Question

Explain the common ion effect with an example.

Answer 1

When a salt of a weak acid is added to the acid itself, the dissociation of the weak acid is suppressed further. Acetic acid is a weak acid. It is not completely dissociated in an aqueous solution and hence the following equilibrium exists.

\[\ce{CH3COOH_{(aq)} ⇌ H^+_{( aq)} + CH3COO^-_{( aq)}}\]

However, the added salt, sodium acetate, completely dissociates to produce Na⁺ and CH₃COO ions.

\[\ce{CH3COONa_{(aq)} -> Na^+_{( aq)} + CH3COO_{(aq)}}\] 

Hence, the overall concentration of CH₃COO is increased, and the acid dissociation equilibrium is disturbed.

We know from Le chatelier’s principle that when stress is applied to a system at equilibrium, the system adjusts itself to nullify the effect produced by that stress. So, in order to maintain the equilibrium, the excess CH₃COO^– ions combine with H ions to produce much more unionized CH₃COOH i.e., the equilibrium will shift towards the left. In other words, the dissociation of CH₃COOH is suppressed. Thus, the dissociation of a weak acid (CH₃COOH) is suppressed in the presence of a salt (CH₃COONa) containing an ion common to the weak electrolyte. It is called the common ion effect.