Common Ion Effect
Suppression of Ionization | Ionic Equilibrium
1. Definition & Principle
Mechanism (Le Chatelier's Principle):
Consider the dissociation of a weak acid (Acetic Acid):
$$ CH_3COOH(aq) \rightleftharpoons CH_3COO^-(aq) + H^+(aq) $$If we add a strong electrolyte like Sodium Acetate ($CH_3COONa$) which dissociates completely:
$$ CH_3COONa \rightarrow CH_3COO^- + Na^+ $$The concentration of acetate ions ($CH_3COO^-$) increases drastically. To restore equilibrium, the reaction shifts BACKWARD, consuming $H^+$ and decreasing the ionization of $CH_3COOH$.
2. Quantitative Aspect (Math Proof)
Let initial concentration of $CH_3COOH$ be $0.1 M$ ($K_a = 1.8 \times 10^{-5}$).
Case A: Pure Acid
Degree of dissociation $\alpha = \sqrt{K_a/C} = \sqrt{1.8 \times 10^{-4}} \approx 0.013$ (1.3%).
Case B: With 0.1 M CH3COONa (Common Ion)
In equilibrium, $[CH_3COO^-] \approx 0.1 M$ (from salt). The weak acid contributes negligible ions.
$$ K_a = \frac{[CH_3COO^-][H^+]}{[CH_3COOH]} $$ $$ 1.8 \times 10^{-5} = \frac{(0.1)[H^+]}{0.1} $$ $$ [H^+] = 1.8 \times 10^{-5} M $$New $\alpha' = [H^+]/C = 1.8 \times 10^{-5} / 0.1 = 1.8 \times 10^{-4}$ (0.018%).
3. Important Applications
A. Purification of Common Salt ($NaCl$)
Pass $HCl$ gas through saturated impure $NaCl$ solution.
$$ NaCl(s) \rightleftharpoons Na^+(aq) + Cl^-(aq) $$Addition of $Cl^-$ (from $HCl$) increases ionic product $[Na^+][Cl^-]$. When it exceeds the Solubility Product ($K_{sp}$), pure $NaCl$ precipitates out, leaving impurities in solution.
B. Salting Out of Soap
Soap ($RCOONa$) is precipitated from solution by adding $NaCl$. The increase in $[Na^+]$ shifts the equilibrium backward, precipitating solid soap.
C. Qualitative Analysis (Salt Analysis)
- Group II Cations ($Cu^{2+}, Pb^{2+}$): Precipitated as sulphides using $H_2S$ in presence of $HCl$. The $H^+$ from $HCl$ suppresses the ionization of $H_2S$, keeping $[S^{2-}]$ low. This ensures only Group II sulfides (low $K_{sp}$) precipitate, preventing Group IV precipitation.
- Group III Cations ($Fe^{3+}, Al^{3+}$): Precipitated as hydroxides using $NH_4OH$ in presence of $NH_4Cl$. The $NH_4^+$ ion suppresses ionization of $NH_4OH$, keeping $[OH^-]$ low to avoid precipitation of higher groups.
4. Buffer Solutions
The common ion effect is the foundational principle behind Acidic and Basic Buffers. A mixture of Weak Acid + Its Salt (Common Ion) resists pH change because the high concentration of the common ion (Salt) acts as a reservoir to consume added $H^+$ or $OH^-$.
Practice Quiz
Test your understanding of the Common Ion Effect.
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