chipped glassware so it is usable, but we can’t fixcut hands.Do not adjust glass tubing connected to rubber stoppers. Severe cuts or puncture woundsmay result.Lubricate rubber tubing. When slipping rubber tubing over connectors, such as filter flasks oraspirators, lubricate with a drop of glycerin (hood) or liquid soap (in lab).Do not force pipette bulbs onto pipettes . Apply just enough pressure to maintain a seal between the pipette and the pipette bulb. Forcing the bulbs may cause the pipette to slip and break, leading to severe cuts or puncture wounds.

WORKING WITH CHEMICALSReagents: Read the label (contents and hazards) before using reagents. Take only as muchreagent as you need - they are expensive and time consuming to prepare. When taking reagents,transfer the amount you need to a clean beaker or other suitable container for taking the material back to your desk. Replace the cap.Never return unused reagents to their storage containers. If you take more than you need,dispose of the excess in the appropriate manner.Clean up spills immediately. The next person to come along has no way of knowing if theclear liquid or white powder on the lab bench is innocuous or hazardous. Neutralize acid spillswith sodium bicarbonate (baking soda) before cleaning them up.Hazard Identification. As part of the UW Laboratory Safety Manual, each laboratory has aChemical Hygiene Plan (CHP). This is available to all students in the lab at all times. As part ofthe CHP, Material Safety Data Sheets (MSDS) must be readily accessible to all students. MSDSare available through the campus computer network on the Lab Safety System (LSS). Thecomputers in the lab have a link to LSS.

M aterial Safety Data Sheets (MSDS) are provided by the manufacturer or vendor of a chemical.They contain information about physical properties of the chemical and identify any hazardsassociated with the chemical. They also identify any special handling precautions and protectiveequipment needed when working with the chemical. You should be familiar with the MSDS before working with any chemical.Read chemical labels carefully . Chemicals are rated from 0 to 4 according to the hazard theyimpose; with 0 representing no hazard and 4 representing high hazard. An example of a hazard

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diamond label is shown below. Each chemical is rated for health, fire and reactivity. Specialwarnings are reserved for the 4th diamond.

3

2 0

Health

Fire

Re ac t i v i t y

Hexa ne

Flam mab le

WASTE DISPOSALDispose of chemical reagents and other materials properly. The proper disposal of chemicalwastes is essential to the health and safety of University faculty, staff, students and thesurrounding community. Chemical wastes must be managed and discarded in the mostresponsible and environmentally sound method available. The University and Metro expect yourcooperation in taking care of the environment. Your laboratory manual will specify how todispose of chemicals used during the laboratory period. Do not put chemicals into glass boxes or

wastebaskets. Only specified non-hazardous water-soluble materials can be rinsed down thedrain. Waste containers for other materials will be provided. If you are unsure of how to disposeof a particular material, ask your instructor.1. ALL NON-CHEMICAL SOLID WASTES used in this class go into the trash cans unless

otherwise noted. Paper towels, matches, pH paper, etc. should NOT be placed in the sinks.2. GLASS (whether broken or not) goes into the glass disposal box. Cleaning up broken

glass is greatly facilitated by using the broom and dustpan (located on the counter under thewindows). Custodial personnel will stop collecting trash after they find broken glass in thetrashcans!

3. BROKEN THERMOMETERS should be taken to the UG stockroom (BAG 271) fordisposal after cleaning up any spilled mercury.

4. ALL ACIDS AND BASES used in this class must be neutralized. Metro requires thatany solutions going down the drain be between pH 5.5 and 12. Acids may be easilyneutralized by the careful addition of sodium bicarbonate; bases may be neutralized by theaddition of dilute HCl and the final pH raised with sodium bicarbonate if necessary. Thefinal pH should be checked with pH paper before it is washed down the drain. Like othersolid wastes, pH paper should be placed in the trash.

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Suggested Procedure for Cleaning up Chemical Spills:Solid Reagents: Wipe up small spills with a damp paper towel; rinse the reagent out of thetowel with water, then dispose of the towel in the trash cans. Clean up large spills using the broom and dustpan (located on the counter under the windows) and dispose of the reagent in anappropriate waste container. If glass is present in the spill, separate the glass from the reagent before disposal. Do NOT place solid chemicals in either the trash cans or the glass box. Spillsin the weighing chamber of the balances should be immediately brushed out using the camel’shair brush provided; the reagent may then be disposed as above.Liquid Reagents (Non-organics of near-neutral pH): Wipe up the spill using a damp papertowel; rinse the reagent out of the towel with water, then dispose of the towel in the trash cans.Acids (including phosphorus trichloride): Neutralize the acid by sprinkling solid sodium bicarbonate over the area of the spill. Clean up the bicarbonate residue with either a damp towelor the broom and dustpan, depending upon the amount used to neutralize the acid. Flush the bicarbonate down the drain with an excess of water.Organic liquids: Wipe up the liquid with paper towels. Do not rinse the paper towels or placethem in the trash. Instead, place them in a hood. Allow the liquid to evaporate and then disposeof the paper towels in the trash cans.Mercury: Obtain a “mercury sponge” from the stockroom. Moisten the sponge with water andthen rub it over the area of the spill (metal side down). The mercury should quickly becomeamalgamated with the metal. When finished, place the sponge back into the plastic bag andreturn it to the stockroom for disposal. During a mercury spill, small droplets may spatter asurprising distance from the area of the spill, especially if the mercury falls from the bench to thefloor. Be sure to check a wide area around the spill to be sure that all the mercury has beenlocated and notify others in the lab to avoid the spill area. If you have a large spill, a specialmercury vacuum may be necessary; ask for assistance.

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Guidelines for Writing Lab Reports1. Report Structure

Introduction:

The introduction of a lab report typically starts with an explanation of what you are tryingto accomplish or observe in the experiment. Include a brief discussion of why this is interesting.You should introduce the compounds to be studied and/or the techniques to be used and theirinterpretation. In the Linkage Isomers lab for instance, you might discuss the long history of thecobalt complexes or the concept of linkage isomerism with reference to the nitro ligand. In theArene Molybdenum lab it would be appropriate to discuss arenes and CO as ligands in contrastto typical (historical) inorganic coordination chemistry, and how1H NMR and IR spectroscopiescan help characterize these compounds. This section should include references to help establishthe context of your work as a small piece in the general body of chemical knowledge.

Results:

This section should tell a story, describing what you did and what results you obtained. It

should describe the experiment as you performed it, not the one described in the manual.Balanced chemical equations are strongly encouraged. Diagrams of the proposed and actual (ifdifferent) structures of the complexes should be shown with labels on the atoms or groups ofinterest. Data tables summarizing your characterizations should be included here, along with anexplanation of the data and corresponding assignments. For instance, in the Arene Molybdenumlab you will record IR spectra for your starting material and products. Assign the peaks you seein the IR spectra of the various compounds and how these spectra relate to each other.When possible, numerical data should be presented in Tables rather than as part of the text.Then the text can refer to the Table, as in “Voltage readings were obtained over the temperaturerange 280 - 340 K and the data are given in Table 1.” All tables and graphs should be labeled aswell as referred to directly within the text. Any problems with data collection should beexplained. Whenever possible include the raw data, and show how the quantities of interest werederived, giving any relevant equations. Always show sample calculations and use units. Erroranalysis should be included in all calculations.The Results section is a factual account only, interpretation should be left for the Discussion section.The grammatical tense in the Results section is generally past tense, but is not strictly adhered to.

Discussion:

The Discussion section should first of all provide your analysis of the results. Did youmake what you wanted? How do you know? If the expected product was not formed, discuss

why this might be the case and whether the actual product can be identified. Discuss what wentwrong, if anything did. In the Arene-Molybdenum lab, you’ll want to analyze your spectral data,for example what the CO stretching frequencies indicate about what happened. Compare thespectral data for the products to those of the starting materials and make comparisons. TheLinkage Isomers lab requires significant data and error analysis which go into the Results andmust be discussed in the Discussion section.

The Discussion section must include a clear description of your conclusions. You couldconclude that the error bars are too big for you to conclude anything or that the procedure didn’t

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work because .... But it’s important that you take a stand − no waffling. The Discussion shouldreturn to the questions, goals, and issues raised in your Introduction . In a way, the Introductionand Discussion are the bookends for the Results .

Experimental:

This section is a completely factual account (in past tense ONLY) of the experiments you preformed and a formalized report of their characterization. The format in which this sectionshould be produced is strict and unwavering. This section is written in “cook book” manner, sothat future chemist could pick up your report and repeat your experimentsexactly using only thedescriptions you provided. It should describe your actual procedure, not the one in the manual.The first section should include general procedural methods although not standard procedures(assume that we all know how to use a balance, glove box, IR spectrometer, etc.) Subsequently,it should include descriptions of the reagents, the amounts of reagents used (grams, mL, eq., andmoles), your yields of the products obtained (both in grams and percent) for each productobtained. A list containing all spectroscopic features and assignments (and specifics of how theywere obtained) and other methods of characterization should conclude the individual compoundsections. In general scientific writing, compounds already reported in the literature are referencedonly, however, you will includeall synthesized compounds in your experimental.

Proper formatting is important in this section more than any other. There is an immenseamount of information which needs to be conveyed in a small amount of space, improperformatting will only add to the confusion which often prevails in this section. The best way tounderstand how to format this section properly is to look at an example from a published journaldisplaying similar information.

References: This section should include a numerical tabulation of all the previously publishedscientific literature which you cited in the above text. It should be in normal ACS (AmericanChemical Society) format, authors, journal , year , volume , pages. For example…

1. Kawamura, K.; Hartwig, J. F., J. Am. Chem. Soc. , 2001 , 123 , 8422-8423.

2. Error Analysis:

It is important to understand and describe the uncertainties (the errors) in any numericalvalues that you report. Errors are typically divided into two types: systematic errors andrandomerrors (and of course there are simple mistakes, like typing a number incorrectly into acalculator). Random errors result from the fact that no measurement is perfect, for instance thatyou can’t measure the temperature on a typical mercury thermometer without an uncertainty of,maybe, ±0.2 ˚ C. You should estimate your random errors when you make a measurement. Forexample, if you measure an absorbance with a UV-vis spectrometer, measure the same sampletwice or three times to get a sense of how reproducible the values are. Systematic errors resultfrom problems that are not random, such as miscalibration of an instrument (e.g., your balancedoesn’t give 100.00 g for the standard 100 g weight). Perhaps your starting compound is only95% pure, so your calculated concentrations are systematically 5% too high – or perhaps your procedure leads to an inadvertent dilution of your samples. Systematic errors are more difficultto identify and analyze but you should be on the lookout for them at all times. Error analysis isdiscussed in more detail in the write-up for the Chelate Effect lab.

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3. Presentation and Style

The ability to write a clear and concise description of what you did and what it means isan important skill, regardless of what job you find yourself in. This is a formal report, a singledocument with a logical lay-out and flow. The arguments must flow within each paragraph andfrom one paragraph to the next. Proper grammar, spelling and formatting must be used.

Scientific writing has its own peculiar styles. The experimental section is typicallywritten in the past passive voice. For instance: “The 0.1 M ammonia solution was added to thesolution of CuSO4 dropwise, with stirring. The solution rapidly turned dark purple....” Youshould not imply that chemicals are active agents, avoiding, for example: “the ammonia turnedthe copper ions purple.” Use the past tense when you are describing what you did at some timein the past. In other places, try to use present tense: “Understanding the chelate effect isimportant because ...” or “A plot of theΔG values free energies versus temperature is shown inFigure Z. ΔS is obtained from the slope of this plot .... TheΔS values imply that ...” Use present tense because what you are describing is independent of time. You imply that anyonelooking at your data would derive the sameΔS and reach the same conclusion. For examples ofscientific writing, refer to the journal articles on reserve in the Chemistry Library or thoseavailable in the laboratory.

You should avoid using personal pronouns, such as “I added ammonia to ....” Thestatement “Addition of ammonia to a solution of Cu2+ causes a rapid change in color to dark purple” is true whether you do it or someone else does it; it is true today and will be true in thefuture. You can indicate that you’re not sure of something with phrases such as “it seems likelythat” or “it is possible that” or “perhaps.”These are guidelines, not firm rules. “The voltages are/were converted to free energies (inkJ/mol) using equation X (see Table Y).” could be present or past tense. The most importantfeatures are clarity of thought and writing. Maintain your focus on the important issues and lead

the reader through your story and your arguments. Ask yourself “what am I really trying to sayhere?”

4. Plagiarism

Reports which are plagiarized (in whole or in part, from published material or from otherstudents) will be automatically assigned a grade of zero, and will be referred to the Vice Provostfor Student Life in accordance with the University of Washington Student Conduct Code. Seehttp://depts.washington.edu/grading/issue1/honesty.htm. Below are some excerpts from theUniversity of Washington web site on this issue.

1. Using another writer's words without proper citation. If you use another writer's words,you must place quotation marks around the quoted material and include a footnote or otherindication of the source of the quotation.

2. Using another writer's ideas without proper citation. When you use another author'sideas, you must indicate with footnotes or other means where this information can befound. Your instructors want to know which ideas and judgments are yours and which youarrived at by consulting other sources. Even if you arrived at the same judgment on yourown, you need to acknowledge that the writer you consulted also came up with the idea.

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3. Citing your source but reproducing the exact words of a printed source without quotationmarks. This makes it appear that you have paraphrased rather than borrowed the author'sexact words.

4. Borrowing the structure of another author's phrases or sentences without crediting theauthor from whom it came. This kind of plagiarism usually occurs out of laziness: it is

easier to replicate another writer's style than to think about what you have read and then put it in your own words. The following example is from A Writer's Reference by DianaHacker (New York, 1989, p. 171).

o Original: If the existence of a signing ape was unsettling for linguists, it was also startling news for animal behaviorists.

o Unacceptable borrowing of words: An ape who knew sign language unsettledlinguists and startled animal behaviorists.

o Unacceptable borrowing of sentence structure: If the presence of a sign-language-using chimp was disturbing for scientists studying language, it was also

surprising to scientists studying animal behavior. o Acceptable paraphrase: When they learned of an ape's ability to use sign

language, both linguists and animal behaviorists were taken by surprise. 5. Borrowing all or part of another student's paper or using someone else's outline to write your own paper.

6. Using a paper writing "service" or having a friend write the paper for you. Regardless ofwhether you pay a stranger or have a friend do it, it is a breach of academic honesty tohand in work that is not your own or to use parts of another student's paper.

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through the canister. (iv) Finally, the box must be able to regulate the pressure inside. If the pressure gets too high the gloves will pop out or the front will break − and if the pressure gets toolow the gloves will suck in. Both are catastrophes. The device that regulates the pressure iscalled the photohelic and it is located above the antechamber. You can recognize it because ithas a pressure gauge on it, calibrated in the unusual unit “inches of water” (more on pressure

units later). The box can take only a few inches of water positive or negative pressure, very littlechange from one atmosphere. The photohelic automatically draws fresh nitrogen from a tank ifthe pressure gets too low and automatically pumps nitrogen out if the pressure gets too high.You can also do these things manually with the foot pedal, labeled R and L, for raise and lower[pressure]. The instructor or a TA will demonstrate the operation of the box and the photohelicin the lab. If the nitrogen cylinder that feeds the box runs out, tell a TA right away.

The drybox is an extremely useful piece of equipment but one that must be treatedcarefully − one accident and you can contaminate the atmosphere and destroy everyone’schemicals. Some researchers in inorganic chemistry use a glove box as if it were a bench top,and do all their work in there. However, this can be tedious, uncomfortable, and inconvenient. Itis difficult to maintain a good atmosphere in the box when you’re working with all sorts ofdifferent solvents and reagents, it is hard to work with gases, and it is hard to do reactions thatrequire heating or cooling (remember, no water for reflux condensers). These boxes are alsoexpensive, costing well upwards of $20,000 each.

For these reasons, we only have two dry boxes in the lab, and they are used mostly forstorage, setup, and workup of experiments. You will do most of your chemistry out in the air,using specialized glassware. In the aggregate, these strategies are called “Schlenk techniques.”II. Schlenk Techniques

The centerpiece of this defense against atmospheric intrusion is the double manifold, orSchlenk line. In the illustration below, the double manifold is in the middle, connected tovarious equipment on both sides. The two long, horizontal tubes of the Schlenk line are called“manifolds.” Each manifold can be filled with a gas, or “un-filled” with vacuum. Reactions andmanipulations are typically done under an atmosphere of an inert gas, usually nitrogen, admittedinto one of the manifolds (the “nitrogen side”). The other manifold (the “vacuum side”) is

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connected to a vacuum pump through a liquid-nitrogen cooled trap. Any solvents that make itinto the vacuum line condense in the trap before they get to the pump, which protects the pumpand the pump oil. Glassware (see below) is connected to the Schlenk line via rubber hoses.Then the glassware is exposed to the vacuum or the nitrogen manifold using the two-waystopcocks. For safety reasons, the Schlenk lines will be set up in the hoods.

Be sure all stopcocks are lightly greased − if you forget this the glass pieces may sticktogether and it can be a real pain getting them apart. The function of the thin film of grease is just to allow the glass pieces to slip over each other. If you use too little grease the glass pieceswill bind. If you use too much grease, it will ooze into the holes and into your line and willdegrade more rapidly. When you’re done, the ground-glass part of the stopcock should lookclear, with no streaks in the grease. Your TA will help you get the hang of this.**NOTE 1** We have replaced the mercury manometer, at left in the Figure above, with 2 pressure gauges: 1. A “Bourdon” diaphragm needle gauge and 2. a digital convection gauge.These gauges attached to the vacuum manifold are your way of seeing what the pressure is in themanifold. The pressure is measured in units of torr. Recall that one atmosphere is 760 torr. TheBourdon gauge has a range of 1-760 torr and is not sensitive to the type of gas in the line. Theconvection gauge has a range of 0.01-760 torr and is sensitive to the type of gas in the line and soit is not as accurate. It is used to measure the low pressure range. These gauges are useful forquantitative gas handling, as gases can be conveniently measured out by PV = nRT. It is alsocritically useful to tell you qualitatively what is going on inside your Schlenk line. Wheneveryou do anything on the Schlenk line, the pressure inside will change. Be sure you know what youexpect to happen whenever you open or close a valve on your line − and watch the gauge to be

sure that this is what happens. If anything surprising occurs, STOP, close the valve, and thinkagain. Note: go slowly and wait for the gauge to respond. These digital gauges are slower toreact than the mercury manometers.** NOTE 2** We have replaced the nitrogen tank (see Figure above) with a link to the house

nitrogen supply located in the hoods.The Nujol bubbler at the end of the nitrogen line is there to prevent air from getting in.

Be careful, if you pull a vacuum on the Nujol bubbler, you will suck Nujol back into your lineand make a big mess. Many groups do this at least once and then have to clean their line (notfun). If you need to pull a vacuum on the nitrogen line, use a pinch clamp on the hose or squeezeit with your fingers to prevent the Nujol from sucking back. You should also place a check valveon the nitrogen line which will prevent the backflow of Nujol into the Schlenk line in case youmake a mistake. When you’re using the line, there should be a slow and steady flow of nitrogenthrough the nitrogen manifold and out the bubbler. The stream of nitrogen bubbles tells youwhat’s going on in this manifold.

You have an array of equipment to help you manipulate solutions without exposing themto air. The lead players are pieces of glassware with sidearms attached, so-called Schlenk ware,as illustrated below.

