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Chemistry · Ch 3 — Ionic Equilibria

Salts of weak acids and strong bases

3.7.5

Salts of weak acids and strong bases

CH3COONa\mathrm{CH_3COONa} is a salt of weak acid CH3COOH\mathrm{CH_3COOH} and strong base NaOH. When dissolved in water, it dissociates completely.

CH3COONa(aq)→CH3COO−(aq)+Na+(aq)\mathrm{CH_3COONa(aq)} \rightarrow \mathrm{CH_3COO^{-}(aq)} + \mathrm{Na^{+}(aq)}

Water dissociates slightly as,

H2O(l)+H2O(l)⇌H3O+(aq)+OH−(aq)\mathrm{H_2O}(l) + \mathrm{H_2O}(l) \rightleftharpoons \mathrm{H_3O^{+}(aq)} + \mathrm{OH^{-}(aq)}

Solution of CH3COONa\mathrm{CH_3COONa} contains Na+\mathrm{Na^{+}}, H3O+\mathrm{H_3O^{+}}, CH3COO−\mathrm{CH_3COO^{-}} and OH−\mathrm{OH^{-}} ions. The Na+\mathrm{Na^{+}} ions of salt have no tendency to react with OH−\mathrm{OH^{-}} ions of water since the possible product of the reaction is NaOH, a strong electrolyte.

On the other hand the reaction of CH3COO−\mathrm{CH_3COO^{-}} ions of salt with the H3O+\mathrm{H_3O^{+}} ions from water produces unionized CH3COOH\mathrm{CH_3COOH}.

CH3COO−(aq)+H2O(l)⇌CH3COOH(aq)+OH−(aq)\mathrm{CH_3COO^{-}(aq)} + \mathrm{H_2O}(l) \rightleftharpoons \mathrm{CH_3COOH(aq)} + \mathrm{OH^{-}(aq)}

Thus, the hydrolytic equilibrium for CH3COONa\mathrm{CH_3COONa} is,

CH3COONa(aq)+H2O(l)⇌CH3COOH(aq)+Na+(aq)+OH−(aq)\mathrm{CH_3COONa(aq)} + \mathrm{H_2O}(l) \rightleftharpoons \mathrm{CH_3COOH(aq)} + \mathrm{Na^{+}(aq)} + \mathrm{OH^{-}(aq)}

As a result of excess OH−\mathrm{OH^{-}} ions produced the solution becomes basic. The solution of CH3COONa\mathrm{CH_3COONa} is therefore basic. …

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