Order and molecularity of a reaction

Elementary reactions are classified according to their molecularity. The number of reacting species, which are involved in simultaneous collision to bring about a chemical reaction is called the molecularity of the reaction. In a unimolecular reaction, a single molecule shakes itself apart or its atoms into a new arrangement.

Example 14:

Decomposition of NH₄NO₂ to N₂ and 2 H₂O

Bimolecular reactions involve the collision of pair of molecules. Bimolecular reactions are common.

Example 15:

Oxidation of nitric oxide by ozone

Intermolecular reactions, three molecules collide simultaneously. Such reactions are rare.

Example 16:

Molecularity differs from order of a reaction. The differences are listed below.

Differences between Molecularity and Order of a Reaction

Molecularity is defined for elementary reactions where as order corresponds to the overall reaction. The overall reaction might involve one elementary reaction or it might involve a sequence of elementary reactions (a sequence of elementary reactions denotes a complex reaction). A bimolecular elementary reaction exhibits second order kinetics. This is easy to comprehend. However, if a reaction exhibits second order kinetics, it is not necessary that the mechanism of the reaction involves bimolecular collision. The reaction might involve a sequence of steps, where the molecularity of one step might be different from another step. Hence, knowing the order of a reaction alone, that is the rate law, is not enough to predict the mechanism of a reaction, where as knowing the reaction mechanism does allow for the rate law to be deduced.

Simple reactions are those, which involve a single elementary reaction. The rate law can be written from the stoichiometry of the chemical equation. One such reaction is the gas phase reaction of ozone (O₃) with nitric oxide (NO). This bimolecular reaction exhibits second order kinetics. However, overwhelming majority of reactions consist of several connected elementary reactions, that is they are complex reactions.

For complex reactions, to account for the kinetic data, reaction mechanisms are formulated. Formulation of complex mechanism involve intermediates. Therefore, the mechanism formulated can be tested with further observations of the intermediates. Then, the rate law is predicted from the mechanism. If the predicted rate law matches the experimental rate law, then the hypothesized mechanism can be accepted as the most probable mechanism of reaction. Elucidation of reaction mechanism is more like detective work where the 'happening' of an 'event' is constructed based on a series of clues. In reaction mechanism studies, identification of intermediates forms an important area of study.

Study of complex reactions is based on certain methods that have been developed over the years. Complex reactions can involve a series of consecutive reactions (series reactions), they can also react via parallel reactions giving rise to multiple products.