Explain common ion effect with an example.

Explanation

Common Ion Effect: 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 aqueous solution and hence the following equilibrium exists.
`CH_(3)COOH (aq) rArr H^(+) (aq) + CH_(3)COO^(-) (aq)`
However, the added salt, sodium acetate, completely dissociates to produce `Na^(+) and CH_(3)COO^(-)`ion.
`CH_(3)COOH (aq) rArr Na^(+) (aq) + CH_(3)COO^(-) (aq)`
Hence, the overall concentration of `CH_(3)COO^(-)` is increased, and the acid dissociation equilibrium is disturbed. We know from Le chatelier.s principle that when a 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 `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. Thus, the dissociation of a weak 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.