Electrochemistry: Voltaic Cells

Electrochemistry: Voltaic Cells

In electrochemistry, a voltaic cell is a specially prepared system in which an oxidation-reduction reaction occurs spontaneously. This spontaneous reaction produces an easily measured electrical potential. Voltaic cells have a variety of uses.

In this experiment, you will prepare a variety of semi-microscale voltaic cells in a 24-well test plate. 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 I and II of this experiment, you will use a Voltage Probe to measure the potential of a voltaic cell with copper and lead electrodes. You will then test two voltaic cells that have unknown metal electrodes and, through careful measurements of the cell potentials, identify the unknown metals. In Part III of the experiment, you will measure the potential of a special type of voltaic cell called a concentration cell. In the first concentration cell, you will observe how a voltaic cell can maintain a spontaneous redox reaction with identical copper metal electrodes, but different electrolyte concentrations. You will then measure the potential of a second concentration cell and use the Nernst equation to calculate the solubility product constant, Ksp, for lead iodide, PbI2.

Figure 1

OBJECTIVES

In this experiment, you will

* Prepare a Cu-Pb voltaic cell and measure its potential.

* Test two voltaic cells that use unknown metal electrodes and identify the metals.

* Prepare a copper concentration cell and measure its potential.

* Prepare a lead concentration cell and measure its potential.

* Use the Nernst equation to calculate the Ksp of PbI2.

MATERIALS

Vernier computer interface 0.10 M Cu(NO3)2 solution

computer 0.10 M Pb(NO3)2 solution

Voltage Probe 1.0 M CuSO4 solution

three 10 mL graduated cylinders 0.050 M KI solution

24-well test plate 1 M KNO3 solution

String 0.10 M X nitrate solution

Cu and Pb electrodes 0.10 M Y nitrate solution

two unknown electrodes, labeled X and Y steel wool

150 mL beaker plastic Beral pipets

PRE-LAB EXERCISE

Use the table of standard reduction potentials in your text, or another approved reference, to complete the following table. An example is provided.

Electrodes Half-reactions Eo Eocell

Zn

Cu Zn(s) → Zn2+ + 2e-

Cu2+ + 2e- → Cu(s) +0.76 V

+0.34 V +1.10 V

Cu

Pb

Pb

Sn

Pb

Mg

Pb

Al

Pb

Zn

PROCEDURE

Part I Determine the Eo for a Cu-Pb Voltaic Cell

1. Obtain and wear goggles.

2. Use a 24-well test plate as your voltaic cell. Use Beral pipets to transfer small amounts of 0.10 M Cu(NO3)2 and 0.10 M Pb(NO3)2 solution to two neighboring wells in the test plate. CAUTION: Handle these solutions with care. If a spill occurs, ask your instructor how to clean up safely.

3. Obtain one Cu and one Pb metal strip to act as electrodes. Polish each strip with steel wool. Place the Cu strip in the well of Cu(NO3)2 solution and place the Pb strip in the well of Pb(NO)3 solution. These are the half cells of your Cu-Pb voltaic cell.

4. Make a salt bridge by soaking a short length of string in a beaker than contains a small amount of 1 M KNO3 solution. Connect the Cu and Pb half cells with the string.

5. Connect a Voltage Probe to Channel 1 of the Vernier computer interface. Connect the interface to the computer with the proper cable.

6. Start the Logger Pro program on your computer. Open the file "20 Electrochemistry" from the Advanced Chemistry with Vernier folder.

7. Measure the potential of the Cu-Pb voltaic cell. Complete the steps quickly to get the best data.

a. Click to start the data collection.

b. Connect the leads from the Voltage Probe to the