Define Molecularity of a Reaction

Q. Define molecularity of a reaction.

Answer:- A reaction’s molecularity is defined as the number of reacting molecules that collide at the same time to produce a chemical reaction. In other terms, the number of reactant molecules that take part in an elementary reaction is defined as its molecularity.

Important points to consider while calculating molecularity:

• Molecularity of an elementary reaction is always a whole number.

• It is never fractional and negative but only a positive number.

• Molecularity is a concept that exists only in theory.

• Molecularity cannot exceed three because more than three molecules cannot collide with one another.

There are three types of molecularity: unimolecular, bimolecular, and termolecular.

Example 1: Decomposition of dinitrogen tetroxide (N₂O₄)

Here only one molecule is present in a reaction, thus it is called a unimolecular reaction, and its molecularity is one.

N₂O₄(g) → 2NO₂(g) 

The reaction rate is precisely proportional to the concentration of dinitrogen tetroxide raised to the first power, making it a first-order reaction.

Example 2: Reaction of Hydrogen iodide (HI)

2HIH₂ + I₂ 

Since there are two molecules of hydrogen iodide involved in this process, it is classified as a bimolecular reaction.

Example 3: Reaction between sulphur dioxide (SO₂) and oxygen(O₂)

2SO₂ + O₂ → 2SO₃ 

Two sulphur dioxide molecules and one oxygen molecule collide and participate in the chemical process. As a result, it’s known as a trimolecular reaction.