Question

How would you define the molecularity of a complex reaction? Explain with an example.

Answer 1

The number of reacting species (atoms, ions, or molecules) taking part in the slowest elementary step (i.e., rate-determining step) of a complex reaction is called the molecularity of the complex reaction.

For example, the reaction between nitric oxide and hydrogen is a complex reaction.

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

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

Step 1 \[\ce{2NO + H2 -> N2 + H2O2}\] (slow)

Step 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.