Reaction Mechanism and Rate Law

Reaction Mechanisms

A reaction mechanism describes the sequence of steps that make up a complex chemical reaction. Each of these steps is known as an elementary process.
Every elementary process involves a single molecular event, such as the breaking or forming of a bond. They can be unimolecular, bimolecular or termolecular, involving one, two or three molecules respectively.
The rate determining step is the slowest step in a reaction sequence. It determines the overall rate of the reaction as it needs to occur before the reaction can proceed.
An intermediary or transition state can be formed during the elementary steps of a reaction mechanism. Intermediaries are highly reactive and unstable and are not found in the overall balanced equation, yet they act as ‘stepping stones’ to the final product.
A catalyst can change the sequence of elementary steps in a reaction mechanism. It reduces the activation energy by providing an alternative pathway for the reaction to occur, thus increasing the rate of reaction.

The Rate Law of a reaction cannot be determined simply by considering the balanced chemical equation. Instead, it can be deduced from the elementary processes of a reaction’s mechanism.
Molecularity of the rate-determining step can provide insight into the reaction order. For instance, a unimolecular step will give first-order behaviour, while a bimolecular step can lead to second-order behaviour.
When the reaction mechanism involves several steps, the rate determining step will determine the Rate Law. The Rate Law will be given in terms of the reactants of the rate-determining step only.
Each step of the reaction mechanism has its own rate constant. The overall rate constant for the reaction is related to these individual rate constants, but the relationship depends on the specific sequence of steps.
Proposed reaction mechanisms are validated by checking if the Rate Law predicted from the mechanism matches the Rate Law determined experimentally.
Misconceptions about reaction mechanisms may lead to inaccurate predictions about the Rate Law or other kinetic properties of the reaction. Therefore, understanding the true mechanism of a reaction is crucial in predicting how the reaction will behave under different conditions.