Salt Hydrolysis & pH | Ionic Equilibrium Class 11

Salt Hydrolysis & pH

Ionic Equilibrium | Interaction of Ions with Water

1. What is Salt Hydrolysis?

                    Definition: Salt Hydrolysis is the reaction of the cation or anion (or both) of a salt with water to produce acidity or alkalinity. It is the reverse process of neutralization.
                

$$ \text{Salt} + \text{Water} \rightleftharpoons \text{Acid} + \text{Base} $$

2. Types of Salts and pH

A. Strong Acid + Strong Base (SA-SB)

Example: $NaCl, KNO_3, Na_2SO_4$.

Behavior: Neither the cation ($Na^+$) nor the anion ($Cl^-$) reacts with water. No hydrolysis occurs.

Result: Solution is Neutral ($pH = 7$ at 25°C).

B. Weak Acid + Strong Base (WA-SB)

Example: $CH_3COONa$ (Sodium Acetate).

Mechanism: The anion ($CH_3COO^-$) reacts with water (Anionic Hydrolysis).
$$ CH_3COO^- + H_2O \rightleftharpoons CH_3COOH + OH^- $$

Result: Solution is Basic/Alkaline ($pH > 7$).

$$ K_h = \frac{K_w}{K_a} \quad ; \quad h = \sqrt{\frac{K_h}{C}} $$ $$ pH = 7 + \frac{1}{2} (pK_a + \log C) $$

C. Strong Acid + Weak Base (SA-WB)

Example: $NH_4Cl$ (Ammonium Chloride), $CuSO_4$.

Mechanism: The cation ($NH_4^+$) reacts with water (Cationic Hydrolysis).
$$ NH_4^+ + H_2O \rightleftharpoons NH_4OH + H^+ $$

Result: Solution is Acidic ($pH < 7$).

$$ K_h = \frac{K_w}{K_b} \quad ; \quad h = \sqrt{\frac{K_h}{C}} $$ $$ pH = 7 - \frac{1}{2} (pK_b + \log C) $$

D. Weak Acid + Weak Base (WA-WB)

Example: $CH_3COONH_4$.

Mechanism: Both cation and anion hydrolyze.

                    Result: pH is independent of concentration but depends on relative strength of acid and base.
                

$$ pH = 7 + \frac{1}{2} (pK_a - pK_b) $$

3. Quick Summary Table

Salt Type	Hydrolysis Type	Nature	pH Range
SA-SB	None	Neutral	$= 7$
WA-SB	Anionic	Basic	$> 7$
SA-WB	Cationic	Acidic	$< 7$
WA-WB	Both	Depends	$\approx 7$

Practice Quiz

Test your ability to predict salt pH.

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