The Schlenk flask is an ordinary round-bottom flask with a sidearm with a stopcock (besure it’s greased!). You can connect this sidearm to the Schlenk line with thick rubber tubing anduse it to admit nitrogen to the flask or to evacuate it. The tubing needs to be thick so that it won’tcollapse under vacuum. You will put something in the neck of the flask, such as a glass stopper(greased) or another piece of apparatus such as a Schlenk addition funnel or a Schlenk filter.

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A cannula is a hollow steelneedle with two sharp ends. It canserve as a sort of express route fortransferring liquids when set up asshown. If the pressure in the flask atright is greater than that in the otherflask, the liquid will be pushed from theright to the left flask.

To address the difficult conundrum of no-air filtrations, some twisted soul invented theSchlenk filter (illustrated below). Its effective use requires some practice and a flair forcontortionism. The filter is placed on top of the flask with the material to be filtered, and on topof it is placed a flask in which to catch the filtrate. The whole assembly is then inverted, and youtry to get as much of the solid as possible to run down on to the fritted glass disk. You can helpthe solid down with the stir bar, which you can move around with a hand-held magnet on the

outside of the flask. Applying a touch of vacuum to the underside of the frit while the top isunder nitrogen will suck the filtrate through just like an ordinary suction filtration. It’sconsidered tacky to pour the solution down one of the sidearms, so make sure you tip the setupso the liquid runs down the other side (as shown). We encourage using more than two hands forthis operation − i.e. , do it with your lab partner.

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III. Gas Handling: Tanks and Regulators

The handling of gases provides a number of challenges. First, all gases are “airsensitive,” to the extent that if they get mixed with air they are no longer pure. (There are alsogases that undergo chemical change because of reaction with oxygen or water.) You will use thevacuum manifold of your Schlenk line to work with gases.

Gases are purchased in thick (usually steel) tubes, either large cylinders or smaller“lecture bottles.” The gas inside is present under pressure, typically pressures much greater thanone atmosphere. This represents the second challenge: a pressurized gas would love to get outof its container and will do so with some force if allowed to. This is a serious hazard, and onethat must always be kept in mind while working with compressed gases. In the mid-90s, therewas a serious incident when someone opened a lecture bottle to their Schlenk line when thestopcock on the line was closed. The high pressure gas was released to the rubber hose but hadno place to go. So what did it do? The gas blew open the thick hose, with sufficient force tosnap a piece of glass off the line. A fair amount of noxious gas was sprayed into the lab andeveryone had to be evacuated. Let’s not have a problem this quarter.

We use a series of valves and regulators to make sure that the flow of gases is controlled(by us). The cylinder or lecture bottle that comes from the vendor has a valve on top, essentiallyan on/off valve (there is a little bit of flow adjustment possible with this valve, but very little).Lecture bottles are metal tubes, roughly 2” in diameter by 15” in length. Gas cylinders come inall shapes and sizes, up to 5’ high and 300 pounds − and from there, gases can be purchased intruck-loads or by the railroad tank car. The primary difference between a cylinder and a lecture bottle (aside from the size) is that we the consumer buy the lecture bottle while the cylinder is the property of the company and we pay demurrage, a “rent” of about 3¢ per day.

The valve on the tank is your first line of defense. If this is closed, no gas can get out.But when that is open, the gas will come out at a high rate (flow): our nitrogen tanks come

pressurized to 2,000 psi (pounds per square inch) or ca. 150 atmospheres. If anything goeswrong while you’re working with a gas,close off this main valve . If the student had donethis (in the mid-90s) when the hose blew, the incident would have been much less serious.

Note: you will be expected to know − perhaps on the laboratory evaluation − all thevarious pressure units used in this class:

1 atm = 1.01 bar = 14.7 psi = 760 torr = 760 mm Hg = ~33 ft or ~400 in of water.

In this lab, you may use large cylinders ofnitrogen as your source of inert gas. To control the flowof gas from a large tank, always use a regulator (such as

the one pictured at right). One other precaution: thesetanks should always be chained to a wall or a lab bench sothat they cannot tip over. If one tipped over and fell onthe regulator, it could snap off the valve on top of thetank. This would turn your demure gas cylinder into asteel torpedo, propelled by the high pressure gas inside.

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IV. Spectroscopy

Spectroscopy deals with the interaction of light with matter. The energy of an atom ormolecule is quantized, that is only a limited number of energy levels are allowed.Spectroscopies examine the gaps in energy between these energy levels, which can be quiteinformative as to the nature and structure of the molecule. In Chem 317 you will use a numberof different kinds of spectroscopies, most if not all of which you have used in other classes.Because of the variety of compounds and situations in this lab, it will be important for you tounderstand what these spectroscopies are actually measuring and what problems could arise. Anoverview is given here; addition information can be found in many of the individualexperiments.

A key issue to always keep in mind in almost all spectroscopic measurements is that yoursample is in a vessel and often in a solvent. The spectrometer measures – so your spectrumshows – the entire sample, including the vessel and the solvent. For instance, the NaCl salt plates that we use for IR spectra are not transparent over the whole IR region. Plastic cuvettesabsorb UV light. NMR solvents often have residual protons that will show up in a1H NMR

spectrum (see table below). For instance chloroform– d is not 100% CDCl3; it probably is about99% CDCl3 and 1% CHCl3. It is always a good idea to take a spectrum of just your solvent, or just your cuvette, or just your salt plate. Then you’ll know what peaks correspond to yoursample and what to the other materials.

Solvent Chemical Shift (ppm)acetone-d 6 2.05 benzene-d 6 7.16chloroform-d 7.26deuterium oxide (D

2O) 4.75 (varies)

A. NMR Spectroscopy

In this lab we will be mainly dealing with two types of NMR – phosphorus and proton.Proton or1H NMR is the most common type of NMR spectroscopy, which you may have studiedin organic chemistry. Recall that1H nuclei have a spin of ½. In a magnetic field, these nucleihave two energy levels, ms = +½ and ms = -½ . Proton NMR looks at the energy of the transition between these two levels. Phosphorus, on the other hand, consists exclusively of a singleisotope, phosphorus-31 (31P), which also has a nuclear spin of ½. So phosphorus or31P NMRspectra have chemical shifts and show coupling, just like proton NMR.

In the Phosphorous Acid lab, coupling is observed between two phosphorus nuclei and

between a phosphorus nucleus and a proton. This P-H coupling can be observed either in the31

Por the 1H NMR. In that lab, we’ll also be looking at deuterium NMR (2H NMR). Similar to witha proton, coupling can be observed between a phosphorus nucleus and a deuterium nucleus.However, deuterium has a quantum spin of 1, so its nuclei can have 3 different orientations in amagnetic field: +1, 0, or -1. This will cause the phosphorus coupling to look different than whenit’s split by a proton. When you get the deuterium (2H) spectrum from this experiment, it willmost likely look very similar to the proton NMR spectrum. The introduction to the PhosphorousAcid lab contains more detailed instructions on how to look at and interpret these spectra.

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NMR Methods

To make an NMR sample, place about 10 mg of your sample in an NMR tube. Addenough NMR solvent (use deuterated solvents unless you’re directed to do otherwise) to reachabout 1 ½ inches (“three fingers”) up from the bottom of your tube. Shake the tube until yoursample is dissolved. Note that if your sample does not dissolve, it will not show up in your NMR spectrum. If necessary, try a new tube with a different solvent to see if you get betterdissolution. When your NMR tube is ready, label it with your name and the NMR solvent usedand give it to your TA.B. Infrared (IR) Spectroscopy

Absorption of light in the infrared region of the spectrum typically corresponds to thevibrations of the molecules in the sample, both stretching and bending vibrations. You probablytook a number of IR spectra in organic labs. IR spectra are usually reported in wavenumbers(cm-1), the inverse of wavelength. Wavenumbers ( ν ) are a unit of energy, and are directly proportional to frequency (ν ): ν = ν × c where c is the speed of light (3× 1010 cm s-1).

IR spectra are useful in the characterization of molecules because each bond typetypically has a characteristic range of frequencies (depending on the molecule). In organicchemistry, there are a limited number of functional groups and the frequencies for each aretabulated in standard texts, for instance Paviaet al. Introduction to Organic LaboratoryTechniques (Appendix 3) or Lambertet al. Organic Structural Spectroscopy (Chapters 8 and 9).Inorganic compounds exhibit a much wider variety than organic ones – we deal with over 100elements! – so one doesn’t find such tables. The best source is Nakamoto, Infrared and RamanSpectra of Inorganic and Coordination Compounds , a copy of which should be found in the lab.IR Methods

There are several different procedures for preparing a sample for IR spectroscopy. Foreach compound you are asked to take an IR of, take a minute to think of which method might be best for this particular compound.I. Nujol Mull

A Nujol mull is probably the simplest method and is recommended if your product hasturned out to be too oily (but not wet). This method requires a mortar and pestle, Nujol (lightmineral oil), and two salt plates. Place a small amount of your solid (perhaps 20 mg) in themortar. Add a tiny amount of Nujol, not more than a couple drops. Use the pestle to grind the Nujol and sample together into a thick and homogenous paste. Scoop up enough of this mull tocoat the center portion of a salt plate and add the other salt plate to make a sandwich. Don’tsqueeze the plates together too strongly or you’ll push the mull out (or worse, crack the salt plates). These really are “salt” plates, made of NaCl, so you need to be gentle with them. Theycrack very easily and dissolve quickly in water (they also cost more than $10 each). To clean thesalt plates, they can be wiped with a Kimwipe™, or rinsed with methylene chloride (CH2Cl2).

Never use water to clean the salt plates. Be sure to take a spectrum of Nujol by itself later toaccount for these peaks in your mull spectra.

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II. KBr pelletsA KBr pellet can provide a much cleaner spectrum, free from pesky Nujol peaks, but may

require a little more effort. You may need to make several pellets to get a feel for the procedureand get a decent IR spectrum. To make a KBr pellet, you will need a mortar and pestle(provided in the lab), some dried KBr (which should be in the oven), and a KBr pellet press(provided in the lab). To begin, place ~1-2 mg of your sample in the mortar. Add 100 times asmuch KBr as your sample (i.e., 200 mg KBr for 2 mg of sample). Grind this mixture into a fine powder, until no individual grains of KBr are visible. Scrape 100 to 150 mg of this powder intothe KBr pellet press. Consult your TA or the nearby directions if you are unfamiliar with thesetup or how to use the press. The pellet press can then be placed directly into the beam of theIR spectrometer after removing the screws.III. Solution cells

Another method involves the use of solution cells (cavity cells). This option is best ifyou already have a product which will dissolve in an organic solvent (i.e., your AreneMolybdenum Tricarbonyl II products). A solution cell is usually a block of NaCl with a cavityof fixed path length created by a thin Teflon spacer. As with the salt plates, care must be takento keep the cells free of moisture (avoid solvents that would contain water or are aqueous – thesecells cost over $100 to repair). A dilute solution (1-2 mg/mL) of your solid in organic solvent is put into the cell. Be sure to also take a solution cell IR spectrum of just your solvent. Empty thecell, rinse with methylene chloride (CH2Cl2) and air-dry after using it. Place the cell back in thedesiccator when you are finished with it.IV. Liquids

An alternate method to get spectra of solutions or liquids is to simply put a drop on a salt plate and cover with another salt plate. If the solvent is volatile, like methylene chloride(CH2Cl2), it may evaporate quickly and you’ll just get a spectrum of your sample. Again, don’tuse water solutions or highly polar solvents that could dissolve the salt plates (you don’t want to pay for new salt plates). When you’re done, wipe the salt plates clean with a KimwipeTM andrinse with CH2Cl2.Using the FTIR Spectrophotometer (Perkin-Elmer 1600 series and similar)

First, notice that directly below the screen there is a row of grey keys which are not marked.These keys are called “softkeys” and the operations or values they perform will be displayedalong the bottom of the screen itself (this will seem familiar to those of you who have usedgraphing calculators).

1. A background scan is not required (it doesn’t change much over time), but can be performed if desired. Make sure the sample holder inside is empty. Press the grayBackg key followed by the greenScan key, and then the key under the number “4”displayed along the bottom of the screen. In 10-15 seconds, the machine should display“Ready” instead of “Scanning” to indicate it has finished taking the background.

2. Place your sample in the instrument and then press theScan key followed by the keyunder the number “4” displayed along the bottom of the screen. When scanning iscomplete, your spectrum will appear on the screen.

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Figure 1. File Toolbar. Left to right, the six buttons are New Spectrum, Open,Save, CopySpectra,Print, and Help. (The ones in bold face are referred to later.)

Figure 2. Spectrum Controls Toolbar. Left to right, the six buttons are:Store Dark Spectrum, Store Reference Spectrum, Snapshot , Single Exposure, Data Acquisition, Emergency Reset.(The ones in bold face are referred to later.)

Figure 3. Part of the Spectral View Toolbar. The five buttons shown areScope Mode, Absorbance Mode, Transmission Mode, Irradiance Mode, and Specular Reflection Mode (theones in bold face are referred to later). Until both a dark and reference spectrum have beenstored, only Scope Mode can be used. Therefore, until both a dark and reference spectrum has been stored, all the buttons except the S are grayed out.

Figure 4. Graph Scale Toolbar. Left to right the three buttons areAutoscale , Set Scale ,Unscale. (The ones in bold face are referred to later.)

4. The instrument will start in Scope Mode which shows a graph of light intensity vs.wavelength (Figure 5 ). You should see a number of peaks on the graph that are fromthe deuterium/tungsten light source, including a major peak at 657nm. If all you see isa flat line or a broad peak in 600-800 nm range, the lamp is not working properly. Note: in the screen shots that follow, the display is of black lines and text on a white background. The display you see may be of white lines and text on a black background.

Figure 5. Ocean Optics software in Scope mode.

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5. In Scope mode, the S icon (Figure 3 ) should be depressed. Press the S icon (or select"Scope Mode" from the Spectrum drop down menu) whenever you need to return toScope mode (you should be in Scope mode when storing either a dark or referencespectrum).

6. Change the wavelength range to appropriate values for your experiment. To do this,click on theSet Scale button (Figure 4 , icon which has 2 perpendicular red arrows) andenter the appropriate x-axis values. (no need to change the y-axis values). To helpselect a wavelength range, you may want to place an empty cuvette into the cuvetteholder and observe the light intensity as a function of wavelength. Plastic cuvettesabsorb some UV light so you want to choose a wavelength range where there issufficient light reaching the detector. Otherwise, you will have noisy regions in yourabsorbance spectrum.

7. Place a reference cuvette filled 3/4 full with the reference solution (usually deionizedwater unless otherwise specified) into the cuvette holder. Be sure that the cuvette isaligned properly so that the light beam will pass through the clear sides of the cuvette.Cover the cuvette with the cuvette cover and make sure that the cover is flush with the base of the cuvette holder (light passes through the sample when either side of thecover is flush with the base).

8. If necessary, adjust the Integration Time until the highest peak in the light sourcetrace is between 3000-3500 Intensity units (usually a value between 8-20 ms is normal).The Boxcar smoothing parameter should be set to 5 (this averages the 5 points beforeand 5 points after each data point). The Average parameter should be set anywhere between 10 and 40. More averaged scans give a better looking spectrum but slightlyslower response time.

9. Calibrate the instrument by recording a reference and a dark spectrum (note that theinstrument should be allowed to warm up for at least 5 minutes before calibrating). Torecord a reference spectrum, make sure the reference cuvette is in the cuvette holder.Click on the yellow light bulb icon (Figure 2 ) or right-click on the screen and select‘Store Reference’ from the menu that appears.

10. To record a dark spectrum, slide the cuvette cover to the side so that it blocks all thelight getting to the detector, which you can see visually on the screen in Scope mode asa flat baseline. For the light sources that have a shutter switch on the front (MINI-DT),you could also move this shutter switch to the ‘OFF’ position. Click on the darkenedlight bulb icon (Figure 2 ) or right-click on the screen and select ‘Store Dark’ from themenu that appears.

11. Select the Absorbance mode by touching the ‘A’ icon (Figure 3 ) or select‘Absorbance Mode’ from the Spectrum menu.

12. Place your sample cuvette into cuvette holder. Select ‘Autoscale’ by touching theicon which looks like an up & down arrow (Figure 4 ) or right-clicking on the screenand selecting ‘Autoscale’ from the menu that appears. The absorbance spectrum ofyour sample should now show full screen.

13. To determine an absorbance measurement, click on the screen at the desiredwavelength. A green cursor will be displayed on the screen. You can fine-tune the

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The short distance and the diamagnetism were explained by proposing that the twochromium atoms form aquadruple bond, consisting of oneσ bond, two π bonds, and oneδ bond. Quadruple bonds are now well established for a number of 4d and 5d transition metals[see D. Shriver & P. Atkins Inorganic Chemistry , 3rd Ed., W.H. Freeman, New York, 1999, pp.304-5.] . However, the presence of quadruple bonds in chromium compounds such as

Cr 2(OAc)4 has been questioned and the compounds have been found to be slightly paramagnetic:see S. Hao, S. Gambarotta, and C. Bensimon J. Am. Chem. Soc. 1992 , 114 , 3556-7 and F. A.Cotton, H. Chen, L. M. Daniels, and X. Feng J. Am. Chem. Soc. 1992 , 114 , 8980-3. Adiscussion can be found in: J. M. MayerChemTracts Inorg. Chem. 1992 , 4, 209-213.

Method

This experiment covers two periods; you should work with your partner. The first day youwill prepare chromous acetate. You will reduce chromium(III) chloride to aqueous chromium(II)with mossy zinc in acidic solution and then add the Cr 2+ solution to a solution of sodium acetate,which will result in the precipitation of brick-red hydrated chromous acetate, Cr 2(OAc)4.2H2O.You will isolate this somewhat air-sensitive solid by Schlenk filtration. The second day you willtake the solid into the glove box, weigh it, and characterize it. The air-sensitive products show the presence of oxygen by changing color, so you will see if any air is leaking in.

Procedure 0. Get Acquainted with your Schlenk Line

Be sure you are familiar with the descriptions of Schlenk techniques and gas handling inthe Introduction (pp. 10-16). Take a moment to compare the actual Schlenk line with the picturegiven. As if dissecting a frog in biology class, try to find the major systems: the line, the bubblers, the vacuum pump, and the nitrogen tank.

Identify the nitrogen manifold. In your best imitation of Sherlock Holmes, trace alongthe manifold in one direction to reach the nitrogen tank. In the other direction, you should findthe Nujol bubbler. Is the nitrogen flowing? We'll assume not, because it’s not good practice tolet the nitrogen flow 24 hours a day (you’re wasting it). You should leave the nitrogen onthrough the lab period, and shut it off when you leave.

The source of nitrogen for the Schlenk lines is so-called “house” nitrogen, rather than anitrogen cylinder. This house nitrogen is plumbed into the fume hoods, and its flow is controlledusing the color coded valves to the left and right of the fume hoods. The house nitrogen comesfrom boil-off from the departmental liquid nitrogen tank, which is at the south end of the loadingdock between Bagley and CHB – take a look at it next time you’re on the road behind Bagley.

