2.10.12 •  If you decomposed 4 moles of hydrogen

peroxide, how many moles of oxygen gas would you produce if you used manganese (IV) oxide as a catalyst?

•  HW – page 301 1-3, and page 311 1-4

•  TYGAGT •  Use mole ratios to predict number of moles of

product in a reaction and mass of products

stoichiometry

•  Comes from the Greek element and to measure

•  This is application of the law of conservation of mass, and to many is the heart of chemistry

•  We are going to be studying reaction stoichiometry, which deals with the mass relationships between reactants and products in a chemical reaction

Example

•  If you have 13.0 mol of Aluminum (III) oxide, how many moles of aluminum can be produced from the electrolysis of molten aluminum (III) oxide to produce aluminum and oxygen gas?

•  Step 1: write this out in an equation •  Step 2: balance this equation •  Step 3: determine mole ratios

Mole ratios

•  Once you have a balanced equation, what you have determined are the molar ratios between all reactants and products

•  You can use these ratios to scale up or down the reaction

•  In our example, the molar ratios are… •  So the number of moles of Al that would

be produced is… •  26.0 mol Al

Using mole ratios

•  You can use the mole ratios to predict the masses produced (ideally)

•  From our example, what would the mass be of the product?

•  Another look: •  13.0 mol Al2O3 X 4 mol Al_ = 26.0 mol

Al 2 mol Al2O3 What cancels in this calculation?

You determine •  What mass of Oxygen was produced in the

example reaction? •  What are your steps? •  What is the mole ratio? •  Can you use the mole ratio between Al2O3

and O2? •  Can you use the mole ratio between Al and

O2? •  Did you get 96.0 g?

Ideal Stoichiometric Ratios

•  These are the stoichiometric predictions for reactions that happen under “ideal” circumstances

•  In any lab, ideal ≠ real •  How do you compute the difference? •  Percent difference!

Example problem

•  Carbon dioxide reacts with Lithium hydroxide to produce lithium carbonate and water.

•  How many moles of lithium hydroxide would be required to react with 20 mol of carbon dioxide?

•  What is the molar ratio between carbon dioxide and lithium hydroxide?

example

•  How many moles of oxygen are required to burn 0.5 mole of methane?

•  Write out and balance equation: •  CH4 + 2O2 2H2O + CO2

•  Mole ratio is 2 moles of O2 : 1 mole CH4

continued

•  How many grams of carbon dioxide are produced from this combustion?

•  Use molar ratio of carbon dioxide to methane

•  Convert to grams by using molar mass of carbon dioxide

example •  In photosynthesis under ideal conditions, what

mass, in grams, of glucose is produced for every 3.00 mol of water?

•  Plan •  Begin with a balanced equation •  Determine mole ratios to get a mole conversion

factor •  Use periodic table to determine mass of glucose •  Multiply conversion factor X molar mass of

glucose

together

•  What mass of carbon dioxide (in grams) is needed to produce that same amount of glucose as in the previous example?

Practice (moles to mass)

•  When magnesium burns in air, what mass of magnesium oxide is produced from 2.00 mol of Magnesium?

•  What mass carbon dioxide is produced from the aerobic respiration of 10 mol of glucose (assume complete oxidation)?

Practice (mass to moles)

•  The first step of the industrial production of nitric acid is the combustion of ammonia to produce nitrogen monoxide and water.

•  In a reaction of 824 g of ammonia and excess oxygen, how many moles of each product are produced?

Practice (mass to moles)

•  Oxygen was discovered by Joseph Priestley in 1774 when he heated mercury (II) oxide.

•  How many moles of mercury (II) oxide are needed to produce 125 g of Oxygen gas?

•  How many moles of mercury are produced?

2.13.12 •  Warmup: Tin (II) fluoride is used in

toothpaste. It is made by reacting Hydrogen fluoride with Tin. How many grams of Tin(II) fluoride are produced from the reaction of 30.00 g hydrogen fluoride with Tin?

