Chemistry, Third edition

110 7 · OXIDATION AND REDUCTION

(consisting of pure Zn metal dipped into 1 mol dm^3 Zn^2 (aq)) connected by a salt

bridge.

If the standard Zn^2 (aq)/Zn(s) electrode is connected to the negative pole of the

voltmeter, the voltage reading is found to be 0.76 V. The positive voltage shows

that the Zn^2 (aq)/Zn(s) electrode is the anode. At the anode, the Zn^2 (aq)/Zn(s)

redox couple undergoes oxidation:

Zn(s)Zn^2 (aq)2e

The electrons cause the H(aq)/H 2 (g) redox couple to be reduced within the cath-

ode of the SHE:

2H(aq)2eH 2 (g)

The overall cell reaction is

Zn(s)2H(aq)Zn^2 (aq)H 2 (g)

The cell diagram is

Zn(s) Zn^2 (aq) Pt H(aq) H 2 (g)

The cell potential is 0.76 V:

E°=E°RE°L

0.76 = E°(H(aq),H 2 (g))E°(Zn^2 (aq)/Zn(s))

0.76 = 0 E°(Zn^2 (aq)/Zn(s))

or,

E°(Zn^2 (aq)/Zn(s)) = 0.76 V

We have now looked at two electrochemical cells. In the first, where the E°of the

redox couple under investigation proved to be positive, the H(aq)/H 2 (g) couple

undergoes oxidation (Fig. 7.5(a)). In the second, where the E°of the redox couple

under investigation proved to be negative, the H(aq)/H 2 (g) couple undergoes

reduction (Fig. 7.5(b)). Generalizing:

1.A negative E° means that a redox couple is a stronger reducing agent than the

H(aq)/H 2 (g) couple.

2.A positive E° means that a redox couple is a weaker reducing agent than the

H(aq)/H 2 (g) couple.

The reverse statements apply to the oxidizing power of these redox couples.

Fig. 7.5The flow of electrons in cells involving the SHE in which (a) Zn^2 (aq)/Zn(s), and

(b) Ag(aq)/Ag(s) form the other electrode.

H 2 (g) + 2Ag+(aq)2H+(aq) + 2Ag(s) Zn(s) + 2H+(aq)Zn2+(aq) + H 2 (g)