A gas regulator and tank are available in the lab if you are interested in learning moreabout their operation. For completeness, we include here the normal procedure for using a gasregulator with a Schlenk line is as follows. Assume that the regulator is full of air, unless youare sure of the contrary. Clear out the air by purging the regulator, passing a lot of gas throughit. First, close the needle valve at the end of the regulator it screws in to close. Then open thelarge valve on the top of the nitrogen tank. The pressure gauge nearer the tank should read between 2200 and 200 psi this is the pressure inside the tank. [If it's lower than this, tell yourTA.] Now cautiously open the little needle valve that sits between the regulator and the rubber

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tubing. Is any gas coming out? Try to do this without blowing Nujol all over your bench.Adjust the diaphragm valve so that the pressure on the second gauge is a moderate 3-6 psi. Thisvalve getsmore open as you screw itin. Use the needle valve to turn the gas flow down until it barely bubbles out, then up to a moderate blast (again, watch the Nujol). Turn the nitrogen flow back down to a slow bubble so you don't empty your tank. In general, the pressure should be set

so that gas will bubble pretty hard if the needle valve is way open. This way, when you need alot of nitrogen, you only have to play with one valve. Now find the vacuum manifold. Trace out the path gas will take when it's pumped out,

starting from the digital vacuum gauge, through the line, through a stopcock into the cold trap, a piece of vacuum tubing and aha! into the vacuum pump. Make sure all the stopcocks andground glass joints are greased (for instance, the cold trap). Close all the stopcocks on thevacuum manifold and turn on the pump. You should be pumping only on the vacuum hoses andthe cold trap. The pump noise will increase, which should subside when the pump gets rid of theair in the tubing and trap. The cold traps are now underdynamic vacuum ; in other words, the pump is actively sucking on them. Place a Dewar flask (these are just giant open Thermos bottles) around each cold trap. The bottom of the Dewar must be firmly supported and youshould be careful not to drop them. The Dewars cost over $100 and shatter easily. Carefully fillthe Dewars with liquid nitrogen.

Liquid nitrogen presents two serious hazards in the lab. It is incredibly cold: 77 K = -196°C = -321 °F! If your skin comes into contact with it for any length of time it will give you anasty cold burn. Fortunately, your skin (and everything else around you) is incredibly hot incomparison to the liquid nitrogen, so the nitrogen evaporates like crazy. This is like drops ofwater on a very hot griddle. So if you get a little onto your hand, you'll have a layer of N2 gas between your skin and the liquid, insulating you. Just don't hold any liquid nitrogen in the samespot on your hand for any length of time. The second hazard is scarier: liquid nitrogen traps are perfectly willing to condense oxygen out of the air. A trap full of blue liquid O2 can explode if

there is any organic matter in the trap (as there often is), so blue liquid in a trap is one of themost chilling sights anyone working at cryogenic [very low] temperatures could encounter.Fortunately, our pumps are powerful and the vapor pressure of the liquid oxygen is fairly large;oxygen will not stick around to give your TA a heart attackas long as the pump is on, or in otherwords, when the system is under dynamic vacuum.1

Returning to your vacuum line, you should not be condensing any oxygen because yoursystem is under dynamic vacuum. You can go ahead and open the vacuum manifold to the pumpand wait until the pump sounds quieter. Watch what happens to the reading on the digitalvacuum gauge. When it stops changing (give it some time to do this), write down the reading. Now close the stopcock to your cold trap so that the line is under static vacuum , without active pumping. If the reading on the digital gauge rises, you have a leak in your line. Try to locatethis leak, checking your stopcocks and their grease, then look to make sure your hoses aren’tcracked. Your TA should probably help with this.

1 In the unlikely event that your trap condenses some liquid oxygen, the prescribed treatment is to tell yourTA, who will help you put the trap back under dynamic vacuum (if the pump is not working, the Dewar flask isremoved and the liquid oxygen allowed to evaporate). While the liquid oxygen is removed, everybody evacuates thelab for a judicious period.

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The process you and your partner have just completed is called “bringing up the line;”now you can impress your friends with lab slang. To take your line down, repeat the above stepsin reverse order.

Quick summary of “bringing up the line”:1.) Turn on the N2 and adjust the needle valve so a gentle, constant

stream of bubbles forms in the bubbler.2.) Close all stopcocks and make sure the trap is on.3.) Turn on the pump and open the stopcock on the trap. Make

sure the reading on the vacuum gauge changes.4.) Check for leaks.5.) Fill the Dewar around the trap with liquid N2.

I. Generation of aqueous chromium(II)In a hood, load a 125 mL Erlenmeyer flask with 5 mL distilled water, 20 mL

concentrated HCl (be sure to check the molarity), 5.0 g chromium(III) chloride hexahydrate, anda magnetic stirbar. Stir the solution to dissolve the chromium(III) chloride. Pour the greensolution into a Schlenk addition funnel (check that the bottom stopcock is closed or the stuff will pour out onto your shoes or onto the floor of the hood). Insert a lightly-greased ground-glassstopper into the top of the addition funnel. Clip it on with a yellow plastic clip (these are called‘keck’ clips) so that when you slightly pressurize the addition funnel with nitrogen the stopperdoesn’t depart for outer space.

You now have to get the oxygen out of the addition funnel, both the oxygen gas and theoxygen dissolved in the solution. Connect the sidearm of your flask to the Schlenk line and openthe two-way stopcock on the Schlenk line to the vacuum manifold. Listen for the pump as youopen the stopcock and watch the vacuum gauge. Now slowly open the stopcock on the addition

funnel, while listening for the pump and perhaps for the gas whistling through the stopcock. Thesolution will start bubbling a little as the water begins to boil at the reduced pressure. Let it bubble for asecond or two , then shut the Schlenk line stopcock.

Very slowly turn the Schlenk line stopcock 180° to admit nitrogen gas to the evacuatedaddition funnel. BE CAREFUL. As you open the evacuated addition funnel to the nitrogenmanifold, the pressure in the manifold will drop dramatically, which will tend to cause the Nujolin the bubbler to suck back into the tubing very, very messy. Many groups wind up having toclean oil out of their line at some point because they forget to worry about this. To minimize this problem, turn up the nitrogen flow to “moderate blast” before you open the two-way stopcock,and open the stopcock as slowly as possible. Putting your finger (or your partner’s finger) overthe exit tube on the Nujol bubbler also helps. Repeat this process of sucking out the gas andrefilling with nitrogen two more times. This is called pump-purge degassing : first you pump theair away and then you purge (fill) with inert gas. Two or three pump-purge cycles are a standardway to rid a vessel of oxygen; the atmosphere in the addition funnel is now essentially purenitrogen.

Add 9 g of mossy zinc (yes, this is really what it’s called; make sure the label says mossyzinc) to a 250 mL Schlenk flask and clamp it to your aluminum rack. Place the addition funnel,lightly greased, into the top of the Schlenk flask, and clamp it as well. Connect the flask to the

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Schlenk line with another one of the hoses. The addition funnel acts as a stopper. Degas theflask with two pump-purge cycles. (Because there’s no solvent to dissolve gases, it’s easier todegas than the first flask.) Once you have finished pumping and purging, turn the nitrogen flowdown to about 100 bubbles/min. Open both the addition funnel and the flask to the nitrogen lineand drip the acidic solution of CrCl3.6H2O in to the flask containing the mossy zinc. As both

heat and hydrogen are generated, the addition must be done drop by drop; if the flask with thezinc feels quite hot, stop adding acid (a little hot is OK). You can also moderate the heatgenerated by periodically applying an ice bath to the bottom Schlenk vessel. It is important that both vessels be open to the nitrogen line to prevent a vacuum from developing in the additionfunnel and to allow the hydrogen gas evolved in the reaction to escape through the gas bubbler.The chromium chloride/HCl solution will undergo a dramatic color change from green to robin’segg blue (almost sky blue) as it is reduced to Cr 2+ (aq).II. Addition of sodium acetate

You now have a solution of a highly air-sensitive intermediate, Cr 2+, in a flask with anaddition funnel protruding from the neck. Turn off the stopcocks leading into the addition funnel

and unclamp it. Have a rubber septum handy. Turn up the nitrogen flow and make sure yourCr II flask is open to nitrogen. Cover the exit of the gas bubbler with a finger and remove theaddition funnel while nitrogen is blasting out of the flask with the chromium(II) solution. NOTE: When you cover the exit of the nitrogen bubbler, you turn the nitrogen manifold into aclosed system with gas pressure building up inside it. If you do not promptly remove theaddition funnel to vent the system, it will create its own vent by popping open a part of yourapparatus. Also, keep a good grip on the addition funnel as you remove it, lest it leap onto the benchtop. This is called doing something against acounterflow of nitrogen .

Immediately cap the flask with the rubber septum. Turn the two-way stopcock to openthe chromium flask to the vacuum and do a pump-purge cycle or two to help clear out anyoxygen that may have been inadvertently introduced. If air gets in, the blue Cr 2+ will beoxidized to green Cr 3+, so you’ll be able to see if you’re doing it right. Any excess zinc that ishanging around will reduce this unwelcome ion back to the Cr 2+ species.

Pour 30 mL of saturated NaOAc solution and a stir bar into a 100 mL Schlenk flask, capwith a septum, and hook it up to the Schlenk line. (Now you know why there are so many ports!)Turn up the nitrogen and do 3 pump-purge cycles to degas the acetate solution. Place one end ofa cannula through the septum on the flask with the chromium solution, but leave the end of thecannula above the level of the solution. Cover the outlet of the mineral oil bubbler on thenitrogen line with your finger to increase the pressure of nitrogen; this should blow a goodstream of nitrogen out the cannula and thus purge it of air. Then place the other end through theseptum on the acetate flask. Place a small needle through the septum on the flask with theacetate. This will serve as an exit for gases to escape. Use your finger to feel that nitrogen isflowing through this needle. Close the stopcock on the flask with the acetate, take your fingerfrom the bubbler, and lower the cannula to the bottom of the flask with the chromium solution.Your apparatus should look like the drawing of cannula transfer in the Introduction (p. 18), andyou are now ready to push the chromium solution over through the cannula.

By covering the bubbler outlet with your finger, you increase the pressure in thechromium solution and push that solution through the cannula into the acetate flask. You can

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adjust the pressure by playing with the flow rate of nitrogen and how much you let out the bubbler with your finger. The goal is to have the chromium solution be delivered slowly youshould see individual drops fall off the end of the cannula, not a jet of material. Do not stir theacetate solution. If the small pressure difference between the two flasks is not enough to makethe liquid flow, you can create a larger pressure difference by placing the acetate solution under a

partial vacuum. To do this, you need to remove the small needle so that you can pull a vacuumon the acetate solution. Apply a slight vacuum to the acetate solution and see if the solutionflows through the cannula. Pull more vacuum as needed.

Once the solution is flowing through the cannula, you can just watch until it’s all done.The solution will turn red-purple. When the addition is finished, lower the N2 pressure to normallevels and remove the cannula and the needle (if still there). If you tilt the flask, you will seesmall brick-red crystals of chromous acetate huddled at the bottom of the flask.III. Filtration of the solid chromous acetate

The next step, a Schlenk filtration, will separate these crystals from their mother liquor.It will also require three arms and your undivided attention. You should review the pictures of aSchlenk filtration in the Introduction (p. 19). First, you have to get the Schlenk filter onto theneck of the flask containing the chromous acetate in place of the septum that is currently keepingout the air. Turn up the nitrogen to a good blast. Hook up both of the stopcocks on the Schlenkfilter to the nitrogen manifold and put a Schlenk or ordinary 100 mL round-bottomed (RB) flaskon one end to catch the filtrate. Be sure the flask has a stir bar in it. Clip the flask on with ayellow plastic clip. Let the nitrogen run for a few minutes to flush out the filter apparatus orcap the open end and pump-purge it. While one hand blocks the nitrogen bubbler, remove theseptum on the chromous acetate flask with the other hand. Your third hand can now sneakaround and place the Schlenk filter (remember to grease the male joint) on the neck of thechromous acetate flask. Secure it with a yellow clip. Close the stopcock on the flask and thelowest stopcock on the filter; open the upper stopcock on the filter to the vacuum manifold. Dotwo pump-purge cycles on the whole apparatus through this top stopcock to suck out any air thatmay have ventured into the system. It’s better to use the top stopcock so that the air would besucked away from the reactive material, rather than over the solution.

Now comes the big moment. Open the stopcock on the flask to the nitrogen line. Thelower stopcock on the frit should still be closed, the upper one still open to nitrogen. Startinverting the filter. Keep the side arms pointed up to avoid pouring solution into the hoses.Untangle any hoses (if you reconnect any, pump-purge them twice.) Use a magnet to twiddle thestir bar around and scrape the solid onto the filter. The solution will fill the top of the filter andslowly come through. To accelerate the process, pumpbriefly on the apparatus below the frittedglass disc. The liquid will flow into the RB flask at the bottom while the solid remains high anddry on the frit. Repeat brief applications of vacuum to drain liquid, if necessary.IV. Washing and Workup of the Chromous Acetate

Your solid is isolated, but it is wet. How can you weigh it out, get an IR spectrum, anddo other things with this air sensitive wet solid? You'll want to weigh it in the glove box, butyou can't take it in there wet, or you'll be bringing water into the box an absolute no-no. Nowater, no oxygen in the box! So first you need to dry the solid. You might have time to do thisin the first period. If not, empty the liquid from the Schlenk filter (think about how you might dothis!), seal the filter back up, pump/purge it a few times and leave it well sealed under nitrogen

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until the next period. If you can, cap and save the filtrate (the liquid that poured through thefilter).

To dry your solid, wash it while it is on the surface of the Schlenk filter, still undernitrogen. First wash with ethanol, then with ether (twice). These organic solvents will washaway any remaining water, and then the last bit of ether is easy to pump away. Volatile solventslike ether (with large vapor pressures) can be pumped away while water evaporates too slowly touse this technique. Choose how you want to do the washing, using a syringe, a cannula, or theaddition funnel. Notice the joints - you may need an adapter. When you've decided on a procedure, talk it over with your TA. Remember, the solvents must be degassed before they areadded. The degassing of water solutions above was done by pump/purge cycles, which alsoworks for organic solvents. But with very volatile solvents like ether, pumping on them causes alot of evaporation. Pump/purge cycles still work just make sure you pump for a very shorttime. You can also freeze the ether in liquid nitrogen and pump on it; at this temperature nonewill boil away. Then thaw the ether to let whatever gas was trapped in the solid escape, coolagain, and pump again. This is called freeze-pump-thaw degassing. [Don't do this with water,since the expansion of water on freezing could crack your flask.]

When your solid is washed, remove the Schlenk flask with the solvents in it and cap theapparatus with a 25 mL round bottom flask or something similar. Make sure you have acounterflow of nitrogen whenever you take apart the apparatus. Pump on it for a few minutes.Close all the stopcocks, disconnect the hoses, and send it into the glove box via the antechamber. Note that if you're sending a sealed piece of equipment into the antechamber it must be undervacuum. When the atmosphere in the box antechamber is pumped out, a sealed flask with gas init will likely pop open the 14.7 psi inside will not be balanced by any pressure outside.Alternatively, you can leave your flask under nitrogen and open a stopcock immediately prior to placing it into the antechamber. The open stopcock will allow the pressure to be equal bothinside and outside the flask while entering the drybox. No matter which method you choose, black electrical tape can be used as a safeguard against exploding flasks. This slightly stretchy

tape will allow an improperly prepared flask to expand in the antechamber, relieving pressure,without the vessel exploding.V. Weighing and Exploring Chromous Acetate; Working in the Glove Box.

Working in the drybox seems simple just like a benchtop but it takes some gettingused to. Think through exactly what you're going to do before you send your stuff into the box.How are you going to scrape out your product? (Don't scrape too hard as this damages thefilter.) How are you going to weigh it? What are you going to weigh it in? Is it labeled? Insum, make sure you bring in all of the things you'll need in the box with you. Note also that therubber gloves are pretty thick (to prevent diffusion of air through them) so you'll be clumsy.Don't plan anything too complicated. Weigh your solid so you can calculate your yield.

Take at least two small samples of your product out of the drybox. Put two samples onthe benchtop, and wet one sample with water. How fast do the two samples react with air? Doyou have any sense of how fast the Cr 2+ (aq) solution reacts with air? Use your other smallsamples to explore other reactions of Cr 2(OAc)4, whatever you think would be interesting (if youhave time). Can you protonate off the acetate ligands with HCl to reform Cr 2+ (aq)?

Make up a Nujol mull of your product for an IR spectrum. Does this have to be done inthe drybox? You'll find a description of how to make up a Nujol mull in the Phosphoric Acidexperiment, along with an introduction to IR spectroscopy. You should also get IR spectra of

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plain Nujol and dry NaOAc (dry, not the aqueous solution - NEVER put water on salt plates) forcomparison. Try to get all of your spectra on the same horizontal (frequency/wavelength) scale.Waste Disposal

All of the waste from this experiment should be placed in a 4-L waste bottle labeledappropriately, “Chromous Acetate Waste”. In this and all other waste issues, the TA’s willinstruct you.

Be sure to clean your Schlenk equipment well as others will be using it shortly.Lab Write-up

A report template will be distributed to you electronically. Fill in the blanks with the requestedinformation and submit a printed copy one week after the completion of the experiment.

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The Chelate EffectBackground

Transition metal cations are Lewis acids; they want to share electron pairs. Themolecules or ions that supply these electron pairs are called ligands, and they coordinate or bindto the metal center. Previous chemistry classes (165, 312, or 416), may have dealt primarily withwater as the ligand, but essentially anything that can act as a base can act as a ligand to a metalcenter. The addition, loss, and exchange of ligands at metal ions form the core ofcoordinationchemistry .

Because of the importance of ligand-exchange reactions, it is convenient to have a way ofassessing the relative thermodynamic stability of various complexes. In aqueous chemistry(chemistry in water), the stability of complexes is typically measured relative to the aquo ion ofthe metal in a given oxidation state. The aquo ion is the complex ion where all ligands are water.In the example shown, addition of ammonia displaces water from the aquo ion to make thetetraammine complex. The more stable a given complex is, the larger its K eq will be.

[Cu(H2O)n]2+ + 4 NH3 → [Cu(NH3)4]2+ + n H2O (1)

K eq =[Cu(NH3)42+]

[Cu(H2O)n2+][NH3]4 (2)

It has long been known that “all ligands are not created equal.” Often the bettersomething is as a base, the better a ligand it will be. Hard/soft acid-base effects are alsoimportant: for example, cobalt(III) has a marked affinity for nitrogen donors while the softermercury(II) is partial to sulfur. But consider the following reaction:

CuNH 3H 3N

H 3N NH 3 NH 2 NH 2

NH 2 NH 2

NH 2NH 2

Cu + 4 NH 3

2+

+ 2

2+

(3)

Ethylenediamine is just about the same as ammonia, as far as the lone pair on nitrogen isconcerned. The major difference between the two is than the nitrogen in ethylenediamine (en) istethered to a second nitrogen which can also bind to copper. Ethylenediamine is termed achelating ligand because it can bind to a metal through more than one site; it is abidentate ligand because it binds in two places. Measurements (such as the ones you will make in this lab) show

that, given the same set of lone-pair donors, a metal will take bidentate or polydentate ligandsover monodentate ones. This is a general phenomenon known as the chelate effect. For a gooddiscussion of the chelate effect, see F. Basolo, R. G. Pearson Mechanisms of Inorganic Reactions 2nd Ed. Wiley 1967, pp. 27ff, 223ff.

The chelate effect is of enormous importance in coordination chemistry and in bio-inorganic chemistry. Nature almost invariably uses a chelating ligand to bind a metal. A classicexample is the heme group in hemoglobin, which strongly binds iron. Chelating ligands are alsowidely used as reagents in the laboratory and even as drugs. For instance, ethylenediamine-

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tetraacetate (EDTA), the hexadentate chelate shown below, is used (as its calcium complex) totreat lead and other heavy metal poisoning. The heavy metal binds to EDTA and the complex isless toxic and is more readily excreted from the body.

Mn+

O

O

ON

ON

O

O

O

O

N N-O2C-O2C

CO 2-

CO 2-

n-4

EDTA 4-

EDTA-metalcomplex

EDTA is also frequently added to foods to retard spoilage (e.g. mayonnaise check your bottleat home). EDTA binds metals (often iron) that would otherwise catalyze the air oxidation(termed autoxidation) of foods.

