Common Ion Effect

When a salt of a weak acid is added to that same weak acid, the acid's dissociation is suppressed further -- called the common ion effect. Adding sodium acetate to acetic acid is the classic case: acetic acid is only weakly dissociated, $CH_3COOH(aq) \rightleftharpoons H^+(aq)+CH_3COO^-(aq)$, while the added salt sodium acetate dissociates completely, $CH_3COONa(aq) \rightarrow Na^+(aq)+CH_3COO^-(aq)$, sharply raising the overall $CH_3COO^-$ concentration. By Le Chatelier's principle, a system at equilibrium responds to a stress by shifting to relieve it -- so the excess $CH_3COO^-$ combines with $H^+$ to form more unionised $CH_3COOH$, shifting the acetic acid equilibrium to the left, i.e. suppressing its dissociation. In general, the dissociation of a weak acid is suppressed in the presence of a salt contain …