Salt Hydrolysis

Carson West

Acids and Bases

Salt Hydrolysis

Salt hydrolysis is the reaction of a salt with water to produce an acidic or basic solution. The pH of the resulting solution depends on the nature of the cation and anion of the salt.

Types of Salt Hydrolysis:

  1. Salt of a strong acid and a strong base: These salts do not undergo hydrolysis. The resulting solution is neutral ( $ pH = 7 $ ). Example: NaCl (sodium chloride)

  2. Salt of a strong acid and a weak base: The anion does not react with water, but the cation will react with water to produce hydronium ions ( $ H_3O^+ $ ), resulting in an acidic solution ( $ pH < 7 $ ).

    Example: $ NH_4Cl $ (ammonium chloride)

    $$ NH_4^+ + H_2O \rightleftharpoons NH_3 + H_3O^+ $$ Weak Acid/Base Equilibria (This will cover $ K_a $ , $ K_b $ , and ICE tables)

  3. Salt of a weak acid and a strong base: The cation does not react with water, but the anion will react with water to produce hydroxide ions ( $ OH^- $ ), resulting in a basic solution ( $ pH > 7 $ ).

    Example: $ NaCH_3COO $ (sodium acetate)

    $$ CH_3COO^- + H_2O \rightleftharpoons CH_3COOH + OH^- $$ Weak Acid/Base Equilibria

  4. Salt of a weak acid and a weak base: Both the cation and anion will react with water. The pH of the resulting solution depends on the relative strengths of the conjugate acid and conjugate base. This requires a more detailed analysis and often requires considering the $ K_a $ and $ K_b $ values.

    Example: $ NH_4CH_3COO $ (ammonium acetate) This requires comparing $ K_a $ of $ NH_4^+ $ and $ K_b $ of $ CH_3COO^- $ .

Calculations:

Calculating the pH of solutions resulting from salt hydrolysis often involves using ICE tables and equilibrium expressions ( $ K_a $ , $ K_b $ , $ K_w $ ). The specific method depends on the type of salt.

ICE Tables and Equilibrium Calculations]]

**Relationship to $ K_a $ and $ K_b $ **:

The extent of hydrolysis is related to the strength of the weak acid or weak base involved. A weaker acid or base will result in a less extensive hydrolysis. The relationship can be expressed through the hydrolysis constant, $ K_h $ .

$ K_h $ for a salt of a weak acid and strong base is related to $ K_a $ and $ K_w $ by: $ K_h = \frac{K_w}{K_a} $

Similarly, $ K_h $ for a salt of a weak base and strong acid is related to $ K_b $ and $ K_w $ by: $ K_h = \frac{K_w}{K_b} $

Acid-Base Constants and Equilibrium