Explain common ion effect with suitable example. - Vidyadip

Question

Explain common ion effect with suitable example.

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Answer

A weak electrolyte dissociates partially in aqueous solution to produce cations and anions. Equilibrium exists between ions thus formed and the undissociated molecules.
$BA \rightleftharpoons B ^{+}+ A ^{-}$
For such an equilibrium, the dissociation constant $K$ is defined as
$K =\frac{\left[ B ^{+}\right] \times\left[ A ^{-}\right]}{[ BA ]}$
$K$ is constant for the weak electrolyte at a given temperature.
Now, if another electrolyte BC or DA is added to the solution $B A$, having a common ion either $B ^{+}$ or $A ^{-}$, then the concentration of either $B ^{+}$or $A ^{-}$is increased. However, as $K$ is always constant, the increase in the concentration of any one of the ions shifts the equilibrium to left. In other words, the dissociation of BA is suppressed. This is called common ion effect. For example, the dissociation of a weak acid $CH _3 COOH$ is suppressed by adding $CH _3 COONa$ having common ion $CH _3 COO^{-}$.
$\begin{aligned}
& CH _3 COOH \rightleftharpoons CH _3 COO ^{-}+ H ^{+} \\
& CH _3 COONa \rightarrow CH _3 COO ^{-}+ Na ^{+}
\end{aligned}$

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