Some reactions occur in a
single step. The reaction in which a chlorine atom is transferred from
ClNO2 to NO to form NO2 and ClNO is a good example
of a one-step reaction.
ClNO2(g) + NO(g)
NO2(g) + ClNO(g)
Other reactions occur by a series of individual steps. N2O5, for example, decomposes to NO2 and O2 by a three-step mechanism.
| Step 1: | N2O5
NO2 + NO3 |
|
| Step 2: | NO2 + NO3
NO2 + NO + O2 |
|
| Step 3: | NO + NO3
2 NO2 |
The steps in a reaction are classified in terms of molecularity, which describes the number of molecules consumed. When a single molecule is consumed, the step is called unimolecular. When two molecules are consumed, it is bimolecular.
Information provided by: http://chemed.chem.purdue.edu