Which of the following are the examples of pseudo unimolecular reactions?

(A) Acid catalyzed hydrolysis of an ester

(B) Inversion of cane sugar

(C) Decomposition of ozone

(D) Decomposition of \(N_2O_5\)

Select the correct answers using the codes given below

A and B

The correct answer is option 1. A and B.

Let us explore the concept of pseudo unimolecular reactions and why (A) and (B) fit this category, while (C) and (D) do not.

A pseudo unimolecular reaction is a reaction that involves two reactants but behaves like a first-order reaction because one of the reactants is present in such large excess that its concentration effectively remains constant throughout the reaction. As a result, the reaction rate appears to depend only on the concentration of the other reactant.

(A) Acid Catalyzed Hydrolysis of an Ester

\(\text{Ester} + \text{H}_2\text{O} \rightarrow \text{Acid} + \text{Alcohol}\)

In this reaction, ester reacts with water to produce an acid and an alcohol. The reaction is typically catalyzed by an acid. Water is usually present in a large excess, so its concentration does not change significantly during the reaction. Therefore, the reaction rate depends primarily on the concentration of the ester. This reaction is pseudo unimolecular.

(B) Inversion of Cane Sugar

\( \text{Sucrose} + \text{H}_2\text{O} \rightarrow \text{Glucose} + \text{Fructose} \)

In this reaction, sucrose (cane sugar) is hydrolyzed into glucose and fructose. The reaction often takes place in the presence of an acid catalyst. Water is present in large excess, so its concentration remains constant throughout the reaction. Hence, the reaction rate depends mainly on the concentration of sucrose. This reaction is pseudo unimolecular.

(C) Decomposition of Ozone

\(2\text{O}_3 \rightarrow 3\text{O}_2 \)

This is a decomposition reaction where ozone breaks down into oxygen. It does not involve a second reactant in large excess and follows a different reaction order. This reaction is not pseudo unimolecular.

(D) Decomposition of \(N_2O_5\)

\(2\text{N}_2\text{O}_5 \rightarrow 4\text{NO}_2 + \text{O}_2 \)

This reaction involves the decomposition of dinitrogen pentoxide into nitrogen dioxide and oxygen. It is inherently a unimolecular reaction because it involves the breakdown of a single reactant molecule. Although it can be treated as a first-order reaction in terms of its kinetics, it does not fit the definition of a pseudo unimolecular reaction because there is no second reactant present in excess. This reaction is not pseudo unimolecular.

From the above analysis, it is clear that (A) Acid catalyzed hydrolysis of an ester and (B) Inversion of cane sugar are examples of pseudo unimolecular reactions because they involve two reactants but behave like first-order reactions due to one reactant being in large excess. The other reactions (C) and (D) do not fit this category.

Therefore, the correct answer is 1. A and B.