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 (N2
) decomposes into nitrogen dioxide (NO2
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 (N2
) is an example of a unimolecular reaction. The general form of a unimolecular reaction is
. The rate of a unimolecular reaction is k
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
A bimolecular reaction can also involve two molecules of the same type. The general form of a bimolecular reaction is
. The rate of a bimolecular reaction is either k
[A][B] or k
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: , , or . The rate of a termolecular reaction is one of three possibilities: k[A][B][C], k[A]2[B], or k[A]3.