2-13-1

Electromotive Force of a Battery

English

Activity:tivity: SetupSetup

￿Equipment

This activity investigates the changes in the electromotive force of a voltaic battery over time.

Theory

A voltaic battery is made of a zinc (Zn) plate and a copper (Cu) plate immersed in a dilute solution of sulfuric acid (H2SO4). Employing the difference in the ionization tendencies between the two metals, the zinc plate is the negative electrode (cathode), while the copper plate is the positive electrode (anode).

Ionization tendency is the tendency of metal to release electrons, which become positively charged ions. As shown below, zinc and copper are on opposite sides of the hydrogen molecule.

K Ca Na Mg Al Zn Fe Ni Sn Pb (H2) Cu Hg Ag Pt Au

Exposing zinc, which has a relatively strong ionization tendency, to the hydrogen ions in a dilute sulfuric acid solution, causes the zinc to dissolve and release electrons, creating a minus charge. When the battery is connected to a circuit, the released electrons flow through the circuit to become electric current, and collect on the copper plate, which has a relatively weak ionization tendency and is not dissolving. The electrons on the surface of the copper plate are consumed when they attach to hydrogen ions in the dilute sulfuric acid solution and give off hydrogen gas. As the electrons on the copper plate are consumed, the zinc plate releases more electrons, which allows the flow of current to continue. The reaction that occurs at the two electrodes at this time is represented by the chemical expression shown below.

Zn → Zn2+ + 2e– (Negative Electrode Reaction)

2H+ + 2e– → H2 (Positive Electrode Reaction)

1Dilute Sulfuric Acid Solution

2Flashlight Bulb

3Negative Electrode (Zinc Plate)

4Positive Electrode (Copper Plate)

It should be noted here that should too much hydrogen gas foam build up on the copper plate surface, a phenomenon called “polarization” can cause the hydrogen gas to start to return to hydrogen ions. Allowing polarization to continue will lead to deterioration of the electromotive force of the battery. If this happens, hydrogen peroxide can be added to the area around the copper plate, which reacts with the hydrogen gas and reduces the amount of foam present. This, in turn, restores the electromotive force of the battery. Repeating this measure when required will theoretically allow the battery to operate properly until the zinc plate is completely dissolved. In actual practice, a voltaic battery will reach the end of its useful life before then.

Stand

Voltaic Battery (Zinc Plate, Copper Plate, Dilute Sulfuric Acid Solution, Cistern)

Flashlight Bulb Hydrogen Peroxide Pipette with a bulb

Voltage Measurement Setup (EA-200, graphic scientific calculator, data communication cable, voltage probe)

￿Setting Up

uTaking care that the two electrodes do not become shorted, setup the equipment as shown in the illustration.

1Stand

2Cistern

3Dilute Sulfuric Acid Solution

4Zinc Plate

5Copper Plate

6Negative Electrode

7Positive Electrode

8Flashlight Bulb

9Voltage Probe (CH1)

0EA-200

20020601