Salt Hydrolysis: Comprehensive NEET Chemistry Notes

1. Introduction to Salt Hydrolysis

Salt hydrolysis refers to the interaction between water and the ions of a dissolved salt. This process plays a critical role in determining the pH of solutions formed by salts. A salt, when dissolved in water, dissociates into its constituent ions. Depending on whether the salt originates from a strong or weak acid and base, its ions may undergo hydrolysis to produce either an acidic or basic solution.

The four main categories of salts based on the strength of the acids and bases involved are:

Salts of strong acids and strong bases: Neutral solution (pH = 7)
Salts of strong acids and weak bases: Acidic solution (pH < 7)
Salts of weak acids and strong bases: Basic solution (pH > 7)
Salts of weak acids and weak bases: pH depends on the relative strength of the acid and base.

2. Hydrolysis of Salts

2.1 Salts of Strong Acids and Strong Bases

Salts formed from strong acids and strong bases, like sodium chloride (NaCl), do not undergo hydrolysis. These salts dissociate completely in water, and neither the cations nor the anions react with water. Hence, the solution remains neutral, with a pH of 7.

Example: $N a Cl (a q) \to N a^{+} (a q) + C l^{-} (a q)$

Real-life Application:

Table salt (sodium chloride) is used in large quantities in cooking and food preservation. Its neutral nature makes it suitable for consumption without altering the pH of food drastically.

2.2 Salts of Weak Acids and Strong Bases

Salts like sodium acetate ( $C H_{3} COON a$ ), formed from a weak acid and a strong base, undergo hydrolysis. The anion of the weak acid reacts with water to produce a weak acid and hydroxide ions ( $O H^{-}$ ), resulting in a basic solution.

Hydrolysis reaction: $C H_{3} CO O^{-} (a q) + H_{2} O (l) ⇌ C H_{3} COO H (a q) + O H^{-} (a q)$

The solution becomes basic (pH > 7) due to the production of hydroxide ions.

Mnemonic:

"Strong Base, Weak Acid = Basic Solution"

NEET Tip:

Remember that in salts formed from weak acids and strong bases, the anion of the salt reacts with water, making the solution basic. Expect basic solutions for salts like sodium carbonate ( $N a_{2} C O_{3}$ ) and potassium acetate ( $C H_{3} COO K$ ).

2.3 Salts of Strong Acids and Weak Bases

Salts such as ammonium chloride ( $N H_{4} Cl$ ), formed from a strong acid and a weak base, undergo hydrolysis. The cation ( $N H_{4}^{+}$ ) reacts with water to form the weak base ( $N H_{4} O H$ ) and $H^{+}$ ions, making the solution acidic.

Hydrolysis reaction: $N H_{4}^{+} (a q) + H_{2} O (l) ⇌ N H_{4} O H (a q) + H^{+} (a q)$

The resulting increase in $H^{+}$ ion concentration makes the solution acidic (pH < 7).

Common Misconception:

Students often confuse the hydrolysis of salts with their simple dissolution. Hydrolysis involves a reaction between the ion and water, which can alter the pH of the solution.

2.4 Salts of Weak Acids and Weak Bases

When both the acid and base are weak, such as ammonium acetate ( $C H_{3} COON H_{4}$ ), both the cation and anion undergo hydrolysis. The pH of the solution depends on the relative strengths (or dissociation constants) of the acid and base. The pH can be calculated using:

$p H = 7 + \frac{1}{2} (p K_{a} - p K_{b})$

If $p K_{a} > p K_{b}$ , the solution will be basic; if $p K_{a} < p K_{b}$ , the solution will be acidic.

Example: $C H_{3} CO O^{-} + N H_{4}^{+} + H_{2} O ⇌ C H_{3} COO H + N H_{4} O H$

3. Factors Affecting Salt Hydrolysis

Nature of the parent acid and base: The hydrolysis of salts depends largely on whether the parent acid and base are strong or weak.
Concentration of the salt: Although the hydrolysis constant ( $K_{h}$ ) is independent of concentration, dilution may cause a slight change in the pH.
Temperature: Increasing temperature can increase the degree of hydrolysis in some cases.

NEET Problem-Solving Strategy:

For salts formed from weak acids and weak bases, always check the dissociation constants ( $K_{a}$ and $K_{b}$ ) to determine whether the solution will be acidic or basic.

Quick Recap:

Salt hydrolysis: Interaction between salt ions and water that affects pH.
Strong acid + strong base: No hydrolysis; neutral pH.
Weak acid + strong base: Hydrolysis produces OH⁻; basic solution.
Strong acid + weak base: Hydrolysis produces H⁺; acidic solution.
Weak acid + weak base: pH depends on the relative strengths of acid and base.

NEET Exam Strategy:

Identify the parent acid and base from which the salt is formed to predict the pH. Time management is crucial—analyze the type of salt first, then determine whether it will undergo hydrolysis. Use the hydrolysis reactions and constants ( $K_{a}, K_{b}$ ) for calculations. For quick decision-making, remember:

Strong acids and bases: Neutral
Weak acids or bases: Hydrolyze and change the pH

Practice Questions

Which of the following salts forms a basic solution upon hydrolysis? a) $N a Cl$
b) $N H_{4} Cl$
c) $C H_{3} COON a$
d) $K Cl$
Answer: c) $C H_{3} COON a$
A solution of ammonium chloride ( $N H_{4} Cl$ ) is: a) Acidic
b) Basic
c) Neutral
d) None of the above
Answer: a) Acidic
Calculate the pH of a solution of ammonium acetate ( $C H_{3} COON H_{4}$ ) if the dissociation constants are $p K_{a} = 4.8$ and $p K_{b} = 4.75$ .
Solution:
Using the formula:
$p H = 7 + \frac{1}{2} (p K_{a} - p K_{b})$
$p H = 7$