Hydrolysis of Salts

When an acid and a base neutralize each other, the salt produced is not automatically neutral in

water — its aqueous behaviour depends on the relative strengths of the acid and base that formed it,

through a reaction with water called hydrolysis. Four combinations arise.

Salt of a strong acid and a strong base (for example, $\text{NaCl}$, from $\text{HCl}$ and

$\text{NaOH}$): both $\text{Na}^+$ and $\text{Cl}^-$ are the conjugate species of a strong base and a

strong acid respectively, and neither has any tendency to react with water. No hydrolysis occurs, and

the solution remains neutral ($\text{pH} = 7$).

Salt of a weak acid and a strong base (for example, $\text{CH}_3\text{COONa}$, from

$\text{CH}_3\text{COOH}$ and $\text{NaOH}$): the cation $\text{Na}^+$ does not hydrolyze, but the

anion $\text{CH}_3\text{COO}^-$, being the conjugate base of a weak acid, reacts with water,

$\text{CH}_3\text{COO}^- + \text{H}_2\text{O} \rightleftharpoons \text{CH}_3\text{COOH} + \text{OH}^-$,

releasing free $\text{OH}^-$ and making the solution basic ($\text{pH} > 7$). The extent of this

hydrolysis, and hence the resulting pH, can be calculated exactly like a weak base's ionization, using

$K_b(\text{CH}_3\text{COO}^-) = K_w/K_a(\text{CH}_3\text{COOH})$ in place of $K_b$.

Salt of a strong acid and a weak base (for example, $\text{NH}_4\text{Cl}$, from $\text{HCl}$ and

$\text{NH}_4\text{OH}$): here the anion $\text{Cl}^-$ does not hydrolyze, but the cation

$\text{NH}_4^+$, the conjugate acid of a weak base, reacts with water,

$\text{NH}_4^+ + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4\text{OH} + \text{H}^+$, releasing

free $\text{H}^+$ and making the solution acidic ($\text{pH} < 7$), calculated analogously using

$K_a(\text{NH}_4^+) = K_w/K_b(\text{NH}_4\text{OH})$.

Salt of a weak acid and a weak base (for example, $\text{CH}_3\text{COONH}_4$, from

$\text{CH}_3\text{COOH}$ and $\text{NH}_4\text{OH}$): both ions hydrolyze simultaneously, the cation

tending to make the solution acidic and the anion tending to make it basic. The net result depends on

the relative strengths of the parent acid and base, and — unlike the previous two cases — is

essentially independent of the salt's concentration, given approximately by

$\text{pH} = 7 + \tfrac{1}{2}(\text{p}K_a - \text{p}K_b)$. If $K_a > K_b$ (the parent acid was

stronger than the parent base), the resulting solution is slightly acidic; if $K_b > K_a$, it is …