Question

Explain the effect of common ions on the ionisation equilibria of weak acids with

the help of a suitable example.

Answer 1

The common ion effect suppresses the ionization of a weak acid by adding more of an ion that is a product of this equilibrium. The common ion effect suppresses the ionization of a weak base by adding more of an ion that is a product of this equilibrium in accordance with Le-Chatlier principle.

For example,$HF$ is a weak acid and it partially dissociate in water as:

$HF(aq)\to H^+(aq)+F^-(aq)$

$K_{eq}=\frac{[H^+][F^-]}{[HF]}$ which remain constant.

If we add $NaF$ in the solution, it will ionize as:

$NaF\to Na^+ + F^-$ and the concentration of flouride ions in the solution increases,and the reaction shifts backward in order to keep equilibrium constant same in accordance with Le-Chatlier principle.

Therefore the concentration of $H^+$ ions decrease causing increase in the $pH$ of solution.