Modern inorganic chemistry

102 ACIDS AND BASES: OXIDATION AND REDUCTION

When either hydrogen ions or hydroxide ions participate in a

redox half-reaction, then clearly the redox potential is affected by

change of pH. Manganate(VII) ions are usually used in well-

acidified solution, where (as we shall see in detail later) they oxidise

chlorine ions. If the pH is increased to make the solution only

mildly acidic (pH = 3-6), the redox potential changes from 1.52 V

to about 1.1 V, and chloride is not oxidised. This fact is of practical

use; in a mixture of iodide and chloride ions in mildly acid solution.

manganate(VII) oxidises only iodide; addition of acid causes

oxidation of chloride to proceed.

Other important effects of ligand and pH changes on redox

potentials will be given under the appropriate element.

USES OF REDOX POTENTIALS

Reaction feasibility predictions

When the e.m.f. of a cell is measured by balancing it against an
external voltage, so that no current flows, the maximum e.m.f. is
obtained since the cell is at equilibrium. The maximum work
obtainable from the cell is then nFE J, where n is the number of
electrons transferred, F is the Faraday unit and E is the maximum
cell e.m.f. We saw in Chapter 3 that the maximum amount of work
obtainable from a reaction is given by the free energy change, i.e.
— AG. Hence
-AG = nFE

or

AG - - nFE

For a half-cell under standard conditions this becomes

where AG^ and E* are the free energy and redox potential under
standard conditions. In Chapter 3 we also noted that for a reaction
to be energetically feasible the total free energy must fall, i.e. AG
must be negative. An increase in free energy indicates that the
reaction cannot proceed under the stated conditions. The relation-
ship AG = -nFE can now be used to determine reaction feasi-
bility. Let us consider first the oxidation of iron(II) to iron(III) by
bromine in aqueous solution, i.e.

2Fe^2 + (aq) 4- Br 2 (aq) -> 2Fe^3 + (aq) + 2Br~(aq)