Reaction Mechanisms

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 low-energy products of an elementary reaction. They are often short-lived, which explains their absence in the product mixture. Reaction intermediates are not the same as activated complexes. Activated complexes are high-energy transition states existing only during the transformation of reactants to products.

Combining the elementary steps gives the equation for the overall chemical reaction. Here, the reaction intermediates are eli