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Explain common ion effect with an example. |
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Answer» Solution :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. |
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