Hydrolysis

Hydrolysis is a chemical process in which a compound reacts with water and breaks down into simpler substances. The word hydrolysis is derived from two words: “hydro,” meaning water, and “lysis,” meaning breaking. In this process, water molecules take part in the reaction and split into H⁺ and OH⁻ ions. These ions then react with the substance and cause the breaking of chemical bonds.

Hydrolysis of Salt

When a salt is dissolved in water, it dissociates into its constituent ions. These ions may react with water to form the parent acid or base again. This reaction of the ions of a salt with water is called the hydrolysis of salts. Hydrolysis mainly occurs in salts that are formed from weak acids and/or weak bases, because their ions have a tendency to react with water. Hydrolysis occurs when ions of a weak acid or weak base react with water.

Example:

Ammonium chloride (NH₄Cl) in water:

NH₄Cl → NH₄⁺ + Cl ^-

NH₄ ⁺ + H₂O ⇌ NH₃ + H₃O ⁺

Here, NH₄⁺ reacts with water, so the solution becomes acidic.

Types of Hydrolysis of Salts

The hydrolysis of salts depends on the strength of the acid and base from which the salt is formed. There are four main types:

1. Salt of Strong Acid and Strong Base

• Formed from strong acid (HCl) and strong base (NaOH)
• Neither cation nor anion undergoes hydrolysis
• No hydrolysis occurs
• Solution is neutral (pH = 7)

Example: NaCl

2. Salt of Strong Acid and Weak Base

• Formed from strong acid (HCl) and weak base (NH₃)
• Cation undergoes hydrolysis

Example: NH₄Cl

Reaction:

NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺

• Produces H₃O⁺ ions
• Solution becomes acidic (pH < 7)

3. Salt of Weak Acid and Strong Base

• Formed from weak acid (CH₃COOH) and strong base (NaOH)
• Anion undergoes hydrolysis

Example: CH₃COONa

Reaction:

CH₃COO ^- + H₂O ⇌ CH₃COOH + OH ^-

• Produces OH⁻ ions
• Solution becomes basic (pH > 7)

4. Salt of Weak Acid and Weak Base

• Formed from weak acid and weak base
• Both cation and anion undergo hydrolysis

Example: NH₄CH₃COO

Reaction:

NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺

CH₃COO ^- + H₂O ⇌ CH₃COOH + OH^-

• If Ka = Kb → neutral
• If Ka > Kb → acidic
• If Kb > Ka → basic

Hydrolysis Constant

Hydrolysis constant is the equilibrium constant for the hydrolysis reaction of a salt in water. It gives the extent to which a salt undergoes hydrolysis. Since hydrolysis is an equilibrium process, it can be represented by an equilibrium constant called K_h.

When a salt reacts with water, an equilibrium is established between reactants and products. The ratio of concentrations of products to reactants at equilibrium is constant, and this is called the hydrolysis constant.

Salt + H₂O ⇌ Acid + Base

K_h = \frac{[\text{Products}]}{[\text{Reactants}]}

Degree of hydrolysis

Degree of hydrolysis is defined as the fraction of the total amount of a salt that undergoes hydrolysis in water. It tells us how much of the salt is actually hydrolysed at equilibrium.

h = \frac{\text{Amount of salt hydrolysed}}{\text{Total amount of salt taken}}

• When a salt is dissolved in water, only a part of it reacts with water.
• The remaining part stays unchanged.
• The ratio of hydrolysed part to total salt is called degree of hydrolysis (h).

Factors affecting Hydrolysis

The extent of hydrolysis of a salt depends on several factors:

• Nature of Salt: Hydrolysis,esence of a common ion suppresses hydrolysis. It shifts the equilibrium in the backward direction. As a result, hydrolysis decreases.
• Temperature: Hydrolysis generally increases with increase in temperature.
• Dilution: Greater dilution increases the extent of hydrolysis.
• Presence of a Common Ion: Presence of a common ion suppresses hydrolysis. It shifts the equilibrium in the backward direction. As a result, hydrolysis decreases.

Related Articles:

• Preparation of Carboxylic Acid
• Corrosion and Rancidity