•  Hint: start by writing it out, diagram it, balance the equation. What info DO you know? What are you trying to find out?

•  T •  Practice stoichiometry, esp mass-mass and

moles to mass problems

From homework

•  Any questions?

Practice (mass to mass)

•  When copper metal is added to silver nitrate (aq), silver metal and copper (II) nitrate are produced. What mass of silver can be produced this way from 100.0 g of Cu?

Today’s assignments

•  Page 108-109 in green book 1-23 odds

2.14.12 •  In the reaction of Zinc metal in hydrochloric

acid, hydrogen gas and zinc chloride are produced. If you start with equal amounts of zinc and HCl, which do you run out of first? What is going to determine how much hydrogen you make?

•  HW - look at problems on 319, bring questions tomorrow

•  TYGAGT •  Calculate yields of reactions if one reagent is

limiting

terms

•  Ideal reactions are difficult to undertake, typically in a chemical reaction one reagent is limiting

•  Limiting reactant – in a reaction, that reactant that will run out first

•  Excess reactant – the unreacted remainder of the non-limiting reactant

•  In our warmup – which reactant was limiting? Which was excess?

problems

•  Keep you head! •  Plan a strategy for each problem! •  Start with what you know! •  Use unit conversions (mole ratios) to get

you from what you know to where you want to get to!

•  ALWAYS ALWAYS ALWAYS include units in your calculations, they are you sign that you are on the right track --- or not!

Examples •  In the following reactions, which reactant is

limiting? Which is in excess? •  Silicon dioxide + hydrogen fluoride yields silicon

tetrafluoride and water. What if 6.0 mol HF is added to 4.5 mol SiO2?

•  Step 1: write out equation and balance •  Step 2: Solve yield for BOTH reactant quantities

using mole ratio as conversion factor •  Step 3: the reactant that gives you the lesser yield

is the limiting reactant •  Solution next slide…

example

•  6.0 mol HF X 1 mol SiF4 = 1.5 mol SiF4 4 mol HF

•  4.5 mol SiO2 X 1 mol SiF4 = 4.5 mol SiF4 1 mol SiO2

•  Because the 6.0 mol HF will only produce 1.5 mol SiF4, HF is the limiting reactant

example

•  N2H4(l) + 2H2O2(l) N2(g) + 4H2O(g) •  If we start with 0.750 mol N2H4 and 0.500

mol H2O2, which reactant is limiting? •  How much of the excess reactant

remains? •  How much of each product (in moles) is

formed?

example

•  Methanol is synthesized from carbon monoxide and hydrogen gas. If 500 mol CO and 750 mol H2 are present, which is the limiting reactant?

•  How many moles of excess reactant remain unreacted?

•  How many moles of methanol are produced?

Mass to mass

•  3Fe(s) + 4H2O(g) Fe3O4(s) + 4H2(g) •  If 36.0 g of H2O is mixed with 67.0 g Fe,

which is the limiting reactant? •  What mass in grams of the iron oxide

compound is produced? •  What mass in grams of excess reactant

remains unreacted?

2.15.12

•  In a reaction between 1.000 mol of sodium hydroxide and 0.500 mol of hydrochloric acid, which reactant is limiting?

•  Today – practice practice practice •  Finish green book problems from both

days and hand in

example

•  Zinc and sulfur react to form zinc sulfide (recall, elemental sulfur is S8!).

•  If 2.00 mol of Zn are reacted with 1.00 mol of S, which is the limiting reactant?

•  How many moles of excess reactant remain?

•  How many moles of product are formed?

example

•  Carbon reacts with steam to produce hydrogen gas and carbon monoxide

•  If 2.40 mol of Carbon are reacted to 3.10 mol of steam, which is the limiting reactant?

•  How many moles of each product are formed?

