Hydrolysis of Salts

What Is Hydrolysis

Hydrolysis: the interaction of salt ions with water to form a weak electrolyte. The medium depends on which base and which acid the salt was formed from.

SB is a strong base, WB is a weak base, SA is a strong acid, WA is a weak acid.

Salt type	Example	Medium
SB + SA	NaCl	pH = 7
SB + WA	Na₂CO₃	pH > 7
WB + SA	CuSO₄	pH < 7
WB + WA	Al₂S₃	complete hydrolysis

Types of Salts

• SB + SA: does not hydrolyze, pH = 7 (NaCl, K₂SO₄).
• SB + WA: hydrolysis by the anion, alkaline (Na₂CO₃, CH₃COOK).
• WB + SA: hydrolysis by the cation, acidic (CuSO₄, NH₄Cl, AlCl₃).
• WB + WA: Al₂S₃, Cr₂S₃: complete, irreversible hydrolysis. Al₂S₃ and Cr₂S₃ cannot exist in aqueous solution.

Equations

• Na₂CO₃: CO₃²⁻ + H₂O ⇌ HCO₃⁻ + OH⁻ (alkaline).
• NH₄Cl: NH₄⁺ + H₂O ⇌ NH₃·H₂O + H⁺ (acidic).
• Al₂S₃ + 6H₂O → 2Al(OH)₃↓ + 3H₂S↑ (irreversible).

Additional Notes

• Ions with multiple charges hydrolyze stepwise (mainly the 1st step).
• To enhance hydrolysis: dilute, heat.
• Hydrolysis by the cation is suppressed by an acid, and hydrolysis by the anion is suppressed by an alkali.
• Litmus: blue in an alkaline solution, red in an acidic solution.