Physical Chemistry Third Edition

596 13 Chemical Reaction Mechanisms II: Catalysis and Miscellaneous Topics

Electrolytic Cells Near Equilibrium

Figure 13.13 schematically shows a particular electrolytic cell. The two electrodes

are made of platinum, and the electrolyte solution is an aqueous HCl solution. The

external power supply drives electrons through the external circuit from left to right. In

the solution positive ions move from left to right and negative ions move from right to

left. An oxidation half-reaction occurs at the left electrode and a reduction half-reaction

occurs at the right electrode.

DC power

supply

Pt Pt

O 2 H 2

HCL

solution

Figure 13.13 An Electrolytic Cell for

the Electrolysis of an HCl Solution.

EXAMPLE13.6

Find the voltage required to produce an infinitesimal electrolysis current at 298.15 K in the cell

of Figure 13.13. Determine what products will be produced. Assume that any gaseous prod-

ucts are maintained at pressureP◦and that the hydrogen and chloride ions are at unit activity.

Solution

The only possible cathode half-reaction is

2H++ 2 e−−→H 2 (g) E◦ 0 .0000 V

If the hydrogen gas is maintained at the electrode at pressureP◦, the cathode is a standard

hydrogen electrode. There are two conceivable anode half-reactions:

2H 2 O−→O 2 (g)+4H++ 4 e− E◦− 1 .229 V

2Cl−−→Cl 2 (g)+ 2 e− E◦− 1 .3583 V

To produce oxygen at pressureP◦with HCl at unit activity, the reversible cell voltage would

be− 1 .229 V. To produce chlorine at pressureP◦, the reversible cell voltage would have to

be− 1 .3583 V. Water will be oxidized and O 2 will be formed at the anode if a voltage slightly

larger than 1.229 V is applied to the cell.

Sir Humphrey Davy, 1778–1829, was a
great English chemist who discovered
several elements, including sodium,
potassium, and magnesium. He was
also famous for inventing a safety lamp
for miners. He hired Michael Faraday as
an assistant in 1811, although Faraday
had no scientific education at the time.

If the products and reactants in Example 13.6 were not at unit activity, the Nernst

equation would be used to find the voltage required to produce each possible

product.

Exercise 13.19

a.Determine what products will be formed if an aqueous solution of sodium chloride with

activity on the molality scale equal to 1.000 is electrolyzed with an infinitesimal current at

298 .15 K, and find the voltage required. Assume that any gaseous products are maintained at

pressureP◦. Write the half-reaction equations and the cell reaction equation.

b.Pure sodium chloride can be electrolyzed above its melting temperature of 801◦C. Write

the half-reaction equations and the cell reaction equation. This is the process used by Sir

Humphrey Davy when he first isolated metallic sodium.

Electrochemical Cells with Finite Currents. The Electrical

Double Layer

We now investigate the molecular processes at the surfaces of electrodes. If a metallic

phase such as an electrode is negatively charged, the extra electrons will repel each