The chelate effect is quite general and also works in main group chemistry. For example,the British developed BAL (British anti-Lewisite) in World War I to combat the arsenic-containing war gas, Lewisite. As a bidentate sulfur donor, BAL binds tightly to the soft arseniccenter. It is still used to treat some kinds of heavy metal poisoning.

AsC l

C l

SH

SH

SO 3-

S

S

SO 3-

As + 2 HCl

BAL

+

Lewisite

Both EDTA and BAL fight heavy metal poisoning because the chelate effect enables them to bind strongly to the undesired metals and the complexes formed are small and charged, hencewater-soluble and easily excreted from the body.

In this lab you will investigate in more detailwhy the chelate effect is observed. In thereaction of tetraamminecopper(II) with ethylenediamine (equation 3 above), three particles areconverted to five particles. This means more translational disorder in the products, and thereaction should have a favorable entropy (ΔS > 0). This has been the traditional explanation of thechelate effect. However, some chemists have suggested that enthalpy may also play a role. Youwill address this question by measuringΔH and ΔS of this chelate-for-monodentate exchangereaction.

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Consider the following electrochemical cell:

Ammonia forms a rather stable complex with copper(II). Another way of saying this is to saythat the copper(II) would like to move from the left-hand beaker, where it is surrounded by merewater molecules, to the right-hand beaker, where it could enjoy the attentions of the more basicammonia molecules. The copper ions can’t make this leap directly, butindirectly the samemovement is accomplished by moving electrons from the right-hand beaker to the left-hand beaker. Simultaneous with this, copper metal is converted to copper(II) in the ammoniacalsolution and, in the water solution, copper(II) is plated out onto the copper electrode. Inert ionsflow through the salt bridge to complete the circuit. Thus:

Left-hand beaker (cathode): Cu2+ (aq) + 2e- → Cu (s)

Right-hand beaker (anode): Cu (s) + 4 NH3 (aq) → [Cu(NH3)4]2+ (aq) + 2e-

______________________________________________

Overall reaction: Cu2+ (aq) + 4 NH3 (aq) → [Cu(NH3)4]2+ (aq)

Note that the overall reaction is not a redox reaction at all, but a Lewis acid-base reaction! Nevertheless, the voltage which can be read on the voltmeter between the two cells is a measureof how desperately the reaction wants to proceed; the larger the voltage, the more spontaneousthe reaction. Put more precisely, the difference inelectrical potential (E) between the two cellsis proportional to the difference inchemical potential (a fancy synonym for the change in freeenergy, ΔG) between the two sides of the net reaction (see the Data Analysis section below).

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Method

This experiment is in two parts, in which you will work with your lab partner. This labmanual describes the first part: a two-period lab experiment. In the first period you willsynthesize and isolate [Cu(NH3)4]SO4.H2O and [Cu(en)2]SO4.2H2O. In the second period youwill use the compounds you have synthesized to make up half-cells and measure the potential between them as a function of temperature. You will then write a lab report describing what youhave done and deriving the thermodynamic parametersΔH and ΔS for each of the threereactions.

There is a common complaint about lab courses that they are “too cookbook" and don'trepresent what scientists really do. A lot of what scientists do is to try something and find that itdoesn't work too well that they overlooked some source of error or that the method wasn't greator that they just messed it up or whatever. By thinking about the problems encountered, wedevise what we hope is an improved method and then we try it. In your lab report based on thefirst two lab periods, you will propose how to make this experiment better. You can decide tocollect the same data, with an improved procedure. Sometimes, just doing the same procedure a

second time is enough to make it work better. Or you can decide that your data from the firstattempt is fine, and propose an extension of this experiment, perhaps using a different complexor different reaction conditions.

After you get your report back you will talk about your ideas with your TA. Then youwill be given a lab period (called Chelate Effect II in the schedule) to put your ideas into practice. No write-up is provided here for this "extra" lab period you have to devise your ownapproach. You will then write a revised lab report, taking into account the comments on the firstversion and describing your work in the Chelate Effect II lab period. You will be graded both onthe quality of the report and on quality of your use of this “extra” lab period.

Experimental

I. Synthesis of tetraamminecopper(II) sulfate monohydrate, [Cu(NH 3)4]SO 4.H 2O, andbis(ethylenediamine)copper(II) sulfate dihydrate, [Cu(en) 2]SO 4.2H 2O.

In each pair of students, one should prepare the tetraammine complex, the other theethylenediamine complex. Each person should follow the procedure below, with their respectiveligand. Work should be done in the hoods instead of on the bench to avoid stinking up the lab.

Dissolve 10.0 g copper(II) sulfate pentahydrate (CuSO4.5H2O; 0.040 mol) in about 10mL of water (heating and stirring will hasten its dissolution). Allow the solution to coolsomewhat. To make the tetraammine complex, add 18 mL conc. ammonium hydroxide to thecopper solution and stir vigorously. To make the ethylenediamine complex adddropwise 5.35mL (4.8 g, 0.080 mol) ethylenediamine with vigorous stirring. The basic amines can cause light blue Cu(OH)2 to precipitate, but this usually dissolves after a few seconds with the formation ofdeeply colored complex ions. If the light blue solid persists, break it up with a glass rod or aspatula; warming the suspensions may also help to get rid of the light blue solid. Adding moreammonia or ethylenediamine may help as well. If there is still light blue solid, filter the solution.

Allow the solution to cool. Add about 20 mL of ethanol to precipitate the complex, andcool further in an ice/water bath. Isolate the solids by suction filtration on a Büchner funnel;

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of the copper solutions into a separate pipette. Place a copper wire electrode into each pipette.Use a rubber septum at the top of each copper electrode to adjust the height of the electrode suchthat it is immersed in the solution but does not touch the Agar plug at the bottom of the pipette.Label your electrodes!! The level of the copper solutions within your electrodes should be belowthe level of the KCl solution. Finally, fill the fourth electrode with KCl solution and place a

digital thermometer inside. This will be used to determine when the temperatureinside theelectrodes has come to thermal equilibrium with the KCl solution.Use a voltmeter (your TA will give you instructions for the specific brand you have) to

measure the potential between A-B, B-C, and A-C. Be sure to record which lead of thevoltmeter was connected to which electrode so you can figure out the signs. The voltagereadings may drift a bit initially, but they should settle down in a few minutes. Check anddouble check that you have good connections between the copper and the voltmeter. Read thevoltages to one more decimal place than you think is meaningful. The potential across A–Cshould be equal to A–B plus B–C (once you get your signs right). In other words, the valuesshould be internally consistent. If they are not, think about what might cause a deviation, andrepeat your measurements. We suspect that the quality of the copper may influence the results,so try to use a clean piece of wire. You can also roughen the surface with sand paper. Youmight want to change the wire during the experiment to see if there’s any effect. Periodicallycheck electrodes for blue/black discoloration. Replace corroded electrodes, or polish them withsand paper. If they don’t look shiny and metallic, they may not work correctly.Note: if your voltmeter is not giving reasonable readings, ensure that there is a direct connection between the agar and the electrolyte solution. If air bubbles are trapped inside the end of the pipette tip there will not be an electrical connection.

You should make these measurements at 6 temperatures (at least) over as wide atemperature range as possible.Only move your electrodes and the white cap between baths! Themore measurements and the larger the temperature range, the more reliable your data will be.

However, keep the solutions below 45 degrees or weird things might happen i.e. , your agar plug (salt bridge) could melt or the amines might evaporate [how would you know?]. If youhave time left, you might repeat measurement(s), in order to check the reproducibility of yourdata.In addition to recording your data in your lab notebook, we will ask you to record it in acomputer spreadsheet provided in the lab. We will use this pooled data during the lecture sectionin our discussions on error.Waste Disposal

All of the copper waste from this experiment (including the solutions in the cells) should be put in a 4-L waste bottle labeled “Chelate Effect Waste”. Agar should be put in a separatewaste container “Agar Waste”. The KCl electrolyte solutions can be poured down the drain(unless a significant amount of copper has leached into it). Used copper wire goes into a specialcollection box.REMEMBER TO SAVE YOUR LEFT OVER COMPOUNDS TO USE IN THECHELATE EFFECT II THAT YOU WILL DO IN A FEW WEEKS.

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Data Analysis

Your goal is to determine the thermodynamic quantitiesΔH, ΔS, and ΔG, for each of the threereactions below (equations 1 & 3 are repeated from above). Also determine K eq at 298 K forCu(NH3)42+(aq) and for Cu(en)22+(aq).

Cu2+(aq) + 4 NH3(aq) Cu(NH3)42+(aq) (1)

Cu2+(aq) + 2 en(aq) Cu(en)22+(aq) (4)Cu(NH3)42+(aq) + 2 en(aq) Cu(en)22+(aq) + 4 NH3(aq) (3)

To obtain these values from the measured voltages, you need the basic thermodynamic relationsin equations 5-7. If these don’t look familiar, you should review the relevant sections onthermodynamics and electrochemistry in your first-year chemistry text (you might look in theindex for free energy or Faraday constant).

ΔG = -nFE (5)where n is the number of electrons involved in the reaction (in thiscase, 2) and F is Faraday's constant, 96.5 kJ/mol V.

ΔG = ΔH - TΔS (6)

ΔG = -RT lnK eq (7)

Equating the expressions forΔG in equations 5 and 6,

-nFE = ΔH - TΔS, or: E = -ΔHnF + T ΔS

nF (8)

Thus a graph of E versus temperature (in Kelvin) should have a slope ofΔS/nF and an intercept

of -ΔH/nF. Make such plots for each of the three reactions. DetermineΔH and ΔS, and thenΔGand K f at 298 K from equations 6 and 7. Make a Table with your results. Are the values youobtain ΔH˚ and ΔS˚ or just ΔΗ and ΔS? Keep in mind that we are using electrochemistry, so youneed to think about the definition of standard state conditions for electrochemistry.

Note that if you reverse the direction of reaction 1 and add it to reaction 4, you getreaction 3. Compare the values of K eq, ΔH, and ΔS that you obtain in this manner for reaction 3with those you calculated from the direct measurement. (Watch out for sign flips! Ask your TAif you aren’t sure about the math.) If there are differences, are they significant? What does the presence or absence of differences mean?

Estimate the uncertainties in the values you report. We’re looking for a qualitative feelfor how reliable you believe your values are there’s quite a difference betweenΔS = 3 ± 2J/mol.K and ΔS = 3 ± 200 J/mol.K. A good way to get this feel is by inspection of your graph.What are the maximum and minimum values that the slope (or intercept) could reasonably be,given the data that you measured?

Think about the reasons for your observed uncertainties. If there is markedly moreuncertainty in the enthalpies than the entropies, or vice versa, say why. What experimentalvalues go into these calculations (concentrations, temperatures, etc.) and how accurate are these

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uncertainty in A is given as the standard deviationσ( A). Rather than giving the full derivation(in terms of partial derivatives∂ A/∂ x etc.), we will just give the results here for three commoncases.

• When A is a multiple of one parameter:A = 3 x then σ( A) = 3 σ( x)

• When A is the sum of other parameters:A = 3 x + 8 y – z then σ( A) = {[3σ( x)]2 + [8σ( y)]2 + [σ( z )]2}1/2

• When A is the product of other parameters:

A = 4 xy/ z then σ( A) = A{[ x

σ (x) ]2 + [ y

σ (y) ]2 + [ z

σ (z) ]2}1/2

or σ( A) = 4 xyz /v{[ x

σ (x) ]2 + [ y

σ (y) ]2 + [ z

σ (z) ]2}1/2

• When A is the logarithm of a parameter:

A = ln( x) then σ( A) =σ (x)

• When A is the exponential of a parameter:A = e x then σ( A) = e x σ( x)

Lab Report

Lab Write-up (first two periods)A report template will be distributed to you electronically. Fill in the blanks with the requestedinformation and submit a printed copy one week after the completion of the experiment.

Please think about the following questions however the answers do not need to be submitted.1) Write balanced equations for all of the syntheses of Cu complexes you performed in this lab.2) Write out balanced half reactions for the redox reactions you measured in this lab.3) Make a table showing all of your electrochemical measurements. See example below.temperature (K) Cu(NH3)4

2+ vs. Cu2+ Cu(en)22+ vs. Cu2+ Cu(NH3)42+ vs. Cu(en)22+

275 0.00 V 0.00 V 0.00 V298 0.00 V 0.00 V 0.00 V

4) Construct plots of E vs. T. CalculateΔ H , ΔS , ΔG and K eq for each plot. It may also behelpful to organize these data in a table. Are these valuesΔ H °, ΔS °, and ΔG° or justΔ H , ΔS , andΔG? In other words, what does the (°) mean? What is the difference betweenQ and K eq?

5) Every reported value ( E , Δ H , ΔS , ΔG, etc.) should include an estimate of error (e.g.Δ H = 10± 1 kcal mol-1). Were there fluctuations in temperature or readings from the voltmeter? Are thereany other sources of error? Some error many not be easily quantifiable (e.g. decomposition ofcopper complexes at high temperature). Are the errors random or systematic? Explain yourreasoning.

Plot these fluctuations as error bars on the above plots. Using these error bars estimate themaximum and minimum slope and intercept of the fits to the data. Translate these fits into error bars forΔ H , ΔS , ΔG and K eq.

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6) Do the error bars account for the scatter observed in the E vs. T data? Do the error bars onΔ H , ΔS , ΔG and K eq seem appropriate? Do the errors seem too large? Too small? In other words, based on observed experimental errors do the propagated error bars make sense? Explain yourreasoning.7) Does the data support the chelate effect? Is the chelate effect enthalpy or entropy driven?

Explain the logic used to make this conclusion. How does the above error propagation effect theconclusion? The correct answer may be that the errors are so big that a conclusion cannot bereached.8) Describe the plan for the final lab period (in a couple of weeks). This should be no longerthan one double spaced page of text! Based on the above questions, particularly the discussionof errors, describe the experimental procedure you plan to pursue during the “extra” lab period.Be sure to address how your approach will address any problems you found. Feel free to discussthis with your TA. This section must be submitted or we can’t let you work in the lab during theChelate II lab period. Be certain to discuss your plan with your lab partner!Final Lab Report

The final lab report should be written in the style of the discussion/conclusion section of a formallab report (3 pages of textmaximum, NOT including plots and data tables). DO NOT write a fulllab report (Introduction, Experimental, etc.). Data should be worked up in the same manner asthe first report (tables and plots where appropriate). You do not need to re-answer the questionsfrom the first report. They should be used as an outline/guideline for the topics to be addressedin your final report. Include data from both the first and second days of the lab. It may be usefulto compare and contrast these data. Did your plan for the final lab period decrease the overallerror in the data? You might choose to omit data from the first set of measurements if you thinkthe values from the “extra” lab period are better;explain your reasoning in the report.Be sure toconcisely address the following points in your report:

Analysis of the E versus T data. Discuss your graphs and your values ofΔG, ΔH, and ΔSand K eq for each reaction. What are the errors on these numbers? If you have notdiscussed how you derived the errors in the results section, do so now.

Did you observe the chelate effect? How do you know?Is the chelate effect enthalpy- or entropy-driven? How do you know?How does uncertainty affect your conclusions? Specifically address the magnitude of the

errors in the calculated values.Think about the magnitude and sign of each of your results. Does the sign ofΔS for reaction

AB match your prediction? Was the most favorable reaction the one you thought it would be?

How do your calculated values (with error) compare to the pooled data you discussed inlecture section? Describe the differences between your values and “pooled” value andhow these differences arise. What are the advantages/disadvantages of increased datacollection?Explain any anomalies or discuss any other items you think are noteworthy.Include a short (~1 paragraph) conclusion, summarizing your findings.

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Phosphorous AcidBackground

Phosphorus lies below nitrogen in the periodic table, and one might expect its chemistryto resemble that of nitrogen. However, phosphorus is able to expand its coordination sphere inways of which nitrogen would never dream. The classic example of this is the violation of theoctet rule found in phosphorus pentafluoride, PF5. This is often explained by invoking the fiveempty 3d orbitals on phosphorus but recent research indicates that the d orbitals are not involved.The ability of phosphorus to exceed the octet rule appears to be due to its larger size (fivefluorine atoms just don’t fit around nitrogen) and to its lower electronegativity.

The lower electronegativity of phosphorous also leads to a very high affinity for oxygen.You may have learned about the Wittig reaction in organic chemistry because it is a versatilecarbon-carbon bond forming reaction. In it, a phosphorus atom exchanges a “:CHR” fragmentfor the oxygen atom of an aldehyde (or ketone):

Phosphorus forms numerous compounds with oxygen and hydrogen and these compoundsdisplay a wide variety of structures and states of aggregation. For example, there are three kindsof oxygen-containing acid with only one P atom: phosphoric acid, H3PO4; phosphorous acid,H3PO3; and hypophosphorus acid, H3PO2. [Note the spelling: the element is phosphorus, but theacids follow the inorganic nomenclature –ic for the highest common oxidation state and –ous forthe next lower; thus the acid you will prepare and study here is phosphorous acid.]

In this lab, you will investigate the structure of phosphorous acid, H3PO3. This acid can be made by adding water to phosphorus trichloride:

PCl3 + 3 H2O → H3PO3 + 3 HCl

The phosphorus acid has two plausible structures, as illustrated below (the dots in the leftstructure indicate a lone pair of electrons):

Modern spectroscopic techniques are of tremendous importance in figuring out thestructures of inorganic molecules, which span a wider range than is found in organic chemistry.This is primarily a spectroscopy experiment; you will take NMR and IR spectra and interpret

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them. You will use a technique called isotopic substitution to confirm the spectroscopicassignments. Hydrogen and deuterium are isotopes of the element hydrogen (abbreviated H andD or 1H and 2H). H and D are chemically very similar but they show up quite differently in NMR and IR spectra.

To understand your spectra, you will need some basic theory of NMR and IRspectroscopies (which you have likely seen something about in other chemistry classes; see alsothe end of the Introduction to this manual). Understanding the basic principles is particularlyimportant in inorganic chemistry because of the wide range of elements, bonds, and structuresfound. Some reference materials can be found in the lab; we encourage you to read themcarefully and ask questions. It is also important that you learn how to prepare samples andoperate the spectrometers. Finally, you will have to interpret your data. A good way to learn tointerpret IR and NMR spectra of unfamiliar molecules is to look at lots of spectra of similarmolecules. The rest of this section will walk you through the spectra of some phosphoruscompounds. NMR Spectroscopy

The most common type of NMR spectroscopy is proton or1H NMR, which you havestudied in organic chemistry.1H nuclei have a spin of ½. In a magnetic field, these nuclei have

two energy levels, ms = +½ and ms = -½ . Proton NMR looks at the energy of the transition between these two levels. Phosphorus consists exclusively of a single isotope, phosphorus-31(31P), which also has a nuclear spin of ½. So it has NMR spectra, just like the proton NMR, atleast in theory. Phosphorus NMR spectra have chemical shifts and show coupling, just like proton NMR. Coupling is observed between two phosphorus nuclei and between a phosphorusnucleus and a proton. This P-H coupling can be observed either in the31P NMR or the1H NMR.

