TOPIC: Electrochemistry

Demo-029

Making a Copper-Zinc Battery

This demonstration illustrates what is necessary to make a simple working battery. One needs only some pieces of copper and zinc metal, water solutions of copper and zinc sulfates, and a voltmeter.

MATERIALS

Thin strips of copper and zinc metals.
250 mL beaker
Two 8 inch test tubes.
Voltmeter (for overhead projector)
Overhead projector (normally in lecture hall)
1 liter 0.5 mol/L copper(II) sulfate.
1 liter 0.5 mol/L zinc sulfate.
Ringstand and two clamps or a buret clamp.
Apparatus for setting up a working galvanic cell. This is best done with a beaker and unglazed porcelain cup (Figure 3). You can also use the H-tube apparatus in Figure 2.

PREPARATION

If you wish to set up a non-working cell, assemble the equipment shown in figure 1. For the working model, use Figure 2 or 3.

PRESENTATION

This demonstration is most effective if the instructor first attempts to make a battery that will not work. Show the class that zinc metal will spontaneously react with a solution of copper(II) ion. Place a strip of zinc into the solution. Withing one minute, the zinc will be coated with a black layer of microscopic crystals of copper. After a few hours, the crystals become large enough to take on the characteristic color of copper metal and the copper(II) solution will have lightened in color.

Explain that the reaction between zinc and copper ion involves the spontaneous transfer of two electrons from zinc metal to copper ion. Write the equation for this process on the board. Explain that the whole idea of a battery or galvanic cell is to cause this spontaneous electron transfer to occur through an external wire by separating the zinc from the copper ion.

Attempt to build a (nonworking) cell by placing the zinc metal in one test tube (containing the zinc sulfate solution) and the copper sulfate solution in the second test tube (also containing a piece of copper metal to serve as an electrode). Figure 1 shows the arrangement. Measure the voltage between the zinc and copper electrodes; there will be none because this cell is an open circuit; there is no pathway for ions in solution to migrate and complete the circuit..

Figure 1. First try battery. It won’t work.

After this "failure," explain the flaw in the design of the cell, namely that there must be a pathway for ion migration to complete the circuit. This can be added by connecting the two test tubes with a filter disk (H-tube in Figure 2), or by separating the two electrodes and solutions with a porous cup (Figure 3). Once this is done, an operating battery is obtained. The measured voltage should be around 1.0 to 1.1 volts. The standard voltage for a zinc/copper cell is 1.10 volts.

Figure 2. H-tube connecting test tubes with a porous partition

 

Figure 3. Copper-Zinc Battery made with porous cup separating solutions.

DISCUSSION

It is important to emphasize that making a battery, i.e., something that produces an electical voltage, is very simple. One does not need a glass apparatus with filter disks or salt bridges; the only thing necessary is two different metals and conducting media. A good example is the unfortunate effect of biting down on a piece of aluminum foil with a tooth containing a filling. The sharp pain associated with this unhappy mistake is the result of a transient electric current set up by placing two different metals in contact. One does not even need different metals; this is explored in another demonstration in this handbook, the concentration cell.

Excellent voltages can also be obtained with a salt bridge, which can be made without much difficulty. This is moving a little more into the "black magic" area. Again it should be noted that nothing fancy is needed for the salt bridge. I have heard that an ordinary hot dog works fine!

HAZARDS

There are few hazards associated with this demonstration. Don't cut yourself when trimming the zinc and copper metal. Do not connect several hundred Zn-Cu cells in series; there is an electrocution hazard there.

The metal strips can be recycled for future use. Clean them up with steel wool.

 

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