2 ANDAMENTAL THEORETICAL PRINCIPLES OF REACTIONS IN SOLUTION

Acid Neutra1 Alkaline

Fig. 2.1

(ii) For any ion 1 of concentration [ I l

pl = log 1/[I] = - log[I]

Thus, for [Na'] = 8 x 10-5molL-1, pNa=4.1. (iii) For a Salt with a solubility product K,:

pK, = log 1/K, = -log K,.

2.18 THE HYDROLYSIS OF SALTS

Salts may be divided into four main classes:

(1) those derived from strong acids and strong bases, e.g. potassium chloride; (2) those derived from weak acids and strong bases, e.g. sodium acetate; ( 3 ) those derived from strong acids and weak bases, e.g. ammonium chloride;

and (4) those derived from weak acids and weak bases, e.g. ammonium formate or

aluminium acetate.

When any of these from classes (2) to (4) is dissolved in water, the solution, as is well known, is not always neutral in reaction. Interaction may occur with the ions of water, and the resulting solution will be neutral, acid, or alkaline according to the nature of the salt.

With an aqueous solution of a Salt of class ( l ) , neither do the anions have any tendency to combine with the hydrogen ions nor do the cations with the hydroxide ions of water, since the related acids and bases are strong electrolytes. The equilibrium between the hydrogen and hydroxide ions in water:

is therefore not disturbed and the solution remains neutral. Consider, however, a Salt MA derived from a weak acid HA and a strong

base BOH {class (2)). The Salt is completely dissociated in aqueous solution:

A very small concentration of hydrogen and hydroxide ions, originating from the small but finite ionisation of water, will be initially present. HA is a weak acid, i.e. it is dissociated only to a small degree; the concentration of A- ions which can exist in equilibrium with H + ions is accordingly small. In order to