Molecularity and Order of Reaction

A chemical reaction that takes place in one and only one step i.e., all that occurs in a single step is called elementary reaction while a chemical reaction occurring in the sequence of two or more steps is called complicated reaction. The sequence of steps through which a complicated reaction takes place is called reaction – mechanism. Each step in a mechanism is an elementary step reaction.

The molecularity of an elementary reaction is defined as the minimum number of molecules, atoms or ions of the reactants(s) required for the reaction to occur and is equal to the sum of the stoichiometric coefficients of the reactants in the chemical equation of the reaction.

In general, molecularity of simple reactions is equal to the sum of the number of molecules of reactants involved in the balanced stoichiometric equation. Or the molecularity of a reaction is the number of reactant molecules taking part in a single step of the reaction.

Chemical Reaction Molecularity
PCl5 → PCl3 + Cl2 Unimolecular
2HI → H2 + I2 Bimolecular
2SO2 + O2 → 2SO3 Trimolecular
NO + O3 → NO2 + O2 Bimolecular
2CO + O2 → 2CO2 Trimolecular
2FeCl3 + SnCl2 → SnCl2 + 2FeCl2 Trimolecular

The minimum number of reacting particles (molecules, atoms or ions) that come together or collide in a rate determining step to form product or products is called the molecularity of a reaction. For example, decomposition of H2O2 takes place in the following two steps:

Overall Reaction H2O2 → H2O + 1/2O2  
Step 1: H2O2 → H2O + [O] Slow
Step 2: [O] + [O] → O2 Fast

The slowest step is rate-determining. Thus from step 1, reaction appears to be unimolecular.

Reactions of higher molecularity (molecularity > 3) are rare. This is because a reaction takes place by collision between reactant molecules and as number of reactant molecules i.e. molecularity increases the chance of their coming together and colliding simultaneously decreases.

The mathematical expression showing the dependence of rate on the concentration(s) of reactant(s) is known as rate-law or rate-expression of the reaction and sum of the indices (powers) of the concentration terms appearing in the rate law as observed experimentally is called order of reaction.

From the study of the kinetics of many simple reactions, it is observed that for a large number of reactions, the molecularity and order are the same. Some examples are given below to justify this point:

Examples Reaction Order Molecularity
Dissociation of N2O5 N2O5 → N2O4 + O2
Dissociation of H2O2 H2O2 → H2O + 1/2O2
Dissociation of HI 2HI → H2 + I2
Formation of NO2 2NO + O2 → 2NO2