Salt Hydrolysis

1. What is Salt Hydrolysis?

• Salt hydrolysis occurs when a salt reacts with water to produce either hydroxide ions $(OH⁻)$ or hydronium ions $(H₃O⁺).$
• This reaction determines whether the solution is acidic, basic, or neutral.
• A salt is formed when an acid reacts with a base, following the general equation: $Acid + Base → Salt + Water$

2. pH of Salt Solutions

a) Strong Acid + Strong Base → Neutral Salt

• The solution is neutral $(pH = 7).$
• Example: $NaCl$ (formed from $HCl + NaOH$)
  • No hydrolysis occurs since both ions are spectator ions.

b) Strong Acid + Weak Base → Acidic Salt

• The solution is acidic $(pH < 7).$
• The salt hydrolyses to produce H₃O⁺ (hydronium ions).
• Example: $NH₄Cl$ (formed from $HCl + NH₃$)
  • $NH₄⁺$ hydrolyses: $NH₄⁺ + H₂O → NH₃ + H₃O⁺$

c) Weak Acid + Strong Base → Basic Salt

• The solution is basic $(pH > 7).$
• The salt hydrolyses to produce $OH⁻$ (hydroxide ions).
• Example: $Na₂CO₃$ (formed from $H₂CO₃ + NaOH$)
  • $CO₃²⁻$ hydrolyses: $CO₃²⁻ + H₂O → HCO₃⁻ + OH⁻$

d) Weak Acid + Weak Base → Variable pH

• The pH depends on the relative strength of the acid and base.
• Example: $NH₄CH₃COO$ (formed from $NH₃ + CH₃COOH$)
  • Can be acidic, basic, or neutral, depending on ion strength.

3. Steps to Determine pH of a Salt Solution

1. Identify the acid and base that formed the salt.
2. Determine if the acid and base are strong or weak.
3. Use the strength to predict if the solution is acidic, basic, or neutral.

Worked Example:

Determine the pH of $NH₄NO₃$ and $K₂SO₄$ solutions.

Salt	Acid	Base	Nature of Solution	Reason
$NH₄NO₃$	$HNO₃$ (Strong)	$NH₃$ (Weak)	Acidic	$Weak base → Acidic salt$
$K₂SO₄$	$H₂SO₄$ (Strong)	$KOH$ (Strong)	Neutral	$Strong acid + Strong base → Neutral salt$

4. Summary Table

Acid	Base	Salt Nature	pH	Example
Strong	Strong	Neutral	$pH = 7$	$NaCl, K₂SO₄$
Strong	Weak	Acidic	$pH < 7$	$NH₄Cl, AlCl₃$
Weak	Strong	Basic	$pH > 7$	$Na₂CO₃, CH₃COONa$
Weak	Weak	Variable	Depends	$NH₄CH₃COO$