First we’ll consider 1H NMR spectroscopy of phosphorus-containing molecules.Consider the spectrum of dimethyl phosphite, P(O)H(OMe)2, shown below. The horizontal scaleat the bottom is given in chemical shifts (1, 2, … in ppm). This spectrum was taken on a 90MHz NMR spectrometer. Since the spectrometer frequency is 90 MHz (90 x 106 Hz), 1 part permillion (1 ppm) = 90 Hz. Recall that Hz is a unit of frequency, seconds-1. In this spectrum, thereis a peak outside the normal range of 0-10 ppm, occurring at 10.6 ppm. On older NMRinstruments you would access this peak by shifting the region the spectrometer looks at by 90MHz (10 ppm); such a peak would appear at the top of the spectrum, at 0.6 ppm, labeled 90 MHzshift. On the much newer Fourier-Transform (FT) NMR instruments we use today, you’ll beable to set the horizontal scale to whatever you want so you won’t need such a shift.

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There are only two kinds of protons in P(O)H(OMe)2, the OCH3 and the PH. There aresix methoxy protons and one PH proton, so two peaks in a 6:1 ratio are expected. But wait –there are four peaks visible! The two big peaks at 3.7 and 3.9 ppm are in fact a doublet centeredat 3.8 ppm. The 0.2 ppm splitting corresponds to an 18 Hz coupling constant (1 ppm = 90 Hz).This splitting is the coupling between the phosphorus nucleus and the methoxy protons. Thesymbol for coupling is J and this coupling is described as3JPH. The 3 refers to the fact that the phosphorus is three bonds away from the methoxy protons: P-O-C-H. ThePH refers to the factthat this is coupling between P and H. The remaining two signals are the small peaks at 2.8 ppmand at 10.6 ppm. These peaks are actually a doublet, centered at 6.7 ppm (halfway between 2.8and 10.6), with the enormous coupling constant of about 700 Hz! (1JPH = 700 Hz). This doublet,due to the PH proton, has 1/6th the area of the methoxy doublet. The one-bond coupling is much

larger than the three-bond coupling because the interaction is larger when the nuclei are closer.Take a look at the next spectrum (next page), which shows phenylphosphinic acid

(C6H5)P(O)H(OH). The aromatic protons appear, as is typical, in a mess between 7 and 8 ppm. Now there are two peaks beyondδ = 10 ppm; one at 12.8 roughly twice the integral of one at10.8. The one at 10.8 has the same integral as the one at 4.2, indicating this to be the doublet dueto the PH proton (what are the chemical shift,δ , and the coupling constant1 J PH?). The large peakat 12.8 ppm is a typical POH peak. It is not split into a doublet by the phosphorus two bondsaway because the proton is very acidic and exchanges very rapidly with other POH protons withthe effect that the coupling is washed out. At very low temperatures, you might be able to slowthe exchange rate down, but at room temperature the POH proton is rather like a carboxylic acid proton.

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Now let’s focus on phosphorus NMR (31P NMR), where we look at the phosphorusnuclei. The chemical shift range for phosphorus nuclei in different magnetic environments ismuch larger than that for protons, ranging over several hundred ppm (proton chemical shiftsusually range from 0 to 15 ppm). The reference material for31P NMR is 70% H3PO4; its single peak is arbitrarily called “0 ppm” (TMS, Me4Si, is assigned as zero for proton NMR). TMS

would of course not give a signal, as it has no P atoms!

Just as we saw P-H coupling in the proton NMR, we also see it in the31P NMR spectrumas well. The origin of this coupling is that the “spinning” proton is a little magnet. As notedabove, quantum mechanics tells us that the spin ½ proton can have two possible orientations (ms values): +½ or –½. The magnetic field felt by the phosphorus atom will depend on the spin stateof the neighboring protons, so the place where you see the signal will be different – thedifference between the two signals is the coupling constant J, which is the energy of theinteraction. Since a proton can have an ms of +½ or -½, the phosphorus signal is split into two peaks, a doublet. The coupling between P and H that you see in the phosphorus NMR isexactlythe same as the coupling between H and P that you see in the proton NMR because it is the sameinteraction energy. Seeing the same coupling constant in both the1H and 31P spectra confirmsthat the coupling is a J PH.

Now what happens if we use deuterium instead of hydrogen? A deuterium nucleus, calleda deuteron, has a nuclear spin quantum number of 1. When it is attached to a phosphorus atom,the 31P NMR really gets interesting. (At the same time, the proton NMR gets pretty dull, as thereare no protons!) A nucleus with a spin of 1 can have 3 orientations in a magnetic field: +1, 0, or –1. Any phosphorus nucleus nearby will feel one of three effective magnetic fields. So the31Psignal will appear as a 1:1:1 triplet. The coupling constant ( J PD) is the distance between twoneighbors in this triplet. The magnitude of the coupling constant between deuterium and anyother nucleus is less than the equivalent hydrogen coupling constant, a factor of 6.51 smaller(this is the ratio of the nuclear gyromagnetic ratios for H and D). If the P-H coupling constant is651 Hz, the P-D coupling constant will be close to 100 Hz.

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Infrared (IR) SpectroscopyIR spectroscopy involves vibrations of a molecule, rather than the nuclear spin flips seen

by NMR. The theory of IR spectroscopy (actually the theory of any vibration) says that thefrequency of vibration (ν) for a bond (or spring) connecting atoms (or objects) a and b is relatedto the stiffness of the spring and the weight of the objects, according to the following equation:

The stiffness of the bond is represented as k ab, the force constant . μ is the reduced mass , a sortof averaged mass. For a system of two particles with masses ma and m b:

With this in mind, consider the spectra of H2O and D2O below.

The spectra look similar, just shifted. The highest frequency band (at the left) is due to O-H (orO-D) stretches. H2O and D2O are chemically very similar, so the force constants are essentiallyidentical (k OH = k OD). The big difference is in the reduced mass, since D is twice as heavy as H:

For O-H,

while for O-D,

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The reduced mass approximately doubles, so the observed bands of D2O should shift tolower frequency by a factor of about 1/√ 2 or roughly 0.7. The stretch involving the heavierisotope occurs more slowly – at lower frequency. This is just as you would expect with ballsvibrating on a spring; heavier balls will move more slowly. Check the spectra of normal andheavy water to see if the bands shift by the factor of 0.7. The lower frequency band is a bending

mode, corresponding to a scissors motion of the two hydrogens.Consider now the spectrum of dicyclohexyl phosphine shown below.

The C-H stretches come at around 3000 cm-1, like all C-H stretches. The P-H bond hasroughly the same reduced mass (check this) but it is weaker so it has a smaller force constant (itsspring is less stiff). Thus P-H stretching bands come at lower frequency thanν CH, usually between 2300 and 2400 cm-1.

The spectrum on the next page shows a strong P=O band at about 1250 cm-1. Can youspot the less intense P-H stretch? Even though the P=O bond is stronger than the P-H (higherforce constant), it has a much larger reduced mass so it appears at a lower frequency. The OHgroup attached to a phosphorus atom shows up in about the same place as other OH groups (e.g.,in water or alcohols), at about 3400 cm-1. The band is often very broad due to hydrogen- bonding.

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IR spectra are often quite crowded below 1000 cm-1, as many different kinds ofvibrations show up there, including bending and rocking motions. So you cannot identify theP-O single bond stretches in this spectrum, which also occur in this region. Assigning bands inthis region for large molecules is very difficult and it not usually attempted without extensivestudy.Method

You will do this lab in groups of two. One member of each pair will synthesize ordinary phosphorous acid, H3PO3, while the other partner will synthesize phosphorous acid in which the

protons are substituted by deuterium, D3PO3. You will then collect and interpret IR,1

H NMR,2H NMR and31P NMR spectra of the normal and deuterated acid to distinguish between the two possible structures of H3PO3 shown on p48 .

This experiment will take two laboratory periods. In the first period you will synthesizethe phosphorous acid. In the second period you will obtain the IR and NMR spectra.Waste Disposal

PCl3 (phosphorus trichloride) is a strong Lewis acid. When it encounters moisture itgives off acidic fumes (HCl) which can burn your lungs and eyes. You are, of course, wearingyour goggles, aren’t you? Do you remember where the eye wash station is? Refresh yourmemory before handling PCl3. Small spills (a few drops) and the residue left in your syringeshould be washed away with sodium bicarbonate solution (NaHCO3 aq); when it stops fizzing,there is no more acid to worry about and you can flush it down the sink or soak it up with papertowels. If you spill a lot, alert your TA.

Needles and other metal “sharps” require special disposal. Please rinse the needles with bicarbonate solution and then put them in the Red “Sharps” container (not the glass waste!). The plastic syringes are disposable, after rinsing. No glass or needles should ever go in the trash!

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Experimental

Synthesis of Phosphorous Acid or Deutero-phosphorous AcidConnect a 50 mL Schlenk flask to the Schlenk line. Add a KOH trap after the Nujol

bubbler at the end of the nitrogen manifold, as shown below. Use ~ 10 pellets of KOH and add

enough water to cover the entry tube. This will catch most of the HCl generated. This trap is not perfect, however, so you should vent it to the back of the fume hood. Then the HCl that makes itthrough the trap won’t get into the lab atmosphere that you have to breathe.

Add a magnetic stirring bar and 20 mL CH2Cl2 to the flask. Flush the flask with nitrogenfor a few seconds and close it with a rubber septum. Purge a syringe by pulling nitrogen into itthrough the septum. This should be done with the flask open to the nitrogen line. After two orthree cycles, leave about 2 mL of N2 in the syringe.

While working in the hood inject the nitrogen in the syringe into the septum-sealedcontainer of PCl3 and withdraw 1.60 mL of the liquid (2.52 g, 18.4 mmol). Inject the PCl3 intothe dichloromethane in the Schlenk flask through the septum. In another syringe, obtain 1.0 mLof distilled water, or D2O if you’re making the deuterated phosphorous acid. Note that you needto treat D2O as an air sensitive material, because it will mix with H2O in the air, diluting theisotopic purity. With the Schlenk flask containing PCl3 open to the nitrogen line, pierce theseptum with the syringe and then slowly drip the water into the stirred, chilled solution of phosphorus trichloride. HCl will be evolved and will flow out into your nitrogen line, where itwill be swept through the Nujol bubbler and mostly trapped by the KOH. Once the addition iscomplete and the reaction has moderated, remove the ice bath and stir the reaction at roomtemperature for about half an hour.

Now turn the stopcock on the Schlenk line to the vacuum, carefully evacuate the flask,and pull off all the volatile compounds. Be sure your trap is full of liquid nitrogen so none of thecorrosive HCl gas gets to your vacuum pump. If the solution is not too thick, you should stir it(with the magnetic stir bar) to avoid bumping. You should eventually get a solid. If you get anoil, try adding some fresh CH2Cl2 and pumping it off. Once the dichloromethane has beenremoved, place the flask in a warm water bath (~60 °C) and continue pumping on it for the rest

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of the period, to remove most of the remaining water. When the end of the period draws near,fill the flask with nitrogen and replace the septum with a glass stopper, against a counterflow ofnitrogen. Remember to grease the stopper so it won’t get stuck. Shut the stopcock on thesidearm and remove the flask from the Schlenk line. Make sure you have your name somewhereon the flask, as it will be placed in a vacuum desiccator along with many other flasks. Put the

flask in the desiccator and open it to the dynamic vacuum so that it can be pumped on until thenext class. This is to get the phosphorous acid product dry as possible.Spectroscopy

Phosphorous acid and deuteron-phosphorous acid are hygroscopic – they suck water outof the air. If you have time, prepare your IR samples in the drybox. Previous 317 students havesaid that making the IR samples out in the air is fine if you work quickly to avoid contaminationwith water; make your own decisions. Assemble a dry agate mortar and pestle, Nujol, and twosalt plates (for a Nujol mull). Pulverize some of the phosphorous acid in the mortar, maybe 20mg; if it is an oil rather than a solid, smear some into the mortar with a clean glass rod or pipette.Mix with a tiny amount of Nujol oil to prepare a Nujol mull; this should be a thick andhomogenous paste. In this experiment, you should make this mull as concentrated as possible inorder to see the bands of interest, which are weak. Once the acid is mixed with the Nujol it isfairly stable to atmospheric water. Grind well, for as much as 10 min (!) according to some previous students, to get the best resolution. Scoop up enough of the mull to coat the center portion of a salt plate and add the other salt plate to make a sandwich. Don’t squeeze the platestogether too strongly or you’ll push the mull out (or worse, crack the salt plates). These reallyare “salt” plates, made of NaCl, so you need to be gentle with them. They crack very easily anddissolve quickly in water (they also cost more than $10 each).

Record the IR spectrum from 4000-500 cm-1 or thereabouts. Also get an IR spectrum of pure Nujol, so that you can identify the peaks in your spectrum just due to the Nujol. Plot allyour IR spectra in this lab on the same horizontal scale (e.g., 4000-500 cm-1), so that you can just

lay one over the other to identify which bands are different. It would probably be useful to getexpansions of your spectra, for instance 1400-700 cm-1; just be sure to plot the same expansionfor all the different spectra.

Make up NMR samples of H3PO3 in D2O, and D3PO3 in H2O (label your tubes). About10 mg of sample is all you need for each. Talk with your TA about obtaining1H, 2H, and 31P NMR spectra.Data Analysis/Lab report

Make sure your lab partner has copies of all your spectra; each of you should analyze both compounds. You should turn in, one week after completion of the lab, a packet containingyour and your partner’s spectra and tables of the important bands with your assignments. Theonly writing required is a paragraph or two stating your conclusion as to what is the structure of phosphorous acid, and your evidence for the structure assignment. Be sure to site specificspectral evidence which either supports or excludes either of the two structures. We’ll discussthe spectra and their interpretation in the Tuesday morning lab discussion hour, so make sure to bring your spectra!IR Spectra: Assign the major bands due to phosphorous acid in both IR spectra (maybe three orfour bands in each spectrum). You don’t need to interpret every wiggle. Use the model spectra

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Phosphorus Acid Chemistry 317 Lab Manual

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included above to help you with the assignments. In particular, you should be able to figure outwhich peaks in the spectra involve a bond to a proton or a deuteron. These will be the modesthat are very different between the spectra of the H3PO3 and D3PO3 - roughly a factor of 0.7 infrequency. Can you figure out from the spectra whether the samples were wet? If so where didthe water come from? Make a table with the assignments and the frequencies of the bands for

both H3PO3 and D3PO3, such as shown below.Observed bands (cm-1)H3PO3 D3PO3 Assignmentxxxx yyyy O-H/O-D stretchetc. … …

NMR Spectra: List all the resonances and coupling constants for each spectrum, and assign allthe peaks. The proper way to do this is to first indicate the chemical shift in units of ppm, thenmultiplicity of the peak – a singlet, doublet, triplet, quartet, and so on – and then, if it is amultiplet, the coupling constant in units of Hz. The coupling constant is separation of two

neighboring peaks in Hz. You may need to get the spectrometer frequencies for the1

H and31

Pspectra from your TA, if they’re not printed on the spectrum. If the frequency is 50 MHz (50 x106 Hz), then 1 ppm (1 part per million) in the phosphorus spectrum equals 50 Hz. Another goodway to calculate the coupling constant, J, is to use the chemical shift difference between the peaks (Δδ , in ppm) and the frequency at which the spectrometer is set (f, in MHz):

J = Δδ × f Note that f will differ depending on the type of NMR (1H, 31P, etc.) and the spectrometer used.

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BF 3 NH 3 Chemistry 317 Lab Manual

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The Lewis Acid-Base Adduct BF 3.NH 3

Background

Boron trifluoride, BF3, is a canonical (prototypical) Lewis acid. In it, boron forms onlythree bonds, leaving a vacant orbital that it could use to share a pair of electrons donated byanother molecule. Ammonia, NH3, is likewise a canonical Lewis base, with a pair of electronsavailable for sharing. Not surprisingly, they combine to form a stable adduct which is paradigmatic of the Lewis acid-base interaction:

BFF F N

H

H

H

N

H

H

H

B

F

F

F

:+ :

In this lab you will prepare this adduct simply by mixing boron trifluoride and ammonia.

There is one unusual technical twist involved here: the reagents BF3 and NH3 are gases atambient temperatures and pressures. They are also both rather hygroscopic and need to be protected from atmospheric moisture. (The adduct, though, is an air- and water-stable solid.)The Schlenk techniques you have learned for protecting compounds from air and moisture needto be adapted to work with gases in particular it does no good to have an inert gas around. Youwill instead use a set of gas-handling strategies known as vacuum-line techniques.

Schlenk methods use differences in pressure to move liquids around, for instance pushingsolutions through a frit or a cannula. In contrast, vacuum-line methods use differences intemperature to move gases around. Say flask A shown below was filled with BF3 and youwanted to transfer the gas into flask B, and both flasks were attached to the vacuum manifold ofyour Schlenk line.

Schlenk vacuum manifold

Flask BFlask A

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result of all of this quantum mechanical stuff (that you may have seen in Pchem) is that avibration of a gas phase molecule will appear in the IR spectrum not as a single band but as a setof lines a broad envelope of more-or-less regularly spaced spikes. Each spike results from atransition that has both a vibrational and a rotational component (e.g., a molecule in the groundvibrational state and the tenth rotational state goes to the first vibrational excited state and the

ninth rotational level).Two such sets of lines are sketched below, one with a central spike (b) and one without

(a). You measure the vibrational frequency at the spike or the center of the open area. The presence of the central spike (called a Q branch) depends on the symmetry of the molecule.Spectrum (a) is plotted as an absorption spectrum (like a UV/Vis spectrum) while (b) is plottedas % transmittance through the sample, as is more typical for IR spectra. In (a), the spacing ofthe lines and the resolution of the spectrometer is such that individual lines are observed, whilein (b) only the overall envelope is seen because the lines are so close together. The distance between the lines is determined by the moment of inertia of the molecule about a rotational axis,such that widely spaced lines (case (a)) are observed only for small molecules with low momentsof inertia. NH3 has a low moment of inertia because only the light hydrogens move on spinningabout the nitrogen, while in BF3 the much heavier fluorine atoms must move on rotation.

(a) (b)

IR spectra of (a) HCl gas at high resolution, plotted in absorbance (peaks go up) and (b) CH3Brgas at low resolution, plotted in % transmittance (peaks go down).

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Method

You will work in pairs for this experiment, which will take one period. You will fill twoSchlenk flasks with BF3 and NH3 gases from gas cylinders; the pressures you measure on thedigital vacuum gauges and the volumes of the flasks will allow you to determine the quantity ofgases. You will also take gas-phase IR spectra of these two reagents. You will make solidBF3. NH3 by vacuum transferring NH3 gas into a flask containing BF3 at liquid nitrogentemperature, and allowing the BF3/NH3 mixture to warm to room temperature. Comparison ofthe IR spectrum of the adduct with that of the starting materials may shed some light on thechanges in bonding that occur when the adduct forms.

Procedure

0. SAFETY!

Please review the section in the Introduction of this manual on gas handling (pp. 20-21).Do not open the lecture bottles unless the TA is working with you. It is imperative that you deal

with these gases safely, as both BF3 and NH3 are toxic. There have been several incidents in thislab that vented a significant amount of BF3 into the air and the lab had to be evacuated.

The key to safe handling of gases is to be sure that pressure does not build up in any portion of your apparatus. Before you let any gas in, trace the path the gas will take. Make surethat the gas can flow through to the manometer and vacuum gauge (i.e., check that all thestopcocks that you think are open actually are open). One of the incidents noted above involvedopening the BF3 when the stopcock on the Schlenk line was closed. Pressure built up in therubber hose until the hose ruptured. Another incident also resulted from too much pressure inthe Schlenk line. In that case, the students did not wait long enough for the digital gauge torespond before adding more gas. To avoid such problems, you should let only a small amount of

gas in at first. Know where the gas is going, and check that you see the gas entered your line bylooking for a response on the gauge. Be sure to wait for the gauge to respond.Both of the gases have built-in leak detection characteristics. BF3 reacts on contact with

the moisture in air, forming a white fog of boron oxides and HF (quite toxic); this is not a goodthing to inhale. NH3 doesn’t leave visual evidence of a leak, but it has a characteristic odor withwhich you should be familiar. If you see white fog seeping from some part of your apparatus orsmell a strong ammonia smell,close the main valve on the lecture bottle and alert a TA.

