Question

What ions hydrolyze in aqueous solutions?

Answer 1

Two types of ions hydrolyze in aqueous solutions: (1) the salts of weak acids and bases and (2) certain metal ions.

Hydrolysis of an ion is its reaction with water to produce an acidic or basic solution.

(1) Sodium acetate is a salt of the weak acid acetic acid. Acetate ion is the conjugate base of acetic acid. It hydrolyzes in water to form a basic solution:

CH₃COO⁻(aq) + H₂O(l) ⇌ CH₃COOH(aq) + OH⁻(aq)

Ammonium chloride is a salt of the weak base ammonia. Ammonium ion is the conjugate acid of the weak base ammonia. It hydrolyzes in water to form an acidic solution:

NH₄⁺(aq) + H₂O(l) ⇌ NH₃(aq) + H₃O⁺(aq)

(2) Metal ions are Lewis acids. In solution, they form aqua ions with the general formula `M(H₂O)_n^(m+)`. Examples are `Be(H₂O)_4^(2+)`, `Al(H₂O)_6^(3+)`, and `Fe(H₂O)_6^(3+)`. Small metal ions with high charge densities undergo hydrolysis to form acidic solutions. For example,

`Al(H₂O)_6^(3+) + H_2O ⇌ Al(H₂O)_5(OH)^(2+) + H_3O^+`