Question

In the following question two statements Assertion (A) and Reason (R) are given Mark. a. If \(\mathrm{A}\) and \(\mathrm{R}\) both are correct and \(\mathrm{R}\) is the correct explanation of \(\mathrm{A}\); b. If \(A\) and \(R\) both are correct but \(R\) is not the correct explanation of \(\mathrm{A}\); c. \(\mathrm{A}\) is true but \(\mathrm{R}\) is false; d. \(\mathrm{A}\) is false but \(\mathrm{R}\) is true, e. \(\mathrm{A}\) and \(\mathrm{R}\) both are false. (A): Hydrolysis of methyl acetate in aqueous solution is a pseudo first order reaction. (R): In this reaction concentration of \(\mathrm{H}_{2} \mathrm{O}\) remains nearly constant during the course of the reaction.

Short answer

a. A and R are both correct, and R is the correct explanation of A.

Step-by-step solution

Understanding Assertion and Reason

The Assertion (A) states that the hydrolysis of methyl acetate in aqueous solution is a pseudo first order reaction. A pseudo first order reaction occurs when one of the reactants is in large excess and its concentration remains nearly constant. The Reason (R) states that water concentration remains nearly constant during the reaction. This description fits the scenario where a reaction can be simplified to first order kinetics.

Evaluating the Truth of Assertion

To verify the truth of Assertion (A), we consider the hydrolysis of methyl acetate, which involves the reaction \[ \text{CH}_3\text{COOCH}_3 + \text{H}_2\text{O} \rightarrow \text{CH}_3\text{COOH} + \text{CH}_3\text{OH} \]. Despite being a bimolecular reaction, if water is present in huge excess, it is approximated as a pseudo first order reaction with respect to methyl acetate.

Evaluating the Truth of Reason

Reason (R) claims that during the course of the reaction, the concentration of water remains nearly constant. This is true if water is present in a large excess compared to methyl acetate.

Connecting Assertion and Reason

Since water is in large excess and its concentration remains nearly constant, the reaction can be simplified to a pseudo first order reaction, making the Reason (R) relevant and an actual explanation of the Assertion (A).

Key concepts

Hydrolysis of Methyl Acetate

Hydrolysis of methyl acetate is a fascinating chemical reaction. Essentially, it's the process where methyl acetate interacts with water to produce acetic acid and methanol. This reaction is often represented by the chemical equation: \[\text{CH}_3\text{COOCH}_3 + \text{H}_2\text{O} \rightarrow \text{CH}_3\text{COOH} + \text{CH}_3\text{OH}\]In this reaction, water is one of the reactants. When it is present in a large amount, we see an interesting dynamic unfold, transforming this seemingly straightforward bimolecular reaction into something known as a pseudo first order reaction.
This highlights a unique characteristic of some reactions - when one reactant is present in large excess (like water in our case), its concentration doesn't significantly change throughout the reaction.
Even though the reaction involves two molecules colliding, because the concentration of water remains persistent, it allows us to treat the reaction as if it depends solely on a single reactant's concentration, thus pseudo-first order. This simplifies the kinetic study and calculations around the reaction, making it easier to focus on the reaction mechanics of methyl acetate only.

Reaction Kinetics

Reaction kinetics is the study of the speed or rate at which chemical reactions occur. Kinetics helps us to understand how different conditions such as concentration, temperature, and presence of catalysts can speed up or slow down a reaction.
In the case of the hydrolysis of methyl acetate, we classify it, under special conditions, as a pseudo first order reaction despite originally seeming bimolecular.

**Understanding Pseudo First Order Reactions**
Pseudo first order reactions are a special class of reactions. To better understand this, let's break it down:

• Ordinarily, a bimolecular reaction would follow second order kinetics, with rate dependent on the concentration of both reactants.
• However, if one reactant is in excess, as is the scenario with water in hydrolysis of methyl acetate, the reaction rate can be simplified to depend primarily on the concentration of the other reactant.
• This approximation simplifies kinetic measurements and is particularly helpful in evaluating reactions in dilute solutions.

It is through reaction kinetics studies that these simplifications aid chemists in predicting how the reaction will proceed under various circumstances.

Assertion and Reasoning Questions

Assertion and reasoning questions are designed to test a student's understanding of core concepts and the relationships between them. In these types of questions, you are typically given two statements: an assertion, and a reason.
The task is to determine whether the statements are true and if the reason correctly explains the assertion.

**Approaching Assertion-Reason Questions**
In the exercise related to the hydrolysis of methyl acetate:

• The assertion was that hydrolysis of methyl acetate in aqueous solution is a pseudo first order reaction.
• The reason given was that water's concentration remains constant during the reaction.
• To solve such questions, consider each statement separately and together. Both assertions need to be evaluated as true or false.
• If both are true, analyze whether the reason appropriately explains the assertion.

In this specific example, the assertion and reason were both correct, with the reason providing a sufficient explanation of why the hydrolysis could be modeled as a pseudo first order reaction. This kind of nuanced understanding is fundamental in analytical chemistry.