Activity 1: A Study of Voltaic Cells
Purpose
To generate an activity series by constructing and comparing several electrochemical cells.
Safety
1. Wear protective goggles throughout the laboratory activity.
2. Any time you spill a solution on your clothes or body, wash it off immediately.
3. Silver nitrate spills on skin will cause dark stains and burns. Handle it with special care.
4. Dispose of all waste materials as your teacher directs.
Procedure
1. Collect the following materials:
Figure 1. Well plate.
2. Examine the well-plate. Use any two adjacent wells to make a voltaic cell. Use the diagram below to design the most efficient arrangement of half-cells so you can measure the electric potential (voltage) of every pair of half-cells. To be most efficient, use one well that is not on the edge of the plate to make your silver half-cell. The silver half-cell involves expensive materials; you will assemble only one of these. If you are unsure about your arrangement, have your teacher check it before you start.
3. Fill each selected well with about 15 drops of appropriate metal ion solution.
4. Identify and label each of the five metals used in this activity according to directions provided by your teacher.
5. Clean each metal strip and wire with sandpaper. Place the cleaned metal samples on a piece of paper next to a chemical symbol that identifies each metal.
6. Make a table to record your data. For example, comparing Mg with Zn is the same as comparing Zn with Mg. You should find that 10 sets of measurements are needed to make all possible comparisons. However, if you are very clever and a little lucky, youÕll be able to answer all the questions with a minimum of four comparisons.
7. Select two half-reactions to be tested. Connect them with a salt bridge. Be sure each end of the salt bridge is immersed in the half-cell solution.
8. Place appropriate metal strips or wires in the half-cell. For example, place Zn metal in the zinc sulfate solution, Ag in the silver nitrate solution, and so forth. NOTE: Always use the correct metal electrode for each solution.
9. Connect the metal strips or wires to the voltmeter. Attach the voltmeter so the cathode of the voltaic cell is attached to the positive lead of the voltmeter. You can tell by seeing which connection of the voltaic cell to the voltmeter gives a positive electric potential. Record the electric potential (in volts) for that pair, and record which metal is the cathode.
10. Continue testing pairs until you can answer all the questions your teacher has assigned.
11. Dispose of all solutions as your teacher instructs. The salt bridges and metal strips or wires can be reused; do not throw them away. Clean them as your teacher directs.
12. Thoroughly wash your hands before leaving the laboratory.
Data Analysis and Concept Development
1. Examine your data. List the half-reactions from highest reduction potential (most likely to be the cathode) to lowest reduction potential (least likely to be the cathode.)
2. If you have measured electric potential energy differences between pairs of half-cells and your teacher has provided the known reduction potential of one of the half-reactions, calculate reduction potentials for the other four half-reactions.
3. Compare your results with a table of standard reduction potentials. Are there any differences between either the order you observed or the values you calculated? If there are differences, what might be possible reasons? (Hint: How were the standard reduction potentials measured, as compared with your procedure?) The reasons you offer to explain differences should be specific enough to be tested in the laboratory, if time allows.
4. Why must the salt bridge be in contact with both solutions for the voltaic cell to generate current?
5. Will zinc metal react when immersed in a solution of copper(II) sulfate? Will silver metal react when immersed in a solution of copper(II) sulfate?
Implications and Applications
1. Cathodic protection is the method most often employed to protect buried iron fuel tanks, iron pipelines, and iron ship hulls. An active metal that oxidizes more readily than iron is attached to the iron object being protected. Which metals used in this activity would be useful for the cathodic protection of iron? The reduction potential for the Fe/Fe 2+ half-reaction is 0.41 V, while the reduction potential for the Sn/Sn 2+ half-reaction is 0.14 V.
2. In 1973, the wreckage of the Civil War ironclad USS Monitor was discovered near Cape Hatteras, North Carolina. (The Monitor and the CSS Virginia, formerly the USS Merrimack, fought the first battle between iron-armored ships.) In 1987, investigations were begun to determine whether the ship could be salvaged. It was reported in Time (June 22, 1987) that scientists were considering attaching zinc anodes to the rapidly corroding Monitor metal hull. Describe how attaching zinc would protect the hull from further corrosion.
 |
TABLE OF CONTENTS | TOPIC OVERVIEW | CONCEPT/SKILLS DEVELOPMENT | LINKS/CONNECTIONS | EXTENSIONS |  |
|---|
