Problem 222 - A Complete Resource Book in Chemistry for JEE Main

Question

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

The assertion is correct, but the reason is incorrect.

Step 1: Understanding the Problem

We need to analyze the behavior of ammonium acetate (\( \text{CH}_3\text{COONH}_4 \)) when it is diluted. We also need to understand the effect of dilution on the degree of hydrolysis and whether the reason statement is accurate based on chemical principles.

Step 2: Identifying Chemical Components

Ammonium acetate (\( \text{CH}_3\text{COONH}_4 \)) is a salt formed from a weak acid (acetic acid) and a weak base (ammonia). The salt undergoes hydrolysis in water to give acetic acid and ammonium hydroxide.

Step 3: Predicting the Effect of Dilution on Hydrolysis

The degree of hydrolysis for salts of weak acids and weak bases is known to increase with dilution. This is because hydrolysis involves a reversible reaction, and Le Chatelier's principle suggests that diluting the solution shifts the equilibrium towards the product side (hydrolysis).

Step 4: Evaluating the Reason Statement

The reason claims that the degree of hydrolysis for this salt is independent of concentration. This claim is incorrect for salts like ammonium acetate where the components are both weak, as shown previously, dilution shifts the equilibrium, increasing the degree of hydrolysis.

Step 5: Decision Conclusion

The assertion is correct because the degree of hydrolysis does increase with dilution. The reason is incorrect because the degree of hydrolysis for salts of weak acids and bases does depend on concentration due to equilibrium shifts upon dilution.

Key Concepts

Degree of Hydrolysis

The degree of hydrolysis is a critical concept in understanding how salts behave in aqueous solutions. It represents the fraction of the salt that undergoes hydrolysis. Hydrolysis is the reaction of the salt with water, leading to the formation of an acid and a base. For salts formed from weak acids and weak bases, such as ammonium acetate (\(\text{CH}_3\text{COONH}_4\)), the degree of hydrolysis is particularly significant.

In weak acid-weak base salts, neither ion is completely dissociated in water. This makes the extent of hydrolysis much more prominent compared to salts from strong acids or bases. The reaction can be viewed as reaching a state of equilibrium between the reactants and the products of hydrolysis. Here's where things get interesting: as the solution is diluted, more water is available to drive the hydrolysis forward. This increased availability shifts the equilibrium, increasing the amount of the acid and base produced, which results in a higher degree of hydrolysis.

Understanding the degree of hydrolysis helps predict the behavior of solutions during dilution, guiding us on how the ionic constituents interact with water and affect the pH.

Le Chatelier's principle

Le Chatelier's principle is a fundamental chemical concept used to predict the effect of changes in concentration, temperature, or pressure on a chemical equilibrium. Simply put, this principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change, re-establishing equilibrium.

When applied to the hydrolysis of ammonium acetate, Le Chatelier's principle provides valuable insight. In the reaction \(\text{NH}_4^+ + \text{CH}_3\text{COO}^- + \text{H}_2\text{O} \rightleftharpoons \text{NH}_3 + \text{CH}_3\text{COOH}\) , dilution involves adding more water, practically increasing the solvent quantity. According to Le Chatelier, the system will shift to oppose the addition of water by favoring the forward reaction (more hydrolysis), thus increasing the formation of ammonia and acetic acid.

This principle is why the degree of hydrolysis increases with dilution: the equilibrium moves to produce more products in response to the increased solvent. It emphasizes the interconnectedness of reaction conditions and equilibrium shifts.

Salts of Weak Acids and Bases

Salts of weak acids and bases form an intriguing category in chemistry, displaying unique behaviors in solution due to the weak nature of their constituent acid and base. Ammonium acetate (\(\text{CH}_3\text{COONH}_4\)) is a prime example, being composed of ammonium (from ammonia) and acetate (from acetic acid).

These salts are not fully dissociated in solution, unlike strong acid or base counterparts. This means that when dissolved in water, they participate in significant hydrolysis, interacting with the water molecules to form the weak acid and weak base from which they originated. The partial ionization leads to an equilibrium state that can significantly change upon conditions like dilution.

The fascinating aspect of these salts is how sensitive they are to changes in concentration. With dilution, as explained by Le Chatelier's principle, the increased amount of water shifts the equilibrium towards more hydrolysis. This is unlike the behavior seen in salts of strong acids and bases, where hydrolysis is typically negligible.