Understanding Molecularity
Molecularity defines the number of reactant molecules, atoms, or ions that participate in an elementary reaction step, directly colliding to form products. It describes a theoretical aspect of an individual step within a reaction mechanism, unlike reaction order which is an experimentally determined value for the overall reaction.
Types of Molecularity
Elementary steps are classified by their molecularity: unimolecular (one reactant species), bimolecular (two reactant species), or termolecular (three reactant species). Unimolecular reactions typically involve a single molecule rearranging, while bimolecular reactions require two species to collide. Termolecular reactions are rare due to the low probability of three species simultaneously colliding.
A Practical Example
Consider the elementary reaction A → products. This is a unimolecular reaction because only one molecule of A is involved. For a reaction A + B → products, it is bimolecular as two distinct species (A and B) collide. If the elementary step were 2A → products, it would also be bimolecular, as two molecules of A are involved.
Importance and Applications
Molecularity is crucial for proposing and validating reaction mechanisms. For an elementary step, the molecularity directly corresponds to the order of that specific step. Understanding molecularity helps chemists predict how changes in reactant concentrations might affect the rate of an elementary reaction, which is foundational to chemical engineering and synthetic chemistry.