Chemistry 203
Lab 6a – Electrochemistry
Objective: To measure the Ecell for two different voltaic cells, measure the Ecell for two different
concentration cells, and experimentally determine the Ksp of an insoluble salt.
Background: In electrochemistry, a voltaic cell is a specially prepared system in where an
oxidation-reduction reaction occurs spontaneously. This spontaneous reaction produces an easily measured
electrical potential. At this point, we are familiar with the path electrons take through an external circuit to
facilitate oxidation and reduction reactions. A sample cell is shown below.
In this experiment, you will prepare a couple of voltaic cells in a series of beakers. A voltaic cell is
constructed by using two metal electrodes and solutions of their respective salts (the electrolyte component
of the cell) with known molar concentrations. In Parts 1 of this experiment, you will use a digital multimeter
to measure the potential of a voltaic cell with copper, zinc, and aluminum electrodes. In Part 2 of the
experiment, you will measure the potential of a special type of voltaic cell called a concentration cell. This is
similar to the work done last week, but with a larger concentration. In Part 3 you will then measure the
potential of a second concentration cell and use the Nernst equation to calculate the solubility product
constant, Ksp, for copper hydroxide.
Procedure
Part 1 - Determine the Eo for a Cu, Zn, and Al Cells
1. In a small beaker, obtain roughly 30 mL of 0.1M CuSO4 solution. In another small beaker, obtain
roughly 30 mL of 0.10M Zn(NO3)2. In another small beaker, obtain roughly 30mL of 0.10M
Al(NO3)3.
2. You will need to make three different galvanic cells with each of these solutions. To prepare your
first, use a piece of the metal in the solution, connect the leads of the digital multimeters to the metals
so that you get a positive voltage. Place a salt bridge (KNO3 soaked paper) between the two
beakers.
3. Remove the metal electrodes, rub them with steel wool and re-immerse them in the solution, record
another reading. Do this one more time so you have three trials.
4. Once you get a stable reading, disassemble the cell, and construct another one with two different
metals. Repeat until you have made three different cells.
Part 2 - Prepare and Test Two Concentration Cells
1. Set up a concentration cell using one of your three metals. Obtain about 25 mL of 1M solution.
Make a 1:20 dilution of this metal (using a graduated cylinder). Using the metal electrodes, digital
multimeter, and salt bridge to set up a concentration cell.
2. Remove the metal electrodes, rub them with steel wool and re-immerse them in the solution, record
another reading. Do this one more time so you have three trials.
3. Disassemble the cell and construct another cell using a different metal system. Repeat for three
more trials.
Part 3 – Ksp of an Insoluble Salt
1. Set up a concentration cell using copper. One cell should have a solution with a concentration of
1.0M CuSO4 (about 25 mL). The other should have a concentration of 0.05M of CuSO4 (about 20
mL). Prepare this through dilution if necessary or use one of the previous set ups.
2. Mix 10mL of 0.05M NaOH with the beaker that contains the 0.05M CuSO4. This will form an
insoluble Cu(OH)2 salt.
3. Insert copper metal and a salt bridge to measure the potential between the two. Remove the metal,
rub it with steel wool, and re-immerse it. Do this to get three trials.
4. Disassemble your cell, and pour all solutions into the waste containers.
Data
In this experiment, you collected several pieces of information. This includes:
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3 Trials for the Ecell of a Cu/Zn cell.
3 Trials for the Ecell of a Cu/Al cell.
3 Trials for the Ecell of a Zn/Al cell.
3 Trials for the Ecell for two concentration cells.
3 Trials for the Ecell for a copper concentration cell with sodium hydroxide.
From this information you will need to calculate the following:
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The average Ecell for Cu/Zn.
The average Ecell for Cu/Al.
The average Ecell for Zn/Al.
The average Ecell for two concentration cells.
The average Ecell for the copper concentration cell with sodium hydroxide.
The concentration of copper in the concentration cell.
The Ksp of Copper (II) hydroxide.
Report
For your report you will need to write a discussion for each of the following results.
1.
Show how your experimentally determined Ecell values for each galvanic cell compare to the
expected values. Explain any difference between the two.
2. Show how your experimentally determined Ecell values for each concentration cell compare to the
expected values. Explain any difference between the two.
3. Show your calculation of the Ksp of Cu(OH)2. Explain any difference between your result and the
expected result.
NOTE: In your explaination, be specific. Unless your result is right on, it will be either high or low.
Look at these numbers are calculated and try to determine factors that might result in a number that is higher
or lower than expected.
This report will be due at the start of class on Tuesday 11/23.