They found that the rate of this reaction is proportional to the concentrations of both reactants. When they ran a similar reaction on a slightly different starting material, they got similar products. But now the rate of reaction was proportional to the concentration of only one of the reactants. Sometimes, the rate law is consistent with what we expect from the stoichiometry of the reaction. Often, however, it is not.
Some reactions occur in a single step.
ClNO is a good example of a one-step reaction. Other reactions occur by a series of individual steps. 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 .