Question

Which of the following are the examples of pseudo-unimolecular reactions? (1) acid catalyzed hydrolysis of an ester (2) inversion of cane sugar (3) decomposition of ozone (4) decomposition of \(\mathrm{N}_{2} \mathrm{O}_{5}\)

Short answer

Options (1) and (2) are pseudo-unimolecular reactions.

Step-by-step solution

Define a Pseudo-Unimolecular Reaction

A pseudo-unimolecular reaction appears to be first-order because it involves two reactants where one of the reactants is present in a large excess compared to the other. This means the concentration of the excess reactant remains relatively constant during the reaction, making it seem like a one-reactant reaction.

Evaluate Option (1) - Acid Catalyzed Hydrolysis of an Ester

This reaction typically involves water and an ester. Water is often used in large excess, so its concentration change is negligible. Therefore, it can be treated as a pseudo-unimolecular reaction because it depends primarily on the ester concentration.

Evaluate Option (2) - Inversion of Cane Sugar

The reaction involves the concentration of water in large excess, making it appear to be first-order relative to only cane sugar. Hence, this is also a pseudo-unimolecular reaction.

Evaluate Option (3) - Decomposition of Ozone

This reaction is not an example of a pseudo-unimolecular reaction because it typically involves a single reactant breaking down without the presence of a large excess of another substance.

Evaluate Option (4) - Decomposition of \( N_2O_5 \)

Similar to the ozone decomposition, this reaction does not involve another reactant in large excess. It is a simple decomposition reaction, not pseudo-unimolecular.

Summary of Evaluations

Based on our evaluations of each option, the pseudo-unimolecular reactions are the acid catalyzed hydrolysis of an ester and the inversion of cane sugar.

Key concepts

Acid Catalyzed Hydrolysis of an Ester

The acid-catalyzed hydrolysis of an ester is a classic example of a reaction that can often be simplified to a pseudo-unimolecular reaction. In this process, an ester reacts with water in the presence of an acid catalyst, such as hydrochloric acid or sulfuric acid. The term "hydrolysis" here means breaking a compound by the addition of water. What happens is that the ester molecule is split into an alcohol and a carboxylic acid.

• Initially, the acid catalyst donates a proton to the ester, facilitating the breaking of its chemical bonds.
• When an excess of water is used in the reaction, it ensures that the concentration of water remains constant throughout the process.
• This allows the reaction rate to primarily depend on the concentration of the ester, as water’s concentration seems invariant.

Therefore, although there are technically two reactants, the reaction can be considered "pseudo-unimolecular" because only the ester concentration significantly changes, driving the reaction forward.

Inversion of Cane Sugar

Inversion of cane sugar, known chemically as sucrose, occurs when this disaccharide undergoes hydrolysis. This reaction breaks sucrose into its two components: glucose and fructose. The term "inversion" stems from the optical activity of sucrose solutions.

• Sucrose is optically active, and upon hydrolysis, the direction of its optical rotation changes (inverts).
• In the presence of an acid catalyst and a large excess of water, the concentration of water does not significantly change.
• This interaction with water, in large excess, allows the reaction to behave similarly to a unimolecular reaction.
• Thus, the rate of reaction appears to depend mainly on the concentration of sucrose.

This simplification to a pseudo-unimolecular reaction helps in easier reaction kinetics analysis.

Reaction Kinetics

Reaction kinetics is the study of the rates at which chemical processes occur and the factors that influence these rates. Understanding kinetics is crucial for predicting how fast a reaction will proceed under various conditions.

• Factors affecting reaction kinetics include temperature, concentration of reactants, catalysts, and the physical state of the reactants.
• Catalysts, like acids in hydrolysis reactions, lower the activation energy needed, thus speeding up the reaction.
• Pseudo-unimolecular reactions simplify kinetic analysis by treating complex reactions as simpler, one-reactant systems.

These insights allow chemists and engineers to design processes that maximize efficiency and control reaction speed by tweaking conditions like concentration and temperature.

Order of Reaction

The order of a reaction relates to how the rate is affected by the concentrations of the reactants. In general, the rate is expressed in terms of a rate law that is determined experimentally.

• A unimolecular reaction has a reaction rate that only depends on the concentration of a single reactant, making it first-order.
• Pseudo-unimolecular reactions, although involving two reactants, behave as if they are first-order when one reactant is in large excess.
• The rate law for such reactions usually omits the large excess component, reflecting its constant presence.

By simplifying complex multimolecular reactions to a lower order, chemists can more easily interpret the kinetics and control the reaction process.