# Reaction Mechanisms and Elementary Reactions

A reaction mechanism may involve a single step or multiple steps. A reaction that occurs in a single step is an elementary reaction.

A chemical equation shows reactants and products of a chemical reaction. Information about how the reaction proceeds must be found experimentally. Some chemical equations show a single chemical change, i.e., a change that can occur in a single step. Some chemical equations are a summary of many individual steps. The reaction mechanism is the exact step or steps required to convert reactants into products.

A reaction that takes place in a single step is an elementary reaction. An elementary reaction can involve only a single molecule as a reactant. For example, dinitrogen tetroxide (N2O4) decomposes into nitrogen dioxide (NO2).
${\rm N}_2{\rm O}_4(\mathit g)\;\rightarrow\;2{\rm{NO}}_2(\mathit g)$
The number of molecules that participate in the rate-determining step of a chemical reaction is the reaction's molecularity. An elementary reaction with one reactant is a unimolecular reaction. The decomposition of dinitrogen tetroxide (N2O4) is an example of a unimolecular reaction. The general form of a unimolecular reaction is ${\rm{A}}\rightarrow\text{Products}$. The rate of a unimolecular reaction is k[A]. A bimolecular reaction is an elementary reaction that occurs between two reactants. An example of a bimolecular elementary reaction occurs between nitric oxide (NO) and ozone (O3).
$\rm{NO}(\mathit g)+{\rm O}_3(\mathit g)\rightarrow{\rm{NO}}_2(\mathit g)+{\rm O}_2(\mathit g)$
A bimolecular reaction can also involve two molecules of the same type. The general form of a bimolecular reaction is ${\rm{A}}+{\rm{B}}\rightarrow\text{Products}$ or $2{\rm{A}}\rightarrow\text{Products}$. The rate of a bimolecular reaction is either k[A][B] or k[A]2.

A termolecular reaction is an elementary reaction that occurs between three reactants. In a termolecular reaction, three molecules or ions collide at the same time. Termolecular reactions are rare compared to unimolecular and bimolecular reactions. They have three possible forms: ${\rm{A}}+{\rm{B}}+{\rm{C}}\rightarrow\text{Products}$, $2\rm A+\rm B\rightarrow \rm{Products}$, or $3{\rm{A}}\rightarrow\text{Products}$. The rate of a termolecular reaction is one of three possibilities: k[A][B][C], k[A]2[B], or k[A]3.