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Casio EA-200 User's Manual

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20020601

English

Activity: SetupActivity: Setup

2-13-1

쐽 Equipment

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)

쐽 Setting Up

u Taking care that the two electrodes do not become shorted, setup the equipment as

shown in the illustration.

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

time.

A voltaic battery is made of a zinc (Zn) plate and a copper (Cu) plate immersed in a dilute

solution of sulfuric acid (H

2

SO

4

). 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 (H

2

) 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 → Zn

2+

+ 2e

–

(Negative Electrode Reaction)

2H

+

+ 2e

–

→ H

2

(Positive Electrode Reaction)

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.

Electromotive Force of a Battery

Theory

1 Dilute Sulfuric Acid Solution

2 Flashlight Bulb

3 Negative Electrode (Zinc Plate)

4 Positive Electrode (Copper Plate)

1 Stand

2 Cistern

3 Dilute Sulfuric Acid Solution

4 Zinc Plate

5 Copper Plate

6 Negative Electrode

7 Positive Electrode

8 Flashlight Bulb

9 Voltage Probe (CH1)

0 EA-200