CBSE Class 12-science Answered

The number of reacting species (molecules, atom or ions) which collide simultaneously to bring about a chemical reaction is called molecularity of a reaction.

So in the given reaction

αA + βB   → Product

Molecularity = α + β

The molecularity of the reaction may be 1, 2,3 etc. according to the participation of one, two or three reactant molecules in the reaction.

On the basis of   molecularity, reactions are classified as:

• Unimolecular reaction
• Bimolecular reaction
• Trimolecular reaction

For example: 2 NO + O₂ → 2NO₂

In this reaction, three reactant molecules are participating, so the molecularity of the reaction is 3.