1.5g1.0g4.0g3.5g3.0g2.5g2.0g Welcome! This page will serve as your home for navigating through this...

1.5g1.0g 4.0g3.5g3.0g2.5g2.0g

Welcome!

This page will serve as your home for navigating through this experiment. Some of the buttons will remain inactivated until you have completed the comprehension quiz, but afterwards you will be able to navigate to any part of the lab. Feel free to skip around at any point in time to better understand part or parts of the experiment, Click on the blue buttons found throughout the lab to navigate through. If you don’t understand something, go back and do it again.

Materials

Step 1

Step 2

Chemical Change: Heat and Quantities of ReactionChem 1100

Hamline University

Introduction Continued

Introduction

Data Table

Safety Information

Quiz

Start Lab

1.5g1.0g 4.0g3.5g3.0g2.5g2.0g

Welcome!

This page will serve as your home for navigating through this experiment. Some of the buttons will remain inactivated until you have completed the comprehension quiz, but afterwards you will be able to navigate to any part of the lab. Feel free to skip around to better understand part or parts of the experiment, Click on the blue buttons found throughout the lab to navigate through. If you don’t understand something, go back and do it again.

Materials

Step 1

Step 2

Chemical Change: Heat and Quantities of ReactionChem 3050

Hamline University


Introduction

Data Table

Safety Information

Quiz

IntroductionScientists often want to measure how far a reaction has progressed. This

can be done by measuring either how much of the reactants have been consumed, how much product was produced, or measuring some other variable that is directly related to one of these quantities.

Most chemical reactions are accompanied by a heat change. The reactions are said to be exothermic if they evolve heat and endothermic if they take it from the surroundings. The heat of the reaction can easily be related to the extent which the reaction has progressed to determine the amount of chemical change.

In this experiment we will be measuring the amount of heat given off when a fixed amount of copper ions in solution react with varying amounts of zinc metal. The net ionic equation is as follows:

Zn (s) + Cu2+ (aq) → Zn2+

(aq) + Cu (s)

As you can see in the balanced equation above, this reaction occurs with a one to one mole ratio of zinc metal to copper ions. The extent to which this reaction progresses is going to be limited by one of the two reactants. If all of the Zinc is used up first it is the limiting reagent, but if all of the copper is used up first it is the limiting reagent.

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As previously discussed, in this experiment you will be reacting a fixed concentration of copper ions with seven different weights of pure zinc metal. This is an exothermic reaction meaning that it gives off heat. The amount of heat that is given off in each reaction is proportional to the amount of reacting that has occurred. The heat that is being produced in this reaction arises from the forming and breaking of chemical bonds. The heat associated with a chemical change that occurs at a constant pressure (atmospheric in this case) is called the enthalpy change of a reaction and is symbolized by ΔH

This program represents a simulation of many actual trials that have been carried out in the laboratory. You will be given the final and initial temperature of the reaction at the end of each trial and will use that data as well as the color changes that you observe in the calorimeter to complete the second portion of this lab which will be done in excel. Have fun and don’t be afraid to repeat certain portions of the lab in order to better understand them; that is the beauty of a computer based laboratory experiment!

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What’s happening on a molecular level?

Zn (s) + Cu2+ (aq) → Zn2+

(aq) + Cu (s)

To help the concept of limiting reagents, we are going to look at what is happening on a molecular level. The reaction that is being carried out is between pure zinc metal and copper sulfate pentahydrate solution. We are going to concentrate on the net ionic equation (shown below) and ignore all other substances present in this reaction. To see the complete reaction click here. Essentially what is happening in this reaction is that zinc metal is becoming a zinc ion and copper ion is becoming copper metal. This change requires a transfer of electrons, but we are not going to dwell on that at this level.

Zn Cu2+ Zn2+ Cu (s)

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Zn Cu2+ Zn2+ Cu (s)

Zn Cu2+ Zn2+ Cu (s)

Zn

Now, lets suppose there are three atoms of zinc metal and two copper ions. Two of the zinc atoms will be able to react with a copper ion to produce the desired reactants, but one will not be able to find a copper ion and will therefore remain zinc metal. In this example, copper ions are the limiting reagent. Once all of the copper ions are consumed, the reaction can no longer proceed and no new product will be produced.

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As you just saw, this reaction involves a change in the chemical nature of the reactants to produce the products. Along with a change in the nature of these substances comes a change in the energy of these substances. This reaction is exothermic, and in an exothermic reaction the products have less energy than the reactants. As you can see in the diagram below, that change in energy can be accounted for by the release of heat. Even though the energy of the products is lower than the energy of the reactants, the final temperature of the solution is higher than the initial temperature of the solution. This is because energy is lost from the reacting system in the form of heat. Therefore, if we measure the amount of heat that is given out we are also measuring the change in energy and therefore the extent to which a reaction has progressed.

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What’s happening on a molecular level?To help explain how the limiting reagent actually stops the reaction when it is used up we are going to look at what is happening on a molecular level. The reaction that is being carried out is between pure zinc metal and copper sulfate pentahydrate solution. Look at the molecular depiction of these two substances below. One copper ion is attached to a sulfate (SO4) molecule which is surrounded by 5 water molecules.

Zn

S

H2O

Cu2+Click to see what happens when Zn

is added to the beaker

Zn + CuSO4·5H2O → ZnSO4 + Cu+ 5H20

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Lets pretend that we are able to add just two atoms of zinc metal to our copper sulfate solution. It is not actually possible to do this because ions are extremely small, but for the sake of simplicity we are going to pretend we can. Those two ions can react via the equation below to form ZnSO4 and 2 Cu ions. However, we have three copper sulfate ions in this beaker, so one of them does not have a zinc ion to react with and the reaction cannot proceed any farther. Zinc is the limiting reagent in this example because it limits how far the reaction can progress. Even if you had 5 Copper sulfate molecules, the reaction cannot proceed any further without more zinc.

