If in an aqueous solution of weak electrolyte ,a strong electrolyte is added having an ion common with the weak electrolyte, then the dissociation of weak electrolyte is decreased. The effect by which the dissociation of weak electrolyte is decreased is known as common-ion effect.
common ion effect is a phenomenon based on the Le Chatelier’s principle
Example of Common Ion effect
Lets take an solution of Acetic Acid
$CH_3COOH(aq) \rightleftharpoons H+ (aq) + CH_3COO^–(aq)$
or we can write this as
$HAc(aq) \rightleftharpoons H+ (aq) + Ac^– (aq)$
$K_a= \frac {[H+][Ac– ]}{ [HAc}$
The common Ion effect can be observed in two cases
1. Addition of acetate ions: If we addacetate ion to an acetic acid solution results in decreasing the concentration of hydrogen ions, [H+].
2. Addition of H+ ions :if H+ ions are added from an external source then the equilibrium moves in the direction of undissociated acetic acid i.e., in a direction of educing the concentration of hydrogen ions,
[H+]
Examples
1. Which of the following pairs will show common ion effect?
(a) NaCl , HCL
(b) $NH_4OH$, $NH_4CL$
(c) AgCN , KCN
(d) $BaCl_2$,$Ba(NO_3)_2$
Solution
(b) as $NH_4OH$ is a weak electrolyte
