What is meant by molecularity of a reaction?

Molecularity of a reaction refers to the number of molecules or ions that participate in the elementary reaction step.

In more detail, the term 'molecularity' is used in physical chemistry to describe the number of molecules or ions that come together to react in a single step of a reaction mechanism. This is different from the order of a reaction, which is determined experimentally and refers to the dependence of the rate of reaction on the concentration of the reactants.

Molecularity can only be 1, 2, or 3. A reaction with molecularity of 1 is called a unimolecular reaction. This is a reaction where a single molecule or ion is involved. An example of a unimolecular reaction is the isomerisation of but-1-ene to but-2-ene.

A reaction with molecularity of 2 is called a bimolecular reaction. This is a reaction where two molecules or ions are involved. An example of a bimolecular reaction is the reaction between hydrogen and iodine to form hydrogen iodide.

A reaction with molecularity of 3 is called a termolecular reaction. This is a reaction where three molecules or ions are involved. Termolecular reactions are rare because it is unlikely for three particles to collide simultaneously with the correct orientation and sufficient energy. An example of a termolecular reaction is the reaction between two molecules of nitric oxide and one molecule of oxygen to form two molecules of nitrogen dioxide.

It's important to note that the molecularity of a reaction is determined from the mechanism of the reaction, not from the overall balanced equation. It is a theoretical concept and is always a whole number.