I. Transfer of gases from cylinders to flasks*The procedure for filling a flask and the IR cell with NH3 is identical to that for BF3. If the

BF3 cylinder is in use, by all means do the ammonia first. Remember to use separate Schlenkflasks for each gas, though! Also, use a slight excess (in pressure) of ammonia over borontrifluoride. At the end of Part I, you should have two closed, gas-filled flasks.Before you begin, your Schlenk line must be brought up to specifications. A good staticvacuum must be demonstrated to your TA and approved before you begin using them tomove these highly toxic gases.

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To begin, connect the cylinder of BF3, a 250 mL Schlenk flask with a glass stopper, and agas-phase IR cell to the Schlenk line, as shown on the following page. Be sure to use the IR cellswith the proper gas written on the label. The cells can contain traces of gas from previous usersand if you combine the two different gases, they will react. You will then have the unenviabletask of cleaning the IR cell. The hose connecting the BF3 to the line should be clamped on, to

help prevent leaks. Also, all of the stopcocks should be clamped if possible we have hadseveral stopcocks that were not clamped become airborne during the course of this lab.

We have two BF3 cylinders, two NH3 cylinders, and four gas phase IR cells in the lab (two foreach gas). Try to be flexible with your procedures as you will need to share these reagents andgas cells. Stay in touch with the TA(s).

Open the vacuum manifold to the pump, then open all the stopcocks in the setup: thethree on the vacuum manifold, the one on the IR cell, and the one on the Schlenk flask. Makesure the main valve on the cylinder is closed but that the needle valve is open. Always use thevacuum manifold of the Schlenk line for reagent gases, because that manifold has the vacuumgauge on it. Let the system pump out for a few minutes, to reach to lowest pressure inside thatthe vacuum pump can attain. Now test to see if there are any leaks. Close the stopcock thatconnects the vacuum manifold to the trap and pump. Watch the level on the vacuum gauge: if itincreases at an appreciable rate, it means that air is leaking into the system from somewhere.You can close various stopcocks to see if you can isolate the trouble; poor hose connections arethe most likely sources of leaks. If you need help finding or fixing your leaks, ask a TA. If thegauge is steady with the manifold cut off from the pump, then your system is not leaky and youare ready to admit the BF3. Write down the reading on the vacuum gauge that corresponds tofull vacuum.CAUTION: Boron trifluoride will corrode your rubber hoses if it is left too long in contact withthem, so work as quickly as possible. (It's also corrosive to lungs.)

With the vacuum manifold and flasks etc. all under vacuum butnot open to the vacuum pump, you're ready to admit the BF3 gas to the manifold+flasks system. Close the needle valveand then open the main valve on the lecture bottle. (Do this carefully every so often you willhave a leak and spew gas into the lab. If there are any problems, close the main valve, close your

Schlenk vacuum manifold

Flask A gas phase IR cellBF 3 cylinder

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hood sash and get your TA.) Check to be sure that the valve on the Schlenk line is open so thatthe gas has a place to go. Open the needle valve a little for an instant, and then close it. Thisshould let a small pulse of gas out. Watch the vacuum gauge on the Schlenk line when you'refiddling with the valve. The reading on the gauge should go up when you let gas in, then stop(you have no leaks, remember?). Make sure to wait for a response from the gauge. This can

take a little time. If what you anticipate does not happen, shut the valve off and think throughwhat you are doing. If you can't quickly figure out what is wrong, get your TA. If you're gettinggas, you're ready for business.

Let in the gas in small puffs, opening the needle valve briefly and then closing it. Wait in between puffs to see a change on the digital vacuum gauge. One partner can work the needlevalve while the other (both?) watch the vacuum gauge. Let in roughly 400 torr of gas. It doesn’tmatter if you get exactly this amount in the system, as long as you record the exact amount youdo let in. At this point, close the stopcocks to the gas IR cell and the Schlenk flask to imprisonthe BF3 therein. Then open the stopcock to the pump, which sweeps out the unused BF3 awayfrom the rubber hosing into the liquid nitrogen-cooled trap. Keep the trap full of liquid nitrogenso that the BF3 is efficiently trapped and not pulled through to corrode the pump. Once all theBF3 has been pumped away from both the line and the rubber tubing, you can detach the cylinderand let someone else use it.

The amount of BF3 in the IR cell is higher than is desired for best resolution. Close allthe stopcocks on the manifold except the one leading to the IR cell. Close the stopcockconnecting the manifold to the pump and open the IR cell to the manifold. We think that the bestresolution and intensity in the IR spectrum is obtained for pressures in the cell of roughly 30-50torr. You may need to pump a little of the BF3 away to get to this pressure range (don't pump itall away!). Re-close the IR cell stopcock and pump the excess gas in the vacuum line into thecold trap. After all the BF3 has been removed, you can disconnect the cell from the line and takethe IR spectrum. When you’re finished, reconnect the cell to the line and pump out all the BF3in it. Please be quick with the IR cell so others can use it.

Repeat the above procedure using NH3.

II. Reaction of boron trifluoride with ammoniaAt this point you should have two closed gas-filled flasks connected to the vacuum line

as shown below. Keep the stopcocks on the Schlenk flasks closed until you want to actuallytransfer the gases (or you’ll pump away your material). Evacuate the system and test for leaks as before. Once you’re satisfied everything is leak-free, put the manifold and hoses under dynamicvacuum and cool the flask with the BF3 in a liquid nitrogen bath. You should see a white solid

condense on the walls of the flask (what is it?). When the flask is cold, open the stopcock on theBF3 flask to dynamic vacuum as well, to remove any air that might have leaked in (why doesn’tthis pump away the BF3?). Once vacuum is established within the flask and manifold, close thevacuum manifold off from the pump. The manifold and hoses are now under static vacuum.

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Schlenk vacuum manifold

NH 3BF 3

You are now ready to transfer the NH3. Open the stopcock on the cold BF3 flask, thenthe stopcock on the NH3 flask. The pressure in the line should rise as NH3 flows into it, then fallas it condenses in the cold BF3 flask. Once you have judged the transfer to be complete, pumpany gases remaining in the line into the cold trap. You can leave the stopcock on the cold flaskopen for this; these condensed gases have a negligible vapor pressure at -196 °C and won't goanywhere. Close the stopcock on the cold flask be sure the stopcock is clamped and removethe liquid nitrogen bath. Let it warm slowly to room temperature; record your observations.Once the flask has come to room temperature, open it to the vacuum line and pump out anyunreacted gases. Then scrape out the solid, weigh it, and take an IR spectrum (preferably as aKBr pellet but a Nujol mull is OK).

Waste Disposal

All of the waste from this experiment should be placed in a 4-L waste bottle labeledappropriately, “BF3 - NH3 Waste”.

CAUTION: Your trap will contain both BF3 and NH3 at the end of the experiment. Be sure tokeep it pretty full of liquid nitrogen. Let your TA shut down your Schlenk line; he or she willremove the trap while still very cold, and let it warm up in the back of the fume hood.

Questions

Please turn in your spectra and answers to the questions below one week after completion of theexperiment.

1. Calculate the number of moles of the reagents you used. The ideal gas law is appropriate forthis calculation. When you measure out solids or liquids for a reaction you have to divide thenumber of grams you used by the molecular weight to get moles, but in this experiment you just use roughly the same pressure of both reagents. Why?

2. What was your yield of product? Where did the rest of it go?

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3. Assign your spectra and prepare a table of IR data with peak positions and assignments. Youwill likely want to consult with the excellent reference book by Nakamoto, Infrared and

Raman Spectra of Inorganic and Coordination Compounds . Spectra for both BF3 and NH3 appear in here, in the section on XY3 molecules in the earlier portion of the book. What dothe IR spectra tell you about BF3. NH3?

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Arene-Molybdenum Chemistry 317 Lab Manual

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Arene Molybdenum Tricarbonyl Chemistry

Background

The metal compounds you have dealt with in this class so far and in Chem 165 or 312or 416 all formally contain metal cations that act as typical Lewis acids. The metals bindtypical Lewis bases such as water, ammonia, chloride, and acetate. Such metal complexes have been a staple of inorganic chemistry for well over a century, with some of the compounds knownsince the days of the alchemists. In the 1950s, a new area of metals chemistry took root,organometallic chemistry, that in a few decades has grown into one of the major areas ofchemistry. As the name suggests, organometallic chemistry is the chemistry of metals bound toorganic groups, mostly compounds with metal-carbon bonds. These organic ligands includealkenes and alkynes, arenes and other aromatic compounds, alkyl and aryl groups, and carbonmonoxide.

The organometallic field has blossomed both because it is new and interesting chemistry,and because it can be very useful, in a practical sense. In both the organic laboratory and theindustrial plant, organometallic reagents and catalysts have led to a number of new processes.For example, acetic acid, a major industrial chemical, is now made in large part by carbonylationof methanol using an organometallic rhodium catalyst. This process (in the reaction shown below) was developed by what was then called the Monsanto Corporation.

[Rh(CO) 2I2]-

I- co-catalyst

CH 3OH + CO CH 3COH

O

A number of polymers are made using organometallic catalysts, for instance polyethylene fromethylene, so called Ziegler-Natta polymerization:

The pentagons with ovals in them stand for the cyclopentadienyl anion, C5H5-, which is aromatic

just like benzene (and is a very good ligand for most transition metals). Billions of dollars arecurrently being invested in alkene polymerization plants using the newest transition metalcatalysts. You can learn more about organometallic chemistry in the course on this topic, Chem417.

Many of the organic ligands found in organometallic chemistry are not classic Lewis bases, for instance the arene and carbon monoxide (CO) ligands you will use in this experiment.Even carbon ligands that are formally anionic, such as CH3

– form quite covalent bonds with mostmetals and are not well described by M+←CH3

– . So the bonding and the nature of

T iCl2 , AlEt 3

n-2

n H 2C = CH 2

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organometallic compounds is in some ways quite different from the bonding in "classical"coordination compounds. Because the compounds are fairly covalent, they resemble organic andmain group covalent compounds. Instead of the octet rule for main group compounds,organometallic compounds usually obey an "eighteen electron rule." Just as the octet rule arisesfrom the filling of the four available orbitals (one s + three p orbitals), metals which have d

orbitals want to have a total of 18 valence electrons in nine orbitals (one s + five d + three porbitals).The molybdenum compounds you will work with in this lab are good examples of the

eighteen electron rule. Your starting material molybdenum hexacarbonyl, Mo(CO)6, has sixcarbon monoxide (CO) ligands bound to the molybdenum. Each CO has a lone pair of electronsthat it uses to bind to the molybdenum, in other words each CO contributes two electrons towardthe molybdenum's goal of eighteen. The molybdenum itself brings six valence electrons, so thetotal electron count is

6 from Mo + 6× (2 per CO) = 18 total electrons about Mo.

The reaction you will do in the first part of the lab is displacing three of the carbon monoxides byan arene ligand, mesitylene:

C

C

Mo COOCC

CO

CMo

CC

+ 3 COreflux+

OOO

OO

O

The product has a so-called "three-legged piano stool" structure as drawn above. The fact that allsix aromatic carbon atoms are bound to the molybdenum is indicated by insertingη 6 in thename: (η 6-C6H3Me3)Mo(CO)3 [η is the Greek letter eta, which rhymes with beta]. The bondingwithin the arene is delocalized, with equal C–C distances. But we can think of the arene as beingcomposed of three double bonds, each of which has twoπ electrons. Thus an arene is sort of achelating tris-alkene ligand, which can donate six electrons to the molybdenum. Therefore, onearene displaces three CO ligands and the compound is still eighteen electrons:

6 from Mo + 6 from the arene + 3× (2 per CO) = 18 total electrons about Mo.

As implied above, arenes and CO act as electron pair donors (Lewis bases) in their bonding to a metal such as molybdenum. Thisσ bonding interaction is shown asA on the next page: the lone pair on carbon monoxide donates into an empty d orbital on the molybdenum.

This σ bond is just like the donation of water or ammonia to a metal center. But there is anadditional interaction, made possible because CO has an emptyπ-antibonding orbital locatedmostly on the carbon (B). The molybdenum can donate its electrons back to the CO, into thisorbital. This empty orbital on CO acts as a Lewis acid it accepts a pair of electrons in a π bond. For this reason, CO is called a "π-acid" ligand. Arenes and alkenes and many otherligands have similar interactions.

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MoC O

Mo

C O

B.A.

π backbondingπ donation from Mo to CO

σ bondingσ donation from CO to Mo

This π-backbonding has a number of critical effects, as explained in Chem 417. Onefeature is that it sucks electron density off of the metal, and therefore stabilizes low metaloxidation states. In the compounds in this experiment, molybdenum is formally in a zerooxidation state! With ligands that cannotπ-backbond, like water or ammonia, molybdenum isonly found in oxidation states +2 and higher.

Another key feature of thisπ-backbonding is its effect on the carbon monoxide. Theπ-backbonding interaction puts some electron density into a CO antibonding orbital, which

weakens the C–O bond. This is most evident in the CO stretching frequency. The C≡O stretchin free carbon monoxide gas occurs at 2143 cm-1, while in Mo(CO)6 this IR band is at lowerfrequency (you'll determine the value in the lab). When three CO ligands are displaced by thearene, the amount of backbonding changes and the CO frequency shifts. Since these shifts aresubstantial and the stretches are quite intense, the CO bands in the IR spectrum provide animportant handle on what is going on. The development of metal carbonyl (M-CO) chemistryhas relied heavily on IR spectroscopy. Mesitylene is a poorerπ acid and a betterσ donor thanCO (almost all ligands are), so the reaction you're doing makes the molybdenum more electronrich. This should cause the stretching frequencies for the remaining CO ligands to shift to lowerfrequency.

The conversion of Mo(CO)6 to (η 6-C

6H

3Me

3)Mo(CO)

3 also changes the symmetry at

the molybdenum center, from octahedral to three-fold symmetry. The symmetry at the metaldetermines, via quantum mechanical selection rules, how many CO stretches you will see in theIR spectrum. Although Mo(CO)6 has six carbonyl ligands, there is only one CO stretching modein the IR. The three CO ligands in (C6H3Me3)Mo(CO)3 should give rise to two IR bands. Thus both the position and the number of CO bands in the IR provide useful information.Method

Four lab periods have been allocated for this experiment, in two blocks. In the first block, you and your lab partner will use one lab period to react Mo(CO)6 with mesitylene andisolate the product. You will also characterize the product by IR spectroscopy and by NMR. In

the second block, three periods have been left open for you and your partner to choose what youwant to do, based on your interests, the papers in the 317 folder and additional inspiration youfind in the library. You might want to explore the reactivity of the (η 6-C6H3Me3)Mo(CO)3 youmade or you might make something else from Mo(CO)6. We expect that you and your partnerwill do two reactions (of 4 proposed) of your own development (or more if you have time andinterest).

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Procedure

0. SafetyCarbon monoxide is a highly toxic, odorless and colorless gas. Since reactions of metal

carbonyls often give off CO, they should be done with care and only in a fume hood or on awell-vented Schlenk line. Metal carbonyl compounds themselves are often highly toxic,especially volatile compounds such as Ni(CO)4 (boiling point 43˚ C). Mo(CO)6 is not so volatileat room temperature and therefore does not present a serious danger, but it should be handledwith care in a hood.

1. Preparation of Mesitylene Molybdenum Tricarbonyl, (C6H3Me3)Mo(CO)3.

Adapted from Angelici, R. J.Synthesis and Technique in InorganicChemistry University Science Books, Mill Valley, CA 1986 pp.129-146.

Put 2.0 g of Mo(CO)6 (0.0072 mol), 10 mL mesitylene and a

stir bar in a 100-mL round-bottom Schlenk flask. Attach acondenser to the flask and connect to a mineral oil bubbler, asshown in the drawing at right. The bubbler should set up into thehood so that you will know that the CO gas evolved is making it tothe hood. Connect the side arm of the Schlenk flask to yourSchlenk line.

Mo(CO)6 is volatile enough that it will sublime out of thereaction flask upon heating. You should let the mesitylene vaporsreflux high into the condenser to wash the Mo(CO)6 down into thereaction flask. Therefore, don't cool the condenser with water theroom temperature will be adequate to condense the mesitylene,since it boils at such a high temperature (b.p. 165

˚

C). Place thereaction apparatus in a heating mantle appropriate for the sizeSchlenk flask you are using.

Mo(CO)6 and (C6H3Me3)Mo(CO)3 are both pretty stable toair at ambient temperatures, but would react with atmosphericoxygen and/or water at elevated temperatures. For this reason youneed to conduct the reaction under an inert atmosphere. This could be done by preparing an allsealed system, by evacuating a flask and closing all the stopcocks. But great care should betaken before ever heating a closed system the system has a tendency to build up pressure and blow up. (In this reaction, you're liberating three equivalents of CO gas, in addition to the

normal increase in vapor pressure of the solvent and the gas expansion on heating from 25 to 165˚ C.) So it's much better to do this reaction open to your oil bubbler, which keeps everything at 1atmosphere pressure. First you need to flush the apparatus with a moderate to fast flow of N2 forca. 5 minutes. To get the nitrogen to flow through your apparatus, you will likely need to prevent the nitrogen from flowing out the bubbler at the end of the nitrogen line. Double checkto make sure that nitrogen is flowing through your apparatus to your new bubbler. Once thesetup is flushed, turn off the nitrogen, close the stopcock (why in that order?), and turn on theheating mantle with the variac.

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(d) React Mo(CO)6 with a different type of ligand.

(e) Do similar reactions with other metal carbonyl compounds.A very valuable class of ligands for organometallic chemistry are phosphorus ligands. They aresoft donors with someπ-acid character, although much poorerπ-acids than CO. We will have

available in the lab triphenylphosphine, PPh3, and trimethyl phosphite, P(OMe)3, in case youwant to use them. Apparatus for various kinds of chromatography can also be made available ifyou need to separate product(s) from your reactions.Before you choose what to do, think about the questions in part II of the lab report description onthe next page. Most importantly,how will you know what happened ? This is a key question inany synthetic procedure, and relates to how you will characterize your products, as described inthe next section.

3. Characterization of (C6H3Me3)Mo(CO)3 and the products of your other reactions.

You should get IR spectra of (i) your starting material Mo(CO)6, (ii) the mesitylene

complex (C6H3Me3)Mo(CO)3, and (iii) whatever materials you obtain in your other reactions.Please refer to the introduction to IR spectroscopy in the techniques section (pp. 23-25) if youhaven’t done that lab already. This section describes, for instance, how to prepare a sample forIR as a Nujol mull. IR spectra of molecules in solution are also very useful, as solution spectraare often better resolved. Solution IR cells will be available (be careful they're very expensive,fragile, and made of salt, so no water!). Dissolve a few crystals in 2 mL heptane or hexane orchloroform. Get a blank of the solution IR cell with solvent in it, and then replace that solventwith your sample solution using a syringe. You should also get spectra of just the solvent usingthe empty cell as a blank. Wherever the solvent absorbs strongly you’re not going to get anyinformation about your compound (why not?). Wash the solution cell by pouring the solutionout (into a waste bottle), and filling it with fresh solvent and decanting at least twice. Dry the

cell with a rapid flush of nitrogen and return it to a desiccator. If any sample gets on the outsidesurface of the cell window, rinse it off immediately with chloroform or methylene chloride.You should also get proton NMR spectra of mesitylene, (C6H3Me3)Mo(CO)3, and your

products from the other reactions. Your TA will advise you on how to make up the NMRsamples something like 10 mg of sample dissolved in CDCl3 (chloroform-d ). (Note thatMo(CO)6 does not give a proton NMR spectrum as there are no protons.) If you use phosphorusligands, you will likely want to get31P NMR spectra as well (see the Phosphorous Acidexperiment, #3, for instructions on obtaining these spectra).

