Electrochemistry PreLab for THURS 4/4

Name:_______________________________ Date:_________________ Hr:____
AP CHEMISTRY
Ch 17 - Electrochemistry LAB 17
ELECTROCHEMICAL CELLS AND THERMODYNAMICS
Introduction:
This investigation focuses on the redox reaction
Zn(s) + CuSO 4 (aq) ZnSO 4 (aq) + Cu(s)
Using calorimetry, one can determine the heat of reaction (∆H) for this system. An
electrochemical method offers another simple and accurate means for the determination of
thermodynamic quantities. During this experiment, a simple electrochemical cell
Zn(s) | Zn 2+ (aq) || Cu 2+ (aq) | Cu(s)
will be constructed as shown below and the overall galvanic cell reaction is essentially the same
as that which will be taking place in a calorimeter.
The quantity of the electrical energy, E, produced or consumed during the
electrochemical reaction can be measured accurately. The free energy change, ∆G, of an
electrochemical reaction is related to the voltage, E, of the electrochemical cell. By measuring
the voltage of our electrochemical cell at several temperatures, one
can obtain a plot of free energy versus temperature.
Assuming that ∆H and ∆S remain constant over a small
temperature range, one can then determine ∆S and ∆H
from the graph, along with ∆G°.
Remember than an ideal calorimeter is a
perfectly insulated vessel which contains a large known
mass of solution in perfect thermal contact with an
accurate thermometer and a small reaction tube. When
measured quantities of reactants are introduced into the
reaction tube, the heat of reaction changes the
temperature of the calorimeter solution. If we assume the
specific heat of the CuSO 4 solution is the same for water,
then the heat of reaction can be determined using the
temperature change and the mass of solution in the
calorimeter.
Equipment and Supplies:
0.5 M zinc sulfate solution
0.5 M cupric sulfate solution
zinc metal strip
copper metal strip
zinc powder
porous cup
coffee cup calorimeter
thermometer
voltmeter & connectors
100-mL graduated cylinder
100-mL beaker
250-mL beaker
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Name:_______________________________ Date:_________________ Hr:____
AP CHEMISTRY
Ch 17 - Electrochemistry LAB 17
Procedure:
PART I – Electrochemistry
1. Pour sufficient 0.5 M ZnSO 4 solution into a clean porous cup to make it about half-full.
Place the porous cup into a clean, dry 100-mL beaker.
2. Pour the 0.5 M CuSO 4 solution into the 100-mL beaker until the level of the CuSO 4
solution in the beaker is slightly above the level of the ZnSO 4 solution in the porous cup.
3. Fold and place a paper towel in the bottom of a 250-mL beaker. Place the 100-mL
beaker into the 250-mL beaker. Add tap water (carefully, so as not to contaminate any
solution) to the 250-mL beaker until the water level is about the same as the level of the
CuSO 4 solution in the 100-mL beaker.
4. Obtain a zinc metal strip and sand it to remove any oxide coating (place the strips on a
paper towel so as to not scratch the lab table). Place it in the ZnSO 4 solution. Repeat
the procedure with the copper metal strip and place it in the CuSO 4 solution. You may
find it handy to bend the top of each metal strip in a “L” near the top of its container.
5. Place the entire assembly on a ring stand. The ring should be adjusted at a sufficient
height for heating with a Bunsen burner.
6. Place a thermometer in the 100-mL beaker of the CuSO 4 solution. Support the
thermometer if necessary.
7. With a Bunsen burner, heat the water bath until the thermometer reads above 75°C.
Stop heating.
8. While waiting for the thermometer to stop rising, make sure the voltmeter is set to read
in the 2 V range (DCV 2000 m). Attach the alligator clips to the metal strips and observe.
Disconnect one of the wires immediately. If a negative number appears, reverse the
wires.
9. Allow the bath assembly to cool naturally. Read and record 5 or 6 voltage-temperature
readings periodically in the 70-50°C range. COMPLETE THE CIRCUIT ONLY AT THE TIMES
OF READING THE VOLTAGE.
10. Return the solutions to the appropriate waste containers.
PART II – Calorimetry
1. Determine and record the mass of a coffee cup calorimeter. Add about 50 mL of 0.5 M
CuSO 4 solution. Mass the calorimeter a second time.
2. Let the apparatus stand so that the components attain the same temperature. Record
the initial temperature.
3. Using the analytical balance, determine the mass of 0.5-0.6 g of zinc powder as precisely
as you can.
4. Add the zinc powder to the calorimeter. Swirl thoroughly and observe the temperature.
Periodically use the thermometer to break up the copper-coated zinc powder. Record
the maximum constant temperature. Be patient: this is not as rapid a reaction as we
have observed before in calorimetry.
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Name:_______________________________ Date:_________________ Hr:____
AP CHEMISTRY
Ch 17 - Electrochemistry LAB 17
Calculations:
Be sure to include an appropriately labeled and referenced graph.
1. Graphically, determine ∆H and ∆S. Calculate ∆G°.
2. Calculate E° from the experimental data.
3. Calculate K at 298K.
4. Determine the ∆H per mole of zinc from your calorimetry data.
Questions:
Discuss the implications of the sign and the magnitude of each of the five experimental
values you determined in this experiment. This discussion should focus on your experimental
values.
Error Analysis:
1. How do your ∆H from the two methods compare (quantitatively)? Discuss possible
reasons for any difference.
2. How does your experimental E° compare (quantitatively) to the theoretical value?
Discuss potential sources of errors and their effects on your experimental E°.
Conclusion:
Be sure to summarize the major results of the experiment as you write your conclusion.
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Name:_______________________________ Date:_________________ Hr:____
AP CHEMISTRY
Ch 17 - Electrochemistry LAB 17
ELECTROCHEMICAL CELLS AND THERMODYNAMICS
Grading Rubric
Item
HEADINGS ON EACH PAGE
Title of Experiment, Date Experiment Started, Successive
Page Numbers, Name & Partner
Points
DATA SHEET
Appropriate Purpose /2
Clear & Complete Procedure /6
Procedure Initialed by Instructor /1
Clear & Complete Data and Observations /5
Data and Observations Initialed by Instructor /1
WRITE UP
Clear & Complete Pertinent Data /2
Clear & Complete Graph Present /8
Clear & Complete Sample Calculations Present
Electrochemistry Calculations /8
Calorimetry Calculations /4
Clear & Complete Answers to Questions /5
Clear & Complete Error Analysis
Accuracy of E° /2
Precision of ∆H /2
Clear & Complete Conclusion /4
TOTAL /50
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