Chemistry, Third edition

7 · OXIDATION AND REDUCTION

Thestandard hydrogen electrode(SHE; Fig. 7.3) is a hydrogen electrode where

H(aq) and H 2 (g) are in their standard states(see page 217). This means that the

concentration of H(aq) is 1 mol dm^3 , and the hydrogen gas is at a pressure of

1 atm. By agreement, the potential difference between the H(aq) solution and the

Pt electrode in the SHE is set to zero at all temperatures. This is written as:

E°(H(aq),H 2 (g)) = 0

where the use of the symbol  shows that the substances are in their standard states.

Other standard electrodes

The electrode of the Ag(aq)/Ag(s) redox couple consists of a rod of silver dipped in

a solution of silver ions. The Ag(aq)/Ag(s) electrode becomes the standard

Ag(aq)/Ag(s) electrode if the silver is pure, and the silver ion concentration is

1 mol dm^3.

The electrode of the Fe^3 (aq)/Fe^2 (aq) redox couple has a different construc-

tion. It consists of a piece of platinum dipped in a mixture of both ions. In an

electrochemical cell, the Pt connects the electrode to the rest of the cell. The Pt also

provides a surface upon which electrode reactions may occur. The standard

Fe^3 (aq)/Fe^2 (aq) electrode contains both ions at a concentration of 1 mol dm^3.

MeasuringE°values by making an electrochemical

cell

1. Measuring E°(Ag(aq)/Ag(s))

We will now see how the standard electrode potential of a redox couple may be

found by experiment. Figure 7.4 shows the electrochemical cell composed of the

standard hydrogen and standard silver ion/silver electrodes. All measurements are

made at 25 °C. The two electrodes are connected by:

1.a wire through which electrons can pass;

Photograph of a standard
hydrogen electrode

108

Electron flow

H+ ions

(1 mol dm–3)

H 2 gas

(1 atm)

Voltmeter

Ag(s)

Salt bridge

containing

saturated

KNO 3 (aq)

Cotton

wool

plug

Ag+ ions

(1 mol dm–3)

+0.80 V

Fig. 7.4Electrochemical cell used in the determination of E°(Ag/Ag(s) ).