PRE-U EXPERIMENT

MAKING AND TESTING A SIMPLE GALVANIC CELL

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PURPOSE

The purpose of this experiment is to introduce the student to electrochemistry and oxidation-reduction reactions through the construction and operation of a simple galvanic cell.

DESCRIPTION

This experiment is appropriate for a general or first-year college-prep course. An inexpensive cell is made from materials obtained locally. Students test the cell by connecting it to a 1.5-volt flashlight lamp and observing whether or not the lamp will light. In addition, four or more of these cells can be connected in series to form a battery that can power a small radio or other device that operates on direct current.

TIME REQUIRED

Two to three labs periods.

MATERIALS

Chemicals

0.5 M copper(II) sulfate (dissolve 125 g CuSO₄·5H₂O in distilled or deionized water and dilute to 1.00 liter)*

0.5 M sodium sulfate (dissolve 71 grams of Na₂SO₄ in distilled or deionized water and dilute to 1.00 liter)

zinc, copper, aluminum, and magnesium strips (approximately 1 cm x 10 cm)*

Equipment

250-mL beaker*

dialysis tubing*

hook-up wire or bell wire

1.5-volt flashlight lamp with less than 100 milliamp rating

alligator clamps

crimping tool

soldering iron and resin core solder

clamps(for dialysis tubing)

sandpaper or steel wool

*See Modifications/Substitutions

HAZARDS

Avoid contact with solutions. Goggles must be worn throughout experiment.

MODIFICATIONS/SUBSTITUTIONS

1. Copper sulfate pentahydrate is available as root killer at garden supply stores.
2. Aluminum, from aluminum cans, may be used if it is sanded well on both sides. Aluminum gutter nails may also be used.
3. Copper tubing or fittings may be used in place of copper strips.
4. Zinc can be obtained from old dry cell battery casings. This should be done carefully to avoid contact with caustic chemicals in battery.
5. Sausage casings can be used in place of dialysis tubing but diffusion is extremely rapid. Dialysis tubing is readily available in biology labs.
6. Baby food jars or other open glass jars may be used instead of beakers.

PROCEDURE

1. Prepare a test lamp by soldering wire test leads to a 1.5-volt flashlight lamp. Attach alligator clips to the ends of the leads.
2. Obtain strips of magnesium and copper, each strip should be 2.5 cm longer than the height of the beaker being used. Sand the strips until they are shiny.
3. Tie a knot with string or use a non-reactive clamp in one end of dialysis tubing that has been soaked in distilled water. The length of tubing should be long enough so that it overlaps the edge of the beaker by 2.5 cm.
4. Fill the dialysis tubing with the prepared copper(II) sulfate solution. Place the copper strip in this piece of dialysis tubing that is now filled with copper(II) sulfate solution and use string or a rubber band to secure the top of the dialysis tubing around the copper. Leave 2.5 cm of copper sticking out of the tubing.
5. Fill the beaker with the prepared sodium sulfate solution, place both the dialysis tubing, containing the copper and copper(II) sulfate solution, and the magnesium strip in the beaker. Place the magnesium strip as far away as possible from the dialysis tubing. Secure both the magnesium strip and the copper strip in the dialysis tubing in the beaker by bending them over the edge of the beaker.
6. Complete the circuit by attaching one alligator clip to the copper strip and the other to the magnesium strip.
7. Observe and record any activity taking place at the metal strips, the solution (especially color changes), and the test lamp.
8. Connect several cells in series and operate a small radio or some other direct current device.
9. Try making cells with zinc or aluminum in place of the magnesium.

DISPOSAL

All solutions may be flushed down the drain with water.

DISCUSSION

The following discussion applies when copper and magnesium strips are used as the metal strips. If other metals are used, similar explanations can be used.

Mg(s) + Cu²⁺(aq) " src="file:///F:/galvanic%20experiment_files/rightarrow.htm" align="BOTTOM" border="0" hspace="4"> Mg²⁺(aq) + Cu(s) + 1.5v

This reaction is an oxidation-reduction reaction in which the magnesium is oxidized by the copper to Mg²⁺ ions. The result of this reaction is a transfer of electrons which provides the power required to light the flashlight lamp.

Students performing this experiment should be familiar with the following terms: anode, cathode, oxidizing agent, reducing agent, and electromotive series. Students should be presently studying a unit on electrochemistry and oxidation-reduction reactions in order to fully understand the "chemistry" of this experiment.

TIPS

1. The metal strips used in this experiment should have a large surface area to minimize resistance within the cell.
2. Use a voltmeter instead of the light bulb to eliminate the need for soldering.
3. Aluminum strips may not work unless sanded thoroughly and acid treated with 6.0 M HCl.
4. Soldering techniques should be mastered by teacher so that any soldering can be

REFERENCE

Summerlin, L.R., and Ealy, J.L. Jr, Chemical Demonstrations-A Sourcebook for Teachers, American Chemical Society, Washington D.C, 1985, p. 115. This experiment is adapted from this source.