Hydrolysis of Salts

Hydrolysis

  • Hydrolysis is the reaction of an anion and/or cation of a salt with water.
  • Salt hydrolysis usually affects the pH of a solution.

Nature of Salts

  • Salts are ionic compounds formed from the reaction of an acid and a base.
  • The strength of the acid/base affects the nature of the salt (neutral, acidic, or basic).

Neutral Salts

  • Have a pH of 7 and equal concentrations of H+H^+ and OH−OH^- ions.
  • Formed from strong acids and strong bases (e.g., NaCl).
  • Ions that don't react appreciably with water:
    • Cations from strong bases: Alkali metal cations of group 1A (Li+, Na+, K+), Alkaline earth cations of group 2A (Mg2+, Ca2+, Sr2+, Ba2+) except for Be2+
    • Anions from strong monoprotic acids: Cl-, Br-, I-, NO3-, and ClO4-

Basic Salts

  • Formed from reactions of weak acids and strong bases (e.g., CH3COONa).
  • Reaction: CH<em>3COONa(s)→CH</em>3COO−(aq)+Na+(aq)CH<em>3COONa(s) \rightarrow CH</em>3COO^-(aq) + Na^+(aq)
  • Hydrolysis: CH<em>3COO−+H</em>2O⇌CH3COOH+OH−CH<em>3COO^- + H</em>2O \rightleftharpoons CH_3COOH + OH^-
  • Hydrolysis Constant: K<em>h=[CH</em>3COOH][OH−][CH3COO−]K<em>h = \frac{[CH</em>3COOH][OH^-]}{[CH_3COO^-]}
  • K<em>b≡K</em>h=K<em>wK</em>aK<em>b \equiv K</em>h = \frac{K<em>w}{K</em>a}

Acidic Salts

  • Formed from a reaction of a weak base and a strong acid (e.g., NH4Cl).
  • Reaction: NH<em>4Cl(aq)→NH</em>4+(aq)+Cl−(aq)NH<em>4Cl(aq) \rightarrow NH</em>4^+(aq) + Cl^-(aq)
  • Hydrolysis: NH<em>4+(aq)+H</em>2O(l)⇌NH<em>3(aq)+H</em>3O+(aq)NH<em>4^+(aq) + H</em>2O(l) \rightleftharpoons NH<em>3(aq) + H</em>3O^+(aq)
  • Hydrolysis Constant: K<em>h=[NH</em>3][H<em>3O+][NH</em>4+]K<em>h = \frac{[NH</em>3][H<em>3O^+]}{[NH</em>4^+]}
  • K<em>a≡K</em>h=K<em>wK</em>bK<em>a \equiv K</em>h = \frac{K<em>w}{K</em>b}

pH of Salt Solutions

  • Example calculation for 0.5M NH4Cl:
    • K<em>a=K</em>wKb=1.0×10−141.8×10−5=5.6×10−10K<em>a = \frac{K</em>w}{K_b} = \frac{1.0 × 10^{-14}}{1.8 × 10^{-5}} = 5.6 × 10^{-10}
    • K<em>a=[NH</em>3][H<em>3O+][NH</em>4+]=x20.50−x≈x20.50K<em>a = \frac{[NH</em>3][H<em>3O^+]}{[NH</em>4^+]} = \frac{x^2}{0.50-x} ≈ \frac{x^2}{0.50}
    • x=(5.6×10−10)×(0.50)=1.67×10−5Mx = \sqrt{(5.6 × 10^{-10}) × (0.50)} = 1.67 × 10^{-5}M
    • pH=−log(1.67×10−5)=4.77pH = -log(1.67 × 10^{-5}) = 4.77

Salts from Weak Acids and Weak Bases

  • The resulting solution can be acidic, basic, or neutral.
  • Examples: NH4F, CH3COONH4, NH4CN
    • If K<em>b>K</em>aK<em>b > K</em>a, the solution is basic.
    • If K<em>b<K</em>aK<em>b < K</em>a, the solution is acidic.
    • If K<em>b=K</em>aK<em>b = K</em>a, the solution is neutral.
  • Example: (NH4)2CO3 solution is basic because K<em>a<K</em>bK<em>a < K</em>b