You and your lab partner should divide up the tasks you need to do. One can stay and setup a reaction while the other is taking NMR spectra. IR spectra can be taken during a slow time

in the lab, for instance while reactions are refluxing. Do a preliminary analysis of your spectraright after you get them, as this will help guide you in what to do next.

Waste Disposal

All of the waste from this experiment should be placed in a 4-L waste bottle labeledappropriately, “Arene Waste”.

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Lab Report

Preliminary Report for Part I - due one week after the conclusion of the first part of the lab

The basic elements of an Introduction, Results, Discussion, Experimental, and Reference

sections should all be included. Follow the formatting guidelines provided for you in theintroduction section of this manualand use the templates from chromous acetate and the chelateeffect as examples. Be sure to include the following items in the appropriate sections of yourreport:

• Background and purpose of the synthesis and subsequent characterization. Include aminimum of 2 literature references (the 317 manual is NOT literature).

• Give and discuss your percent yield.

• Prepare tables summarizing your IR and NMR spectra, including any assignments youcan make. (In addition to appropriate characterization in the Experimental section.)

• Compare and contrast the complexes you made and/or starting materials. Is there anystarting material in your product? Does it look clean (by IR, NMR, and visualappearance)?

Proposal for Part II - due one week after the conclusion of the first part of the lab

• A template for this assignment will be distributed electronically. Please complete thistemplate and attach the requested supporting documents. Note that this assignmentshould NOT be attached to your part I report as it may be sent to a different grader.

Final Report Parts I and II

Return your graded reports and grade sheet from the first part of this lab, in addition to the final

report. Do not include the proposal documents. Refer to the guidelines given in the introductionof this lab manual for the correct formal report formatting and use the templates from chromousacetate and the chelate effect as examples.

The format of this report should remain the same as the preliminary report, but willreflect the syntheses and characterization done in all four lab periods.

Use comments received on the preliminary report and incorporate the following ideasfrom part II to form a comprehensive final lab report:

Describe the experiments that you designed. What did you hope to accomplish and didyou succeed? Provide references for any sources you used. Answer, to the best of yourknowledge, the key questions for any synthesis:

i) Was a single product formed?ii) Is there any starting material left?iii) What did you make?iv) How do you know?v) Compare and contrast the characterization you obtained with those reported in theliterature. (Remember to appropriately cite this literature in your report.)

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A full analysis can probably be done inroughly three to five pages of text , not

including attached spectra and other data. But clarity and completeness are most important, so ifyou need to write a longer report you should do so. You will be graded on how well youapproached and analyzed your synthesis, and how clearly you wrote it up. Please don’t do

something just because you think it will be easy. We want you to be imaginative, adventurous,and to have some fun here.

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Linkage Isomers Chemistry 317 Lab Manual

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Linkage Isomers of Nitro-Pentammine-Cobalt(III)Background

Coordination chemistry is the study of how ligands (bases) coordinate to (bind to) metal

cations (Lewis acids). Common ligands include water, as in aquo ions [M(H2O)x]n+, ammonia(NH3), and pyridine ( N ). The nitrogen bases are in general much better ligands than water because they are better bases. You have already seen in Experiment 2 that adding ammonia to alight blue aqueous solution of Cu2+ causes rapid formation of the dark purple copper ammoniacomplex. A complex with ammonia is called an ammine complex.

Ammine complexes were the first coordination complexes studied in detail, back at theturn of the 20th century. Alfred Werner and others made a large number of ammine complexesof cobalt(III), such as [Co(NH3)6]3+. Studies of the chloro-ammine complexes showed that[Co(NH3)5Cl]2+ exists as one isomer, [Co(NH3)4Cl2]+ has two isomers, and Co(NH3)3Cl3 has

two isomers. This proved (or at least strongly suggested) that these compounds have octahedralstructures. The structures of the two geometrical isomers of Co(NH3)3Cl3 ( facial andmeridional ) are shown below. [You can find translations of some of Werner’s classic papers inClassics in Coordination Chemistry , G. B. Kaufman, Ed., Dover, New York, 1968.]

Cl

Co

NH3

ClCl

H3 NH3 N

Cl

Co

Cl

Cl NH3

H3 NH3 N

meridonal facial

Another form of isomerism common in coordination chemistry islinkage isomerism .This can occur when a ligand has two different basic sites, so it can bind in two different ways.The oldest recognized example of linkage isomerism involves the nitrite anion, NO2

¯ . In 1894,Jørgensen reacted [Co(NH3)5Cl]Cl2 with sodium nitrite, NaNO2, to give a red solution fromwhich he isolated red crystals. If the red solution was boiled with acid, it changed color anddeposited yellow crystals on cooling. No problem, thought Jørgensen, who doubtless thought hehad made some other cobalt complex. However, the red and yellow complexes had identicalelemental compositions. Jørgensen and Werner proposed, based on the colors of other cobaltammine complexes with nitrogen and oxygen donor ligands, that the red compound contains a“nitrito” ligand, bound to cobalt through an oxygen atom while the yellow compound has a“nitro” ligand, bound through its nitrogen atom. The isomers [Co(NH3)5ONO]Cl2 (nitrito) and[Co(NH3)5 NO2]Cl2 (nitro) are drawn below. The formula [Co(NH3)5 NO2]Cl2 means that there isa [Co(NH3)5 NO2]2+ dication, in which the cobalt is coordinated to (bound to) five ammonialigands and one nitro group, and that there are two chloride counterions to make the compoundelectrically neutral.

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This experiment probes the mechanism of the isomerization of the nitrito to the nitrocompound, by studying the kinetics of the reaction. One can never prove a mechanism, but onecan distinguish between possible options. For instance, consider the two mechanisms drawn below. Does the cobalt slide across the nitrito ligand from O to N (path A below), or does thenitrogen atom of the nitrito ligand reach out and grab another cobalt before the oxygen atom lets

go of the first cobalt (path B)? In these drawings, the square brackets indicate intermediates thatare not observed or transition states, marked with‡.

A goal of any mechanistic study is to figure out what is the rate-determining or rate-limiting step in the reaction. In the one step mechanism (A), the rate limiting step is as shown,while in (B) the rate limiting step could be loss of nitrite to generate Co(NH3)5

3+ or attack of thesecond cobalt complex on the nitrite nitrogen. One common way to address these questions is tostudy thekinetics of the reaction.

Reaction kinetics are usually interpreted using a model called transition state theory. Inthis theory, a chemical reaction occurs when the reactant(s) come together with and havesufficient energy to climb an energy barrier. The top of this barrier is called the transition state,the point at which the reactants cross the barrier to the product side. The transition state is like a pass connecting one valley to another (reactants to products). The lifetime of the transition state

is very short it is not an intermediate and it is never present in any significant concentration. Ina multistep reaction, the rate limiting step is the one that goes over the highest energy barrier.

The first information you get from the kinetics is the order of the reaction: is it first orderin nitrito complex or second order? First order would mean that the transition state contains onlyone nitrito complex, while second order means that two nitrito complexes (or two speciesderived from the nitro complex) have to come together to do the reaction. Nitrite dissociation,

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k = e-ΔG‡/RT (1)

k is the measured rate of reaction, T is the temperature (in degrees Kelvin, K), and k B, h, and Rare the Boltzmann, Plank, and gas constants:

k B = 1.381× 10-23 J K -1; h = 6.626× 10-34 J s; R = 8.314 J K -1 mol-1

κ (Greek letter kappa) is the transmission coefficient, the fraction of the time that species at thetransition state really make it over to products as opposed to falling back to the starting materials(you can assume it’s 1). As mentioned above,

ΔG‡ = ΔH‡ - TΔS‡ (2)

Substituting into equation (1) gives

k = e-(ΔH‡ - TΔS‡)/RT (3)

or: = [eTΔS‡/RT] e-ΔH‡/RT

or: = [eΔS‡/R ] e-ΔH‡/RT (4)The right side of equation (4) has an initial term, in square brackets, that is independent oftemperature and a second term that is temperature dependent. So with a batch of rate constantsdetermined at different temperatures you can determineΔH‡ and ΔS‡, as described in the DataAnalysis section below.Method

This experiment is in two parts. This write-up describes the first part while in the second part, a couple of weeks later, you will execute your own plan to extend this study. This is thesame pattern that we followed in the Chelate Effect experiment.

The part described here is a four-period lab in which you will work in pairs. You willsynthesize the cobalt(III) amine complex [Co(NH3)5Cl]Cl2 and convert it into the nitrito andnitro linkage isomers. You will then study the kinetics of the isomerization of the nitrito to themore stable nitro complex to determine the reaction order and the rate constant, k, at varioustemperatures. To follow the progress of the reaction in your kinetic studies you will use UV/Visspectroscopy.Procedure

I. Synthesis of [Co(NH3)5Cl]Cl2References: W. L. Jolly,The Synthesis and Characterization of Inorganic Compounds , 1970 p.461-462. G. Schlessinger Inorganic Syntheses 1967 , 9, 160. [ Inorganic Syntheses is awonderful set of volumes containing syntheses that really work not only do they work for theresearchers that submitted them for publication but they have been checked by another group.]The original references are S. M. Jørgensen Z. Anorg. Chem. 1894 , 5, 147 and1898 , 17 , 455.

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out a quartz cuvette from the stockroom but the special UV plastic cuvettes available in the labwork just as good for this experiment.] Leave the flask, stoppered, in the water bath, put thecuvette in the spectrophotometer and record the spectrum. Be sure to save the spectrum at eachtimepoint. Don’t change any spectrometer settings once the run has started as this will invalidateyour data. In your notebook, record the absorbance at the peak and the time of this spectrum.

Use a stopwatch if one is available. What should you use as tinitial for the reaction, the time ofmixing or the time of the first spectrum or ...?Pour the contents of the cuvette into the experiment waste container, then rinse the

cuvettes with distilled water and dry with a Kimwipe (or use a free nitrogen line at a slow rate).Repeat the process of taking aliquots and measuring their spectra at noted times. At the beginning, the absorbance will change rapidly and you should try to take a measurement as oftenas possible. Later in the reaction the absorbance will change at a more sedate pace and you cantake a measurement every quarter hour or so. You can judge the progress of the reaction byoverlaying the saved spectra on the screen of the spectrometer.

When you have time during the kinetics, view some of the spectra using the overlay

function. Only eight spectra can be plotted at once so you should pick the first and last spectrum,as well as six in the middle. Make sure you get a plot like this when you’re all done. As youlook at your set of spectra on the screen, are there any wavelengths at which the absorbancedoesn’t change over the course of the reaction? These are calledisosbestic points , wavelengthswhere the starting material and the product have the same extinction coefficients (ε’s). Their presence indicates that there are only two species with absorbance in the visible region in thereaction mixture in any appreciable concentration. Do your spectra go precisely through theisosbestic point(s)? Or is there some scatter or a systematic deviation, such as the isosbestic point appearing to drift up or down?

The data analysis (see below) requires a final absorbance for the reaction, Af . Yourreaction may be fast enough that it is complete by the end of the lab period (the absorbance stopschanging significantly). But in most cases, you will need to help the reaction along tocompletion. About half an hour before the end of the period, heat your flask to 80˚ (not hotter!)for a couple of minutes to fully convert the nitrito to the nitro complex. You can monitor this bytaking aliquots, as above. Spectra of the final solution, cooled to room temperature, give the Af value. Boiling or overheating the solution could change the concentration of the nitrito productor cause decomposition and thus give an incorrect Af .

Be sure that all spectra are preceded by a blank spectrum. You can use the same blankfor multiple spectra only if you're sure no-one else has used the spectrometer in between.Because your samples are 10 mL complex in DI water plus 90 mL buffer, your blank should be a1:9 mixture of DI water and buffer.

C. Three Kinetics Runs at OnceIn the last lab period, you will follow the kinetics of three reactions simultaneously on

one spectrophotometer, measuring the absorbance at the same wavelength as the first kineticsrun.

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As in part B, make up a buffer solution 0.1 M in both NH3 and NH4Cl, enough to put100.0 mL of buffer solution in each of three labeled 250 mL Erlenmeyer flasks. (You mightwant to make 500.0 mL of buffer solution just because you have a 500 mL volumetric flask).Preheat the stoppered flasks in water baths at three temperatures that are different from thetemperature you ran in part B. Find three UV/Vis cuvettes and label them something catchy like

25˚

, 30˚

, and 35˚

(don’t put labels on the optical windows!). As above, make 50 mL of a stocksolution of [Co(NH3)5ONO]Cl2 and store it in an ice bath. Add 10.00 mL of the stock solution toeach of the preheated flasks with 90.0 mL of buffer solution. Swirl to mix, record the time, andtake an aliquot from each flask into the appropriate cuvette. Leaving the flasks in theirrespective water baths, quickly measure the absorbance at your chosen wavelength, recording thevalue and the time in your notebook. Collecting data for three concurrent kinetic runs is hectic atthe beginning when the measurements are taken most frequently. You can monitor the rate ofchange by making a rough plot of absorbance vs. time in your notebook as you collect the data.As before, toward the end of the period you should heat each of your flasks to 80˚ for a couple ofminutes to finish the reactions and obtain final absorbance values.Waste Disposal

All of the waste from this experiment should be placed in a 4-L waste bottle labeledappropriately, “Linkage Isomers Waste”.Data Analysis

1. Determination of the reaction order and the rate constant (k )You measured absorbance (A) as a function of time. You convert the absorbance to

concentration of the nitrito isomer as follows. At time t, the measured absorbance At is the sumof the absorbance due to the nitrito complex and that due to the nitro complex. [CoONO]t and[CoNO2]t are the concentrations of the complexes at time t, andεx is the extinction coefficient ofcomplex X at the wavelength chosen. Square brackets are used to indicate concentrations and,for convenience, the formulas are written omitting the five amine ligands and the charge.

At = εCoONO [CoONO]t l + εCoNO2 [CoNO2]t l (5)

Assume that the nitrito starting material was 100% pure so that the initial absorbance, Ai is:

Ai = εCoONO [CoONO]i l

If the nitrito cleanly converts to the nitro isomer, then by mass balance the total amount of cobaltcomplexes at any time is equal to the initial concentration of nitrito, [CoONO]i, which is equal tothe final concentration of the nitro, [CoNO2]f .

[CoONO]t + [CoNO2]t = [CoONO]i = [CoNO2]for: [CoNO2]t = [CoNO2]f – [CoONO]t (6)

Substituting eq 6 (the ‘mass-balance’ equation) into eq 5,

At = εCoONO [CoONO]t l + εCoNO2 ([CoNO2]f – [CoONO]t) l

At = εCoONO [CoONO]t l + εCoNO2 [CoNO2]f l – εCoNO2

[CoONO]t l

The middle term is simply the final absorbance, Af :

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At = εCoONO [CoONO]t l + Af – εCoNO2 [CoONO]t lAf - At = (εCoNO2

– εCoONO) [CoONO]t l

Multiplying both sides by [CoONO]i,

[CoONO]i (Af - At) = (εCoNO2 [CoONO]i l – εCoONO [CoONO]i l ) [CoONO]t

Since [CoONO]i = [CoNO2]f , the terms inside the parentheses on the right are Af and Ai:

[CoONO]i (Af - At) = (Af - Ai) [CoONO]tor: [CoONO]t = [CoONO]i (7)

Check this equation to make sure it looks right: does it give the right answers at t = tinitial and t =tfinal?

Think about the assumptions that went into this equation. Is your nitrito complex really100% pure? What effect would it have on the equations below if it isn’t? Does the reactionreally proceed in 100% yield? (The tightness of your isosbestic points might help you answerthis.) It turns out that the most critical assumption in using eq 7 is that you know the finalabsorbance, Af . Make a plot of your absorbances vs. time (your raw data) and estimate wherethis would level off. Calculate the Af you would expect based on theεCoNO2 at this wavelengthfrom your UV/Vis spectrum of the nitro complex (Af = εCoNO2

[CoNO2]f l = εCoNO2 [CoONO]i

l ). Do these estimates of Af agree with the one you got from heating the solution? Compare theε’s you get from the kinetic studies with those from your UV/Vis spectra of the solids. Which doyou think are more accurate? With absorbances andε’s at two wavelengths, you can calculatethe concentrations of both species without the mass balance assumption (eq 6). Can you seehow?

Now onto the kinetic analysis. If the reaction is first order in [CoONO], then

= -k [CoONO]t (k has units of s-1)

Integrating: ln[CoONO]t = -k t + ln[CoONO]i (8)

Using eq 7 to recast eq 8 in terms of the direct observable, At, gives:

ln= -k t + ln[CoONO]ior: ln= -k t (9)If the reaction is second order in [CoONO], then

= -k [CoONO]t2 (so k has units of M-1 s-1). Integrating:

= k t + (10)As above, use eq 7 to recast eq 10 to give an equation that relates At to t. In these equations,k isthe rate constant for the reaction at a particular temperature.

Pick a temperature at which you think you have good data and make some plots. If thereaction is first order, a plot of ln[(Af - At)/(Af - Ai)] vs. t should be linear, with a slope of -k (eq9). Work out what plot should be linear for a second-order reaction following your recast

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version of eq 9. See which best approximates a straight line. The one that is best will have somescatter above and below the best line, but will not look like a curve. If you are unsure of your Af values (see above), try making plots with the various values. Alternatively, you can have a program like Microsoft Excel fit your raw At vs. t data. If the reaction is first order, it should fitthe exponential form of eq 9:

At = a e-k t + b wherea = Ai - Af and b = Af (11)

You can derive the equivalent expression for a second order process. These direct fits have theadvantage that they don’t require you to input Af or Ai values, but it’s harder to tell whether thefit is good or not because the program is adjusting three parameters. Make sure that the Af or Ai values you get are reasonable.

Repeat the first-order vs. second-order analysis for another temperature. The order of thereaction should not change with temperature (unless there is a change in mechanism which isquite unlikely). The distinction between first and second order will be most clear for data setsthat extend farthest toward the end of the reaction. If you only have one half life of data (the last

point corresponds to only 50% of starting material converted), it'll be very difficult to tell firstorder from second order. The plots will be much clearer if you have two or three half lives (75%or 87.5% completion). From your plots determinek at each of the four temperatures. Be sure(always!) to indicate the units ofk .2. Determination of the enthalpy and entropy of activationIn the background section, we “derived” equation (4):

k

T

h

kk B

= [eΔS‡/R ] e-ΔH‡/RT (4)

Taking the natural logarithms of both sides, we obtain

ln + ln hkk = –

A plot of ln(k /T) vs. 1/T an Eyring plot should be linear (you should have four data points).The y intercept isΔS‡/R - ln(h/κk B) and the slope is -ΔH‡/R. Watch out for units when you dothis plot (in fact, whenever you use an equation): T must be in degrees Kelvin, K, and the rateconstant k should be in s-1 (for a first order rate constant; s = second) or M-1s-1 (for secondorder). If you get confused or want to check your constants, there's a nice summary inTheChemists Companion A. J. Gordon, R. A. Ford, 1972, Wiley, New York.

Estimate the error in each rate constant (see page 12). Considering your various values of

Af , how much does k depend on which value you chose? This provides another measure of errorin k . Put these error bars on your Eyring plot and crudely estimate your errors inΔS‡ and ΔH‡.

