Multi-Step Reactions

A chemical reaction is often represented by an overall balanced chemical equation indicating the reactants and products.

However, the actual reaction is often more complex and transpires in multiple steps. For instance, this reaction of nitric oxide with hydrogen forming nitrogen gas and water takes place in three distinct, successive steps. These steps are called the reaction mechanism.

Each step in the reaction mechanism is called an elementary reaction and represents the interaction, such as bond breakage or formation, between the reacting species.

Specific molecules, like dinitrogen dioxide and nitrous oxide, are formed during one elementary step and consumed during another. Such species are called reaction intermediates.

Reaction intermediates are not the same as transition states, which exist only during the transformation of reactants to products.

Reaction mechanisms are hypothesized based on their balanced chemical equations, and experimentally determined rate laws of each elementary step.

Each step has a specific reaction rate, rate constant, and activation energy. The slowest step is called the rate-determining step and influences the net reaction rate. It can be used to verify the rate law for the overall reaction and to validate a proposed reaction mechanism.

Consider the decomposition of nitrous oxide to nitrogen and oxygen. The experimentally determined rate law does not correspond to the rate expression of a single-step reaction, which is corroborated by the observed presence of oxygen atoms—a reaction intermediate.

Hence, a reaction mechanism is proposed where all steps cumulate to give the overall reaction.

First, the rate constants indicate that the first step is the rate-limiting step. It is the slowest, and thus influences the overall reaction rate. A rate law proposed from this step can be set equal to the overall rate law.

This proposed rate law, directly derived from the molecular concentration of the elementary reactant, matches the experimental rate law and verifies the predicted reaction mechanism.

This text is adapted from Openstax, Chemistry 2e, 12.6: Reaction Mechanisms.