Question

Answer the following in brief :

The aqueous solution of sodium carbonate is alkaline whereas the aqueous solution of ammonium chloride is acidic. Explain.

Answer 1

i. Sodium carbonate (Na₂CO₃) is a salt of weak acid H₂CO₃ and strong base NaOH. When dissolved in water, it dissociates completely.

\[\ce{Na2CO3_{(aq)} -> 2Na^+_{ (aq)} + CO^2-_{3(aq)}}\]

ii. The Na⁺ ions of salt have no tendency to react with OH⁻ ions of water since the possible product of the reaction is NaOH, a strong electrolyte.

iii. On the other hand, the reaction of `"CO"_3^{2-}` ions of salt with the H₃O⁺ ions from water produces unionized H₂CO_3.

\[\ce{CO^{2-}_{3{(aq)}} + 2H2O_{(l)} ⇌   H2CO_{3(aq)} + 2OH^-_{ (aq)}}\]

Thus, the hydrolytic equilibrium for Na₂CO₃ is,

\[\ce{Na2CO_{3(aq)} + H2O_{(l)} ⇌ H2CO_{3(aq)} + 2Na^+_{ (aq)} + 2OH^-_{ (aq)}}\]

iv. As a result of excess OH⁻ ions produced, the resulting solution of Na₂CO₃ is alkaline.

v. Similarly, ammonium chloride (NH₄Cl) is salt of strong acid HCl and weak base NH₄OH. When NH₄Cl is dissolved in water, it dissociates completely as,

\[\ce{NH4Cl_{(aq)} -> NH^+_{4(aq)} + Cl^-_{ (aq)}}\]

vi. `"Cl"_{("aq")}^-` ions of salt have no tendency to react with water because the possible product HCl is a strong electrolyte.

vii. The reaction of `" NH"_4^+` ions with OH⁻ ions form unionized NH₄OH. The hydrolytic equilibrium for NH₄Cl is then written as,

\[\ce{NH^+_{4(aq)} + 2H2O_{(l)} ⇌ NH4OH_{(aq)} + H3O+_{ (aq)}}\]

viii. Due to the presence of an excess of H₃O⁺ ions, the resulting solution of NH₄Cl is acidic.