Suggest reasons for your observed uncertainties. Is there more uncertainty in theenthalpies than the entropies, or vice versa? Why? Think about all the experimental values thatgo into these calculations concentrations, temperatures, absorbances, etc. and how uncertainthey are. How would small errors in these values affect your calculated enthalpies and

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entropies? Do you think your errors are random errors, caused by uncertainties in measurement,or systematic errors that result from your particular experimental method and equipment?Lab Report

A lab report on this experiment is due a week after the experiment is completed. Itshould have the long report format described in the beginning of the lab manual, focused on thekinetics portion. You will revise this lab report after the "extra" lab period. Try to be conciseand to the point, but completeness is more important than staying within the suggested guidelinesfor the length of each section.

I. Introduction (several paragraphs).A. describe why this experiment was done and why it is of interest. Useappropriate literature references.

II. Experimental (try to keep to 1-2 pages, not including spectra & tables).A. briefly describe the syntheses of all synthesized products, with yields and colors

1. IR spectra, with assignments.2. visible spectra, withλmax and ε values.

B. describe how the kinetics was done.1. plots of raw data: overlay plot, At vs. t, etc.2. determination of Af

III. ResultsA. Tables summarizing IR and UV/Vis characterization (attached original UV/Vis and

IR spectra for all synthesized products at the end of your report).B. An overlay plot of one kinetic run (pick a good one) illustrating an isosbestic pointC. Plots of raw data (At vs. t) for all temperatures.D. Plots for 1st and 2nd order fits for all temperatures, showing linear fits.E. Eyring plot, showing error bars.

a. calculations for determination ofΔH‡ and ΔS‡

IV. Discussion (aim for ~5 pages of text).A. analysis of the kinetic data.

1. determination of reaction order, rate constants.B. analysis of the rate constants: determination ofΔH‡ and ΔS‡

C. Discussion of errors.D. Interpretation of reaction order, rate constants, and activation parametersin terms of the reaction mechanism (the most important part).

V. Plan for the final lab period (~1 page)

Describe an experimental procedure for what you will do in the additional lab period,with a clear rationale for your choice. Consult the discussion below.The "Extra" Lab Period

As in the Chelate Effect and (Arene)molybdenum experiments, you plan your own procedure for an "extra" lab period. We suggest that you extend this study, exploring themechanism of the reaction by doing kinetics under different conditions. You should plan onrunning at least two kinetic runs in the extra period, one under your new conditions and one with

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the same solutions but under the “old” conditions. This is the best way to tell if the change youmade had any effect, comparing two runs on the same solutions in the same temperature bath. Ithas the additional value that you will see how reproducible your data are from day to day.Suggestions:

You might vary the concentration of the starting cobalt complex, to confirm your firstorder/second order conclusion. If the reaction is first order, its half life (the time for half of thematerial to go away) will be independent of concentration. Conversely, a second order reactionwill go faster at higher concentrations. Note that the concentration you choose must beappropriate for your experimental conditions. For instance, proposing to increase theconcentration ten times will not work because your Co complexes will not be soluble at thisconcentration.You could determine the order of the reaction with respect to nitrite anion, by doing kinetics runsin the presence of added NaNO2. If the mechanism requires an initial equilibrium dissociation ofnitrite, then adding nitrite should slow the reaction down by shifting this pre-equilibrium to theleft and reducing the amount of reactive stuff. (It's called a pre-equilibrium because it comes before the rate limiting step in the reaction.)

[Co(NH3)5(ONO)]2+ [Co(NH3)53+] + NO2-

You could examine the effect of pH on the reaction, by changing the ratio of the components ofthe buffer. Be sure to check the pH of your reactions with a pH meter. [You'll also need toworry about pH if you add NO2-, as this is a weak base.] Note that one unit change in pHcorresponds to a factor of ten change in the concentration of H+ and OH-.You could add excess Cl – , I – , or some other ligand to try to trap [Co(NH3)5

3+]. Or you could find your own questions (usually the best way). Whatever approach you take, besure to discuss it with your TA.

One week after the extra period, turn in your revised lab report. This should takeinto account the comments on the first version and incorporate what you did with the extra period. You should describe what you hoped to learn from the experiments you planned andwhether you actually did learn those things.You will turn in this revised lab report, alongwith your first lab report and grade sheet.

Questions for thought (no need to turn in answers):• If you had some isotopically labeled nitrite, say Na15 NO2 (too expensive to give you), what

experiments might you do test the hypotheses described above?• You could also try to make other cobalt complexes of the form [Co(NH3)5X]Cl2. What X

groups might you want to use? Would you expect linkage isomers from nitrate (NO3 – )?

What about cyanate (OCN – )? How might you do these syntheses? Do you think you couldtell from the visible spectra what the structures of the complexes are?

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Doped Phosphor Chemistry 317 Lab Manual

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Preparation of a Doped Phosphor: ZnSBackground

The emission of light from matter is a fascinating and very useful phenomenon. All

matter glows when it is heated to high temperatures, which is incandescence (think of anincandescent light bulb or the heating elements in an electric oven that turn orange when hot).Incandescence is an example ofblack body radiation and it occurs over a very broad continuumof wavelengths (it has a broad spectrum). Emission occurs in the visible region only attemperatures above about 1000°C. Certain minerals and insects, on the other hand, emit light atroom temperature; this phenomenon puzzled natural philosophers hundreds of years ago. In1603, an Italian shoemaker heated the mineral barite (BaSO4) with charcoal and made materialthat glowed in the dark after exposure to sunlight; he called this a “phosphor”, which means“light bearer” in Greek. Today, luminescent materials of all kinds, including phosphorescentones, are used in TV screens, X-ray films, fluorescent lights, light emitting diodes (LEDs),semiconductor lasers for fiber optics, invisible hand stamps, anti-counterfeiting inks for

currency, and many other applications.In order to get light out of a luminescent material, you have to put energy in. There are a

myriad of sources of energy: sunlight, for those “glow-in-the-dark” articles found in cereal boxes; beams of electrons, for the luminescent coating which produces the picture on your TV orcomputer screen (unless you have a flat panel screen); high-energy chemicals, such as the ATPwith which fireflies fuel their glow. The phosphor which you will make in this lab is stimulated by photons in the ultraviolet part of the spectrum. A UV photon comes in, excites the material,and a photon of lower energy (visible light) is emitted; the difference in energy is lost to thematerial as heat.

The photon serves to boost the luminescent system up to an excited state. This takes

about 10-18

seconds (this is the time it takes a photon, at the speed of light, to travel 300 pm!).The excited state may go back to the ground state by releasing thermal energy (heat) or byrapidly re-emitting light of roughly the same frequency (this takes 10-12 to 10-6 seconds). Thisfast re-emission of light is called fluorescence; it is exemplified by the coatings on fluorescentlight bulbs. Another option for a molecule in an excited state is to decay to another excited state.This sounds pointless, but sometimes it has dramatic effects. The interconversion of photons,excited states, and the ground state is called the photophysics of the problem. The excited statecan also use its energy to break bonds or to react with something, in other words to do

photochemistry . You might use or have used UV light to do a reaction of a molybdenumcarbonyl compound in experiment 6.

A sodium atom provides a simple illustration of photophysics. The ground state of asodium atom is described by the electronic configuration [Ne]3s1. Excitation of a sodium atom by a photon in the yellow region of the visible spectrum gives the lowest energy excited state,configuration [Ne]3p1. In the energy level diagram below, the most stable level is at the bottomand the highest energy one is at the top. Events that involve a photon are drawn with wavyarrows while the transfer of heat energy is depicted with straight arrows. Excitation with a photon of energy hν1 gives the excited state which then either emits the light back out byfluorescence or decays to the ground state without emitting a photon. Decay without emission is

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called radiationless decay, and the energy of the excited state is just dumped as heat (perhaps in acollision with another sodium atom).

This isn't a very exciting system: you put light in and you get light of the same energyout, just in lower yield. But let's say you could excite the sodium atom to the next lowest excitedstate, [Ne]4s1, perhaps by collision with an electron. This is shown in another energy leveldiagram on the next page. The [Ne]4s1 excited state has a very low probability of emitting to theground state, because of quantum mechanical selection rules. So it waits around until it is hit byanother atom or something else happens to allow it to decay to the [Ne]3p1 excited state. This isnow a much more interesting photophysical system: the energy of the electron (or whatever isused for excitation) is converted into a photon of wavelengthν1. This is the principle of sodiumvapor lamps, the yellow lights you see in tunnels and elsewhere around town. Another exampleof interesting photophysics, the ruby laser, is described in the Appendix to this experiment.

The system you will work with in this experiment is zinc sulfide, commonly employed inthe electronics industry as a coating for cathode ray tubes (such as TV tubes or computerscreens a “cathode ray” is a beam of electrons). ZnS (the minerals zinc blend and wurtzite) can be thought of as an ionic solid (Zn2+S2– ), but based on its optical and electronic properties it is asemiconductor. Solids such as ZnS have energy levels, just as molecules do. In a molecule, arelatively small number of atomic orbitals interact to give molecular orbitals of specific energies.In a solid, ca. 1023 atomic orbitals interact and give rise to ca. 1023 molecular orbitals. Theseorbitals typically fall into groups with similar energies, which are calledbands of orbitals. Theelectrons, of course, occupy orbitals, so in a solid, electrons occupy bands of orbitals (see the

drawing on page 79). In solids that are electrically conducting such as metals, the highest energy band is only partially filled, leaving the electrons mobile. In insulating solids, there is a largeenergy gap between the highest filled band (thevalence band ) and the lowest unoccupied band(the conduction band ). [This is like the typical gap in a molecule between bonding (typicallyfilled) orbitals and antibonding (empty) orbitals.] In a semiconductor such as silicon, there is asmall band gap .

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Defects in a crystal bring their own electrons and orbitals. Often the energies of these

orbitals fall within the band gap and therefore influence the optical and electronic properties.Defects can be lattice vacancies (e.g., a missing zinc ion in ZnS), added atoms in the holes withinthe lattice, or impurities that are substituted for lattice atoms, for instance a Cu2+ replacing Zn2+.This is how the electronic structure of silicon is varied to make transistors: if a silicon atom isreplaced by a phosphorus atom, there is an extra electron in the solid but if Si is replaced by Ga,there is one too few electrons (a hole). These aren-doped and p-doped silicon (negative and positive), respectively.

Zinc sulfide is a semiconductor with a fairly large band gap between its valence andconduction bands. An ordinary photon of visible light doesn’t have enough energy to excite anelectron across this gap (that's why the solid is white or yellow). However, crystal defectsintroduce energy levels that lie within the energy gap. Absorption of a UV photon promotes anelectron to the conduction band, leaving a positivehole in the valence band. If this electronrelaxes to a defect energy level, light can be emitted at a lower frequency than the excitatorylight. This photophysics is similar to that in the sodium case above. In the ZnS phosphor, UVlight goes in and visible light comes out. (The case drawn shows defect energy levels that are atleast partially empty so electrons can be transferred into them; it is also possible to have filleddefect levels into which the hole relaxes up.) Photons promoting electrons to the conduction band is the essence of “photoconductivity” in semiconductor photocells.

So how would one go about making a doped solid? One can make ZnS solid by mixingaqueous solutions of Zn2+ (aq) and HS- (aq) but the product still has water and hydroxide ions init and is not very crystalline. Thus it has lots of defects and is not very pure, so it is not usablefor semiconductor applications. (The formation of pure, crystalline solids under such mildconditions is a challenge, and a topic of much current research.) The problem with making a

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Semiconductorwith Defects

Pure Semiconductor(no defects)

(UV)

unfilled(empty)

filled

Defect energy levels hν (visible)

solid at room temperature is that if the Zn2+ or S2- ions don’t fall in exactly the right placeinitially, it’s hard to move them into their proper crystal locations. Diffusion in a solid istypically very slow at room temperature.

The synthesis of solids is typically done at high temperatures, to facilitate solid statediffusion. The classic way to make ZnS is to mix solid zinc and solid sulfur in a sealed tubeunder vacuum and heat to about 450°C, the boiling point of sulfur, making this a gas/solidreaction. To make crystalline samples, you might then heat the crude solid to 1000°C to getsolid diffusion to occur with any speed. High temperatures are especially needed in cases where both reactants are solids even at 1000°C, as reaction then can take place only where the particlestouch. In this case, finely powdered reagents are often used to maximize particle contact.

This experiment starts with commercial "electronics grade" zinc sulfide and a dopantsuch as Cu2+ is introduced. Cu2+ can substitute for Zn2+ in the lattice and it has a partially filledd shell. The dopant is added to the ZnS powder as a solution, to disperse the cationic dopantfairly evenly over the surface of the ZnS particles. Imagine the problems if you just placed onecrystal of CuSO4 next to all the ZnS particles! You would never get an even distribution.Dispersing the dopant over the particles helps achieve a random distribution of defect sites.Initially the dopants are only on the surface of the ZnS particles, so the solid must be heated to900 °C to allow the dopants to move (diffuse) through the lattice.

Even at 900 °C, this reaction is slow so you will use a flux, the solid state chemist’sequivalent of a solvent. You will use NaCl as your flux. While most chemists think of NaCl as awhite solid, it is a liquid at 900°C. A flux can act like a typical solvent and dissolve all thereactants, or, more commonly, it dissolves only a small surface layer but this is enough to assistthe reaction. NaCl is a nice flux because when everything has cooled back to room temperature,it can be easily washed away from the product with water.

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Method

This is a one-period lab in which you should work individually, but coordinating with therest of your lab section. Choose a cation as a dopant, checking with your TA to see what’savailable. Each person should try to choose a different dopant, as well as one person who will

not use any dopant at all, also consider using iron as a dopant. You grind together the dopant,the NaCl flux, and the ZnS into a fine powder to get the particles as well mixed as possible.Then you load the powder into a small “boat” for heating in the tube furnace at 900°C. Once itcools, you wash away the flux with water. You place the doped ZnS under a UV light to see ifthe product phosphoresces (“glows in the dark”), or if your dopant quenches (prevents) emissionof light.

Experimental

First, choose a dopant to use. This experiment was originally written using Cu2+ as thedopant, but ZnS can be doped with a variety of metal ions. You might use salts of manganese,silver, gold, chromium, nickel, uranium, lead, tungsten, etc. Luminescence is claimed to becompletely quenched by iron and nickel, even in concentrations as low as 10-6 mole per mole ofZnS (one part per million, or 1 ppm). You could test whether this quenching really happens. Itis also reported that when ZnS is heated with no dopant, it loses some sulfur and becomes a non-stoichiometric compound (ZnS1-x), which might reasonably have lots of crystal defects andluminesce.

Clean everything that will touch the phosphor materials with 1 M HCl. Rinse well withdeionized water, and then with acetone. Remember that even traces of iron are reported toquench the emission.

Prepare a dopant solution of molarity roughly 10-5 M in water (there may be somealready made). If you’re making one up, mark the concentration on the bottle and leave it in casesomeone else wants to use it (remember, there are lots of lab sections). Weigh out about 1 g NaCl (don’t use a metal spatula!) and put it in a clean mortar. Add 1.0 mL of your dopantsolution using a clean 1 mL pipette. Put the wet salt into an oven at about 120°C until it is dry,about 1/2 hour. [Why do you think the Na+ from the flux does not dope the ZnS, but divalentcations and Ag+ will?]

Weigh out about 1 g electronics grade ZnS. Grind it and the doped salt together in themortar until you have a fine powder. Add a few drops of ethanol and continue grinding until youget a smooth paste. Pack this paste into a clean vitreous boat and dry in the 120˚ oven for about20 minutes.

Note which boat is yours before you put it in tube furnace (how? you can’t use a markeror tape as this will just burn off at 900°C). Let your phosphor heat in the furnace under aconstant stream of dry nitrogen; your TA will set this up. The nitrogen is critical because anyoxygen present at 900° would convert the ZnS to ZnO, which doesn’t emit much (it has a verylarge band gap). Allow the boats to cool in the nitrogen stream.

Check the crude product for luminescence in the UV light box. You might also examinesome of the starting material, ZnS. Check out the samples others have made.

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Waste Disposal

All of the waste from this experiment should be placed in a 4-L waste bottle labeledappropriately, “Zinc Sulfide Waste”. Be sure to clean your weigh boats at the end of lab. Thesolid from the weigh boats can also be placed in the 4-L waste bottle.Lab Report:

Observe which dopants effect phosphorescence using the UV light box. Record this forall doped ZnS samples made in your lab period. Write a short (approx. 1 single-spaced page t)news article, suitable for the science column of a newspaper or a science-oriented magazine.Your article should include enough background to enable a scientifically literate reader tounderstand it, and it should communicate the latest research in the field undertaken by students inChem 317. The dry scientific writing you have done in previous reports is not appropriate toreach a more general audience, as we are asking you to do here. Part of the challenge for thisreport is providing enough background to bring your readers up to speed, while leaving enoughspace to explain the actual “news story,” aka, what your class achieved in the lab period.Although the format is different, be sure to discuss relevant details of your experiment. Besuccinct, funky, and imaginative - have some fun with this one.

Questions for you to think about (but not turn in.):

Why is it that the ZnS must be heated under nitrogen, yet the products may be handled in the air?

Why should you grind your starting material to a fine powder?

Why are high temperatures needed?

Why should you not use a metal spatula? (What metal do you think it is made of?)

Why type of glass is used in the tube furnace? (Yes, there are different types.) We normally usePyrex® glass in the lab. What is the maximum temperature that Pyrex remains useful? How canyou tell different types of glass apart in the laboratory?

Appendix :Another Example of Photophysics: The Ruby Laser.

Ruby contains chromium(III) ions doped into a crystal of Al2O3 (so the correct formula isAl2-xCr xO3). The Cr 3+ ions are isolated from each other and therefore interact with light asisolated ions. We can therefore discuss what happens to only one ion and we’ll have most of the picture. The Cr 3+ ions are octahedral holes of the oxide lattice so that the d orbitals split into thefamiliar “2 above 3” pattern. The ground state of octahedral Cr 3+ is labeled4A2 in the symmetrylanguage used for such problems; the4 stands for 3 unpaired electrons plus 1 (2S + 1 in quantummechanics-ese) and the A2 refers the orbital symmetry of the state. A photon of light can excitea chromium atom to an excited state (4T2), also with three unpaired electrons, as shown in thediagram below. As above, events that involve a photon are drawn with wavy arrows while thetransfer of heat energy is depicted with straight arrows. The absorption of light (4A2 → 4T2) iswhat gives the aquo chromium(III) ion, Cr(H2O)63+, its forest green color (remember, in the

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Chromous Acetate experiment?). When the chromium is bound to oxide ions in a ruby lattice,instead of water molecules in Cr(H2O)63+, the energy gap between the4A2 and 4T2 states is suchthat blue/green light is absorbed and rubies look red.

Energy

2E Excited State

4T2 Excited State

Ground State

excitationE=hv 1

reemisionE=hv 1

thermaldecay

thermal decay"intersystem crossing"

emissionE=hv 2

The 4T2 chromium excited state in ruby rapidly decays predominantly by intersystemcrossing to another excited state, where one of the electrons has flipped its spin and there is nowonly one net unpaired electron (2E). To get back to the ground state, this electron has to flip itsspin back again, but since photons are not trained to help out with changes in electron spin,emission of light is a difficult process; it is said to be “spin-forbidden.” (This quantummechanical “forbiddenness” is why incoming light does not excite ground state Cr 3+ (4A2)directly to the2E excited state ) “Forbiddenness” doesn’t stop emission but it slows it down