•  What is the mass of each product formed?

practice

•  Green books – page 119 1-7

*also should finish up problems from the other day!*

2.21.12 •  Chlorobenzene (C6H5Cl) is produced by

reacting benzene (C6H6) with chlorine gas. Hydrogen chloride gas is a by-product.

•  Write out and balance the equation representing this reaction.

•  If 36.8 g benzene react with excess chlorine, what is the expected mass of chlorobenzene?

•  If your actual yield was 38.8 g of chlorobenzene, what happened?

Homework questions?

•  Tonight’s homework – look at questions on page 318

•  Quiz on Thursday on limiting reactants and stoichiometry topics

•  Last call for “penny” labs

Today •  Topic – percentage yield, the ratio of the

amount of product we actually get over the amount of product we predict to get

•  Express as a percentage •  Will never be >100% (or something really

screwed up your reaction – then it’s a do-over)

•  There are important reasons why <100% is a rule that have nothing to do with error, we will explore them later (reaction equilibrium)

2.22.12

•  If 1.85 g of aluminum react with excess copper (II) sulfate, which reactant is limiting?

•  How much copper (II) sulfate was actually consumed in the reaction? IN moles? In grams?

•  If the actual yield of the sulfate product was 3.70 g, what was the percentage yield?

The scientific method

•  How research is done •  Undergraduate and secondary – totally under

direction of supervisor, research ideas 85-100% from supervisor

•  Option A – from BS to MS thesis, ideas 50% yours, support 100% from supervising PhD

•  Option B – from BS/MS to PhD, ideas 90+% yours, support 100% from supervising PhD

And then •  Post-doctoral positions and research •  Research ideas your own, but YOU have to

find a lab that will support your research ideas – this is not a given. So are you really doing “your own” research?

•  Funding comes externally, so you have to be researching something that has already gotten funding

•  You are still under supervision and support of a PhD (even though you have one of your own)

And then •  You may become a staff scientist (or go back to

academia) •  You direct post-docs and provide material,

administrative and other research support •  Support post-docs in publishing •  You still do not really have any research autonomy •  Why not? •  If in academia, you have to secure your own

funding, and then you do your own research and have your MS or PhD students carry it out (see previous slide)

Why supported?

•  MS and PhD are supported because their supervisor GOT the funding, so they are for the most part carrying out their supervisor’s research

•  The supervisor has a team working on a project

•  The team members learn how to do science so they can one day direct their own work

finally

•  You may be a supervising scientist •  Why am I discussing this?

Lab

•  Limiting reagents lab and formal lab report •  Will be done in groups of four - no more

than five •  Group leader – responsible for designating

tasks among groups •  Group leader must turn in to me in writing

the assignments for their group •  Groups turn in single paper

Paper format •  Title page:

– Title – Authors (I am also an author, but should be listed

last) – Date, institution – Abstract

•  Introduction – Explanation of stoichiometry, calculations, activity

series, solubility rules of double replacement reactions, percent yield

– Fully referenced!

continued •  Methods – written in passive voice (something nearly all of

you lost points for in STEM fair!) –  Should include illustration/diagram of set-up –  Description of how error will be managed –  Description of how data will be collected

•  Data –  Present in table format the averages, standard deviations, and

percent yields all to same decimal value and show units •  Conclusions

–  Error analysis and discussion of percentage yield, review hypothesis

–  Did results support hypothesis? Why or why not based on discussion of previous bullet

finally

•  Works cited in APA format •  Use MS word to create this (note: this

requirement is PART of the assignment! You must use this format!)

•  Also – pages should be numbered, use “headers” feature to include title of paper on every page

grading •  No rubric •  As a contributing author and mentor, I will

only accept quality submissions •  If your paper needs work, you will have to

continue working on it •  When it is ready for submission, I will accept

it •  However, you MUST have an acceptable

paper ready for submission by March 9 – or you are “fired”

Questions

•  Is there a page length requirement? •  No – it must be “complete” •  Can I do a different project? •  No •  Can I work by myself? •  No •  Is there any room for negotiation here? •  No