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 eliminated and hence do not appear in the overall chemical equation.

The different elementary reactions may progress at varying speeds. The slowest elementary step determines the overall reaction rate. Here, the reaction of dinitrogen dioxide with hydrogen gas is the rate-limiting step.

Elementary reactions can be commonly characterized as three types, depending on the number of reacting molecules or molecularity.

In a unimolecular reaction, a single reactant molecule transforms into one or more products. In a bimolecular reaction, two distinct molecules react. A termolecular reaction, though very rare, involves three individual molecules reacting to yield intermediates or products.

Unlike the rate law for an overall chemical reaction, which is determined experimentally, rate laws for elementary reactions can be predicted from stoichiometric coefficients of their reactants. In short, the molecularity of an elementary reaction corresponds to the overall reaction order of the elementary step.

Hence, unimolecular reactions are often first-order reactions, bimolecular reactions are second-order, and termolecular reactions are of the third order.

An understanding of reaction mechanisms and kinetics helps chemists in identifying and optimizing chemical reactions.