Maharashtra BoardEnglish MediumSTD 12 ScienceChemistryIonic Equilibria3 Marks
Question
Explain the common ion effect on dissociation of a weak acid.
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Answer
(1) Consider the dissociation or ionisation of a weak acid, $CH _3 COOH$ in its solution. $CH _3 COOH _{( aq )} \rightleftharpoons CH _3 COO _{( aq )}^{-}+ H _{( aq )}^{+}$ The dissociation constant $Ka$ for $CH _3 COOH$ will be, $K _{ a }=\frac{\left[ CH _3 COO ^{-}\right] \times\left[ H ^{+}\right]}{\left[ CH _3 COOH \right]}$ $K _{ a }$ is constant for $CH _3 COOH$ at constant temperature.
(2) If a strong electrolyte like salt $CH _3 COONa$ is added to the solution of $CH _3 COOH$, then on dissociation it gives a common ion $CH _3 COO ^{-}$. $CH _3 COONa \rightarrow CH _3 COO ^{-}+ Na ^{+}$
(3) Due to common ion $CH _3 COO ^{-}$, overall concentration of $CH _3 COO ^{-}$in the solution is increased which increases the ratio, $\frac{\left[ CH _3 COO ^{-}\right] \times\left[ H ^{+}\right]}{\left[ CH _3 COOH \right]}$. In order to keep this ratio constant, the concentration of $H ^{+}$is decreased, by shifting the equilibrium to the left hand side according to Le Chatelier's principle.
(4) Thus the ionisation of a weak acid is suppressed by a common ion.
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