Modern inorganic chemistry

ACIDS AND BASES: OXIDATION AND REDUCTION 87

THE EFFECT OF THE SOLVENT

If, for a given acid, we wish to increase the acid strength, then we

choose a solvent which has a greater affinity for protons than has

water. If we add ammonia to a solution of hydrogen chloride in

water, the essential equilibrium is

H 3 o

+

+ NH 3 - H 2 o + NH;

and clearly here ammonia has a stronger affinity for protons than

water — it is a stronger base. Hence if we dissolve an acid which is

weak in water in liquid ammonia, the strength of the acid is increased,

i.e. pKa decreases. Thus methanoic (formic) acid is a weak acid in

water but a strong acid in liquid ammonia.

When we use any substance as a solvent for a protonic acid, the

acidic and basic species produced by dissociation of the solvent

molecules determine the limits of acidity or basicity in that solvent.

Thus, in water, we cannot have any substance or species more

basic than OH~ or more acidic than H 3 O+ ; in liquid ammonia,

the limiting basic entity is NH7, the acidic is NH^. Many common

inorganic acids, for example HC1, HNO 3 , H 2 SO 4 are all equally

strong in water because their strengths are 'levelled' to that of the

solvent species H 3 O*. Only by putting them into a more acidic

solvent do they become weak acids, with determinate pXa values

which differentiate their strengths. Thus in glacial ethanoic (acetic)

acid as solvent, the order of strength of some common strong acids is

H 2 SO 4 > HC1 > HNO 3

As we shall see later, the limitations imposed by most solvents may

prevent us from being able to utilise the very strong basic character-

istics of some anions. However, at this point it is more useful to

consider other factors affecting the strengths of acids.

THE EFFECT OF STRUCTURE: ACID STRENGTH IN

WATER

Consider first two substances which have very similar molecules,
HF, hydrogen fluoride and HC1, hydrogen chloride; the first is a
Weak acid in water, the second is a strong acid. To see the reason
consider the enthalpy changes involved when each substance in
Water dissociates to form an acid: