Understanding Salt Hydrolysis
When salts dissolve in water, they can produce acidic or alkaline solutions even though the salt itself is neutral. This phenomenon is called salt hydrolysis. Let’s explore how and why this happens.
What is Salt Hydrolysis?
Definition: Salt hydrolysis is the reaction of ions from a salt with water molecules to form the undissociated weak acid or weak base.
General Principle: The salt of a weak acid and strong base produces an alkaline solution, while the salt of a strong acid and weak base produces an acidic solution.
Types of Salt Hydrolysis
1. Hydrolysis of Salt of Weak Acid and Strong Base
Example: Sodium Acetate (CH₃COONa)
Dissociation in water:
CH₃COONa(aq) → CH₃COO⁻(aq) + Na⁺(aq)
Hydrolysis reaction:
CH₃COO⁻(aq) + H₂O(l) ⇌ CH₃COOH(aq) + OH⁻(aq)
Explanation: The acetate ion acts as a Bronsted-Lowry base, accepting a proton from water. This produces OH⁻ ions, making the solution alkaline (pH > 7).
Why this happens: The acetate ion is a stronger base than water is an acid, so the hydrolysis reaction is favored.
The equilibrium constant for hydrolysis (Kh):
Kh = [CH₃COOH][OH⁻] / [CH₃COO⁻] Since Kw = Ka × Kb and Kb = Kw / Ka: Kh = Kw / Ka Or more generally: Kh = Kw / Ka (for salt of weak acid)
Worked Example: A solution of 0.1M sodium acetate is prepared. Calculate the pH if Ka for acetic acid = 1.8 × 10⁻⁵
Kh = Kw / Ka = 1×10⁻¹⁴ / 1.8×10⁻⁵ = 5.6 × 10⁻¹⁰ Since acetate hydrolysis: [OH⁻] = √(Kh × [salt]) [OH⁻] = √(5.6 × 10⁻¹⁰ × 0.1) = 7.5 × 10⁻⁶ mol/dm³ pOH = -log(7.5 × 10⁻⁶) = 5.1 pH = 14 - 5.1 = 8.9
2. Hydrolysis of Salt of Strong Acid and Weak Base
Example: Ammonium Chloride (NH₄Cl)
Dissociation in water:
NH₄Cl(aq) → NH₄⁺(aq) + Cl⁻(aq)
Hydrolysis reaction:
NH₄⁺(aq) + H₂O(l) ⇌ NH₃(aq) + H₃O⁺(aq)
Explanation: The ammonium ion acts as a weak acid, donating a proton to water. This produces H₃O⁺ ions, making the solution acidic (pH < 7).
Why this happens: The ammonium ion is a stronger acid than water, so the hydrolysis reaction is favored.
The equilibrium constant for hydrolysis:
Kh = Kw / Kb (for salt of weak base)
Worked Example: A solution of 0.1M ammonium chloride is prepared. Calculate the pH if Kb for ammonia = 1.8 × 10⁻⁵
Ka = Kw / Kb = 1×10⁻¹⁴ / 1.8×10⁻⁵ = 5.6 × 10⁻¹⁰ For NH₄⁺ hydrolysis: [H⁺] = √(Ka × [salt]) [H⁺] = √(5.6 × 10⁻¹⁰ × 0.1) = 7.5 × 10⁻⁶ mol/dm³ pH = -log(7.5 × 10⁻⁶) = 5.1
3. Hydrolysis of Salt of Weak Acid and Weak Base
Example: Ammonium Acetate (CH₃COONH₄)
This salt can undergo both acidic and basic hydrolysis:
CH₃COO⁻(aq) + H₂O(l) ⇌ CH₃COOH(aq) + OH⁻(aq) [Kb] NH₄⁺(aq) + H₂O(l) ⇌ NH₃(aq) + H₃O⁺(aq) [Ka]
The pH depends on which hydrolysis is stronger:
- If Ka(NH₄⁺) > Kb(CH₃COO⁻), then solution is acidic
- If Ka(NH₄⁺) < Kb(CH₃COO⁻), then solution is alkaline
- If Ka(NH₄⁺) = Kb(CH₃COO⁻), then solution is neutral
4. Hydrolysis of Salt of Strong Acid and Strong Base
Example: Sodium Chloride (NaCl)
No hydrolysis occurs because:
- Na⁺ is a very weak conjugate acid
- Cl⁻ is a very weak conjugate base
Therefore, NaCl solution is neutral with pH ≈ 7.
Summary Table: Salt Hydrolysis Types
| Salt Type | Example | Hydrolysis Reaction | Solution Type | pH |
|---|---|---|---|---|
| Weak Acid + Strong Base | NaCH₃COO | A⁻ + H₂O ⇌ HA + OH⁻ | Alkaline | > 7 |
| Strong Acid + Weak Base | NH₄Cl | B⁺ + H₂O ⇌ B + H₃O⁺ | Acidic | < 7 |
| Weak Acid + Weak Base | NH₄CH₃COO | Both reactions possible | Depends on Ka and Kb | Variable |
| Strong Acid + Strong Base | NaCl | No hydrolysis | Neutral | = 7 |
Factors Affecting Hydrolysis
- Strength of Acid or Base: Weaker the conjugate acid/base, stronger the hydrolysis
- Concentration: Higher concentration favors more hydrolysis
- Temperature: Higher temperature favors hydrolysis (endothermic process)
- Dilution: Dilution favors hydrolysis (increases degree of hydrolysis)
Practical Applications
- Water Treatment: Salts are used to adjust pH of water
- Pharmaceutical Industry: Salt selection important for drug formulation
- Agriculture: Soil pH adjustment using salts
- Industrial Processes: Control of solution pH in various processes
Key Points to Remember
- Salts of weak acids and strong bases are alkaline
- Salts of strong acids and weak bases are acidic
- Salts of weak acids and weak bases may be acidic or alkaline
- Salts of strong acids and strong bases are neutral
- Hydrolysis involves reaction of salt ions with water to form weak acid or base
