Explain common ion effect with an example.

Explanation

The dissociation of a week acid `(CH_3 COOH)` is suppressed in the presence of a salt `(CH_3 COONa)` containing an ion common to the weak electrolyte. It is called the common ion effect.
Eg: Acetic acids is a weak acid. It is not completely dissociated in aqueous solution and hence the following equilibrium exists.
`CH_3COOH_((aq)) hArr H_((aq))^(+)+CH_3 COO_(((aq))`
However, the added salt, sodium acetate, completely dissociates to produce `Na^+` and `CH_3 COO^(-) "ion"`.
`CH-3 COONa_((aq)) to Na_((aq))^(+)+CH_3COO_((aq))^(-)`
Hence, the overall concentration of `CH_3 COO^(-)` is increased , and the acid dissociation equilibrium is disturbed. According to Le chatelier's principle, the excess `CH_3 COO^(-)` ions combines with `H^+` ions to produce much more unionized `CH_3 COOH` i.e., the equilibrium will shift towards the left. In other words, the dissociation of `CH_3 COOH` is suppressed.