Question

How is the molecularity of a reaction determined for a complex reaction?

Answer 1

Complex reactions are those that take place in two or more steps. The stoichiometry of their balanced equations cannot determine the molecularity of complex processes. This is due to the possibility that their balanced equations contain a significant number of reactant molecules. For example,

\[\ce{2NO + 2H2 -> N2 + 2H2O}\]

This reaction is supposed to take place in the following two steps.

1. \[\ce{2NO + H2 -> N2 + H2O}\] (Slow)
2. \[\ce{H2O2 + H2 -> 2H2O}\] (Fast)

Step 1 determines the rate because it is slow. Three molecules of reactants are involved in the chemical reaction which is similar to step 1. As a result, the slowest elementary step's molecularity is 3, which is really understood as the reaction's molecularity.

It is to be noted that although the molecularity and order of a reaction may be similar in some cases, the two are quite different from each other.