Physical Chemistry Third Edition

8.2 Electrochemical Cells 355

HCI

solution

AgCl(s)

Ag(s)

Pt Pt

H 2

Load

(resistor, potentiometer, etc.)

Figure 8.2 An Electrochemical Cell.

porous platinum) to increase its surface area. Hydrogen gas is bubbled through the

solution around the electrode and is adsorbed on the platinum, where it can undergo

the oxidation process

H 2 (g)−→2H++ 2 e− (8.2-1)

This process is called ahalf-reactionbecause it cannot take place unless another

process accepts the electrons that are produced. This half-reaction is an oxidation

half-reaction. An electrode for which an oxidation half-reaction occurs is called an

anode.

The electrode at the right in the cell of Figure 8.2 is a silver–silver chloride electrode.

It is a piece of silver that is coated with solid silver chloride. Thereduction half-reaction

AgCl(s)+e−−→Ag(s)+Cl− (8.2-2)

can occur at this electrode and can accept the electrons produced by the oxidation

half-reaction. The electrode for which a reduction half-reaction is written is called a

cathode. The platinum metal of the hydrogen electrode forms a terminal. We attach a

terminal of platinum to the right electrode in order to have two terminals of the same

material. In the English language you can remember the names “anode” and “cathode”

by noting that “anode” and “oxidation” both begin with vowels, and that “cathode”

and “reduction” both begin with consonants.

We multiply the reduction half-reaction equation by 2 and add the two half-reaction

equations to obtain thecell reaction equation:

H 2 (g)+2AgCl(s)+ 2 e−(R)−→2H++2Ag(s)+2Cl−+ 2 e−(L) (8.2-3)