Problem 22 - A Complete Resource Book in Chemistry for JEE Main

Question

In the following questions two statements (Assertion) (A) and Reason (R) are given. Mark (a) If both \(\mathrm{A}\) and \(\mathrm{R}\) are correct and \(\mathrm{R}\) is the correct explanation of \(\mathrm{A}\). (b) If both \(\mathrm{A}\) and \(\mathrm{R}\) are correct but \(\mathrm{R}\) is not the correct expalnation of \(\mathrm{A}\). (c) \(\mathrm{A}\) is true but \(\mathrm{R}\) is false. (d) A is false but \(R\) is true. (e) \(\mathrm{A}\) and \(\mathrm{R}\) both are false. Assertion: When aqueous solution of \(\mathrm{CH}_{3} \mathrm{COONH}_{4}\) is diluted, then its degree of hydrolysis increases. Reason: Ammonium acetate is the salt of weak acid and weak base, its degree of hydrolysis does not depend on the concentration.

Step-by-Step Solution

Answer

Option (b): Both A and R are correct, but R is not the correct explanation of A.

Step 1: Understanding the Assertion

The assertion states that when the aqueous solution of \( \mathrm{CH}_{3} \mathrm{COONH}_{4} \) is diluted, its degree of hydrolysis increases. This is because dilution generally increases the degree of hydrolysis in salts derived from weak acids and weak bases, as there is more water to interact with the ions.

Step 2: Understanding the Reason

The reason claims that ammonium acetate is a salt of a weak acid and a weak base, and that its degree of hydrolysis does not depend on the concentration. However, this is incorrect; the degree of hydrolysis for such salts does indeed depend on concentration, and it typically increases with dilution.

Step 3: Evaluating the Statements

Both the assertion and reason are individually correct: the degree of hydrolysis of a salt like ammonium acetate increases with dilution, and ammonium acetate is indeed a salt of a weak acid and weak base. However, the reason incorrectly states that the degree of hydrolysis does not depend on concentration, which contradicts the assertion.

Step 4: Determining the Correct Option

Given that both assertion and reason are correct statements, but the reason does not accurately explain the assertion, the correct option is (b): both \( \mathrm{A} \) and \( \mathrm{R} \) are correct, but \( \mathrm{R} \) is not the correct explanation of \( \mathrm{A} \).

Key Concepts

Understanding the Degree of Hydrolysis

The degree of hydrolysis refers to the extent to which a salt reacts with water to form the parent acid and base. It is a vital concept when discussing salts formed from weak acids and bases. When a salt like ammonium acetate ( \( \text{CH}_3\text{COONH}_4 \)) dissolves in water, it partially hydrolyzes due to the weak nature of both its constituent acid (acetic acid) and base (ammonium hydroxide). Hydrolysis is affected by the ionicity of the salt; as a result, not all the salt molecules dissociate completely in water, leading to an equilibrium between the ions and the unreacted molecules. Thus, the degree of hydrolysis directly influences the pH and the behavior of the solution.

• The weaker the acid and base, the higher the degree of hydrolysis.
• More extensive dilution typically increases the degree of hydrolysis by providing more water to interact with the salt ions.

Understanding Ammonium Acetate

Ammonium acetate ( \( \text{CH}_3\text{COONH}_4 \)) is an interesting compound because it is a salt derived from acetic acid (a weak acid) and ammonium hydroxide (a weak base). Being a salt of a weak acid and weak base, ammonium acetate does not completely dissociate into its ions in water. Instead, it reaches a dynamic equilibrium, where some of it remains as unreacted molecules. This partial dissociation is a key factor affecting many properties of its solution, including the degree of hydrolysis.

• The interaction between the ions can lead to a neutral or very slightly acidic solution, depending on the relative strengths of the acid and base.
• Its behavior is quite different from salts of strong acids and bases, which fully dissociate in water.

Exploring the Dilution Effect

The dilution effect in the context of salts pertains to how diluting the solution impacts equilibrium reactions, such as hydrolysis. For ammonium acetate, diluting the solution provides more water molecules, which means more opportunities for the ammonium and acetate ions to interact with water. As a result, more extensive hydrolysis can occur. This increased interaction leads to a rise in the degree of hydrolysis, which is why dilution often results in observable changes in the solution properties, like pH.

• Dilution of salts of weak acids and bases increases the extent of hydrolysis.
• More water reduces the ionic strength of the solution, which in turn enhances the hydrolysis process.

Concept of Weak Acid and Weak Base

The interaction of weak acids and weak bases forms the core of understanding hydrolysis in salts like ammonium acetate. A weak acid only partially ionizes in water, meaning not all the molecules donate protons to water. Similarly, a weak base does not fully accept protons from water. This partial reaction sets up a dynamic equilibrium resulting in only weakly ionic solutions.

Equilibrium and Hydrolysis

The hydrolysis reaction for ammonium acetate can be represented as: \[ \text{CH}_3\text{COONH}_4(aq) + \text{H}_2\text{O}(l) \rightleftharpoons \text{CH}_3\text{COOH}(aq) + \text{NH}_4\text{OH}(aq) \]

• Weak acids and bases undergo this reversible reaction, resulting in the characteristic degree of hydrolysis.
• The weak nature implies higher susceptibility to changes in conditions, like concentration changes through dilution.

Understanding Concentration Dependence

Concentration dependence refers to how the concentration of a solution affects the degree to which reactions, like hydrolysis, occur. For a salt of a weak acid and weak base such as ammonium acetate, the degree of hydrolysis is not independent of concentration. In fact, as the concentration of the salt in the solution decreases, typically through dilution, the degree of hydrolysis increases. This increase happens because the dilution gives ions more opportunity to interact with more water molecules, facilitating more extensive hydrolysis.

• Concentration changes can shift equilibria in solutions involving hydrolysis.
• This phenomenon plays a crucial role in determining the properties of weak acid and weak base solutions.