Question

(a) What is meant by the term elementary reaction? (b) What is the difference between a unimolecular and a bimolecular elementary reaction? (c) What is a reaction mechanism?

Short answer

(a) An elementary reaction is a single-step chemical process in which reactants directly change into products in one collision event, with a consistent reaction rate depending on the concentrations of reacting species. (b) A unimolecular elementary reaction involves the rearrangement or decomposition of a single molecule, with the reaction rate depending on the concentration of that reactant. A bimolecular elementary reaction involves a collision event between two reacting species, with the reaction rate depending on both reactants' concentrations. (c) A reaction mechanism is a series of elementary reactions that describe the pathway of a chemical reaction from reactants to products, providing a detailed description of molecular collisions, bond rearrangements, and energy changes during the reaction process.

Step-by-step solution

(a) Definition of Elementary Reaction)

An elementary reaction is a basic chemical process that represents a single step in a reaction mechanism. In an elementary reaction, reactants directly change into products in one collision event, with a consistent rate of reaction depending only on the concentrations of reacting species.

(b) Unimolecular vs Bimolecular Elementary Reactions)

A unimolecular elementary reaction involves the rearrangement or decomposition of a single molecule or reactant. The rate of a unimolecular reaction depends solely on the concentration of this single reactant. An example of a unimolecular reaction is the isomerization of an alkene, where one isomer transforms into another. A bimolecular elementary reaction, on the other hand, involves a collision event between two reacting species or molecules. The rate of a bimolecular reaction depends on the concentrations of both reactants. An example of a bimolecular reaction is a nucleophilic substitution reaction, in which a nucleophile replaces a leaving group on a molecular structure.

(c) Reaction Mechanism Definition)

A reaction mechanism is a series of elementary reactions or steps that describe the pathway of a chemical reaction, from reactants to products. It provides a detailed description of how molecular collisions, bond rearrangements, and energy changes occur during the reaction, helping chemists understand the underlying principles and kinetics of the reaction process. Reaction mechanisms may include one or multiple steps, with each step being an elementary reaction.