Zn

S

H2O

Cu2+

Zn + CuSO4·5H2O → ZnSO4 + Cu+ 5H20

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mmorgan03

I like this slide, especially the animation, but I have a problem because it doesn't show the zinc metal changing to zinc ion. This will be tricky because we haven't introduced redox yet, I'm not sure what to do about this.

As you just saw, this reaction involves the breaking of a bond between the sulfate molecule and the copper ion and the formation of a bond between that same sulfate molecule and a zinc ion. As these bonds are broken and created, heat is released because this reaction is exothermic. The energy diagram below shows a graph of the energy as the reaction progresses and shows that the difference in energy between the products and the reactants can be accounted for as energy lost in the form of heat. Measuring the heat that is given off gives us information about how far a reaction has progressed or how many bonds have been broken and formed.

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Safety Information


Obviously, when carrying out a lab on the computer there is no real risk to you or others around you. However when you are in the laboratory it is important to have a good understanding of the chemicals that you are working with and whether or not they are harmful by themselves or when reacted with other chemicals. One of the best sources of information about the safety and reactivity of chemicals is the material safety and data sheets (msds’s) for each chemical. Laboratories are required to have these sheets on file for all of the chemicals in stock and the lab manager would be happy to provide them to you upon request. They can also be found online, and links to the msds’s for the chemicals used in this experiment can be found below. Also, remember to wear googles at all times in the laboratory.

Zinc Metal: http://sciencelab.com/xMSDS-Zinc_Metal-9925476

Copper Sulfate Pentahydrate: http://www.sciencelab.com/xMSDS-Copper_sulfate_pentahydrate-9923597

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Note: you will have to exit and re-enter the laboratory demonstration in order to view these links. If you click on them they will open in an internet browser behind the demonstration so you can view them later.

http://sciencelab.com/xMSDS-Zinc_Metal-9925476

http://www.sciencelab.com/xMSDS-Copper_sulfate_pentahydrate-9923597

Comprehension Quiz

1.) The reaction between aqueous copper ions and zinc metal is an______ reaction.

a.) exothermicb.) endothermicc.) extremothermicd.) temperate

Click on the answers below to test your knowledge. You will not be able to navigate anywhere else in the lab until you have answered all questions correctly.

Comprehension Quiz

2.) The enthalpy change of a reaction is the heat associated with the chemical reaction which occurs at constant _________.

a.) fluidityb.) molar concentrationc.) pressure d.) temperature


Comprehension Quiz

3.) The limiting reagent is the one that is completely consumed in the reaction.

a.) trueb.) false


7 styrofoam cups to use as calorimeters and one to use as insulation

Copper Sulfate Pentahydrate (CuSO4·5H20) Graduated Cylinder

Vernier LabQuest with Temperature Probe

Materials

Zinc Metal


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Trial 1 2 3 4 5 6 7

Mass of Zinc 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Initial Temp

Final Temp

Δ TSolution Color

Before we begin the lab, copy the following data table into your notebook. You will be completing the empty rows of the table as you proceed through the lab.


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Step 1: Click Add Zinc button below to weigh out 1.0g of Zinc. The rest of the masses will be weighed out for you.

Quantities of zinc used:1.0g, 1.5g, 2.0g, 2.5g, 3.0g, 3.5g, 4.0g

0.0000 g

Zinc Metal


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Add Zinc

Remove Zinc



0.0000 g0.2810 g

Zinc Metal


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Add Zinc

Remove Zinc



0.2810 g0.5631 g

Zinc Metal


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Add Zinc

Remove Zinc



0.5631 g0.7501 g

Zinc Metal


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Add Zinc

Remove Zinc



0.7501 g1.235 g

Zinc Metal


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Add Zinc

Remove Zinc



1.235 g

Zinc Metal


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Add Zinc

Remove Zinc

1.536 g



1.235 g1.012 g

Zinc Metal


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1.0g

4.0g3.5g3.0g2.5g2.0g1.5g

copper sulfate pentahydrate (CuSO4·5H20)

Step 3: Click the Pour button below to add 50 mL of copper sulfate pentahydrate to each of the seven calorimeters using a graduated cylinder


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Pour

1.0g 4.0g3.5g3.0g2.5g2.0g1.5g

copper sulfate pentahydrate (CuSO4·5H20)

Click next slide to continue with lab


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Click Add Zinc button below to add 1.0 g sample of zinc to calorimeter


Add Zinc

Collecting Data…

Temp Initial: 22 .0 º CTemp Final: 35.6 º C

It is important to stir with the temperature probe while the reaction progresses. Watch for color changes inside of the calorimeter and record the final and initial temperature of each trial. Click next slide once you have gathered your data.


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Add Zinc

Collecting Data…

Temp Initial: 22 .0 º CTemp Final: 43.6 º C

Remember to observe color changes and to record the final and initial temperatures. Click next slide once you have recorded your data.


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Add Zinc

Collecting Data…

Temp Initial: 22 .0 º CTemp Final: 49.4º C


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Add Zinc

Collecting Data…



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Add Zinc

Collecting Data…



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Add Zinc

Collecting Data…



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Add Zinc

Collecting Data…



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Calculate the change in temperature (ΔT) by subtracting the initial temperature from the final temperature in each trial and record this number and the final color of each solution in your data table.

Trial 1 2 3 4 5 6 7

Mass of Zinc 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Δ T

Solution Color


Trial 1 Trial 2 Trial 3 Trial 4 Trial 5 Trial 6 Trial 7

This is the end of the lab. Hit the escape key to exit the demonstration and follow the instructions on the lab handout to complete the rest of the lab in excel.

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