Short Answer

The molecularity of a reaction refers to the number of reactant molecules or particles that take part in a single elementary step of a reaction. It tells us how many molecules must collide at the same time for the step to occur.

Molecularity can be one, two, or three, depending on whether one, two, or three reactant particles participate in the elementary step. It is always a whole number and is used to understand how reactions happen at the molecular level.

Detailed Explanation :

Molecularity of a Reaction

The molecularity of a reaction describes the number of reacting particles—atoms, ions, or molecules—that come together in one elementary step to form products. It is a fundamental concept in chemical kinetics, particularly when studying reaction mechanisms. Molecularity only applies to elementary steps, not to the overall reaction. This is because the overall reaction may be made of many steps, but each step occurs through simple molecular interactions.

Molecularity helps chemists understand how a reaction proceeds at the smallest level, how many molecules must collide successfully, and why some reactions are faster or slower. Since molecularity deals with collisions, it directly relates to the probability of reactant particles meeting at the same time with proper energy and orientation.

1. Types of Molecularity
2. a) Unimolecular Reaction (Molecularity = 1)

Only one molecule is involved in the reaction step.
The molecule rearranges or breaks apart on its own.
Example: decomposition of N₂O₅.

1. b) Bimolecular Reaction (Molecularity = 2)

Two molecules or particles collide to form the products.
This is the most common type because two-particle collisions are frequent.
Example: HCl + OH⁻ → H₂O + Cl⁻.

1. c) Termolecular Reaction (Molecularity = 3)

Three particles collide simultaneously in one step.
This is very rare because the chance of three molecules meeting at exactly the same time is extremely low.

2. Important Characteristics of Molecularity
3. a) Always a Whole Number

Molecularity cannot be zero or a fraction. It must be 1, 2, or 3 because it counts actual particles.

1. b) Applies Only to Elementary Steps

It cannot be used for complex reactions because those occur in multiple steps.

1. c) Cannot Be Determined Experimentally

Molecularity is based on the proposed mechanism, not on experiments like the rate law.

1. d) Related to Collision Theory

More molecules in the step mean collisions must happen in a proper way.

1. e) Probability Decreases with Number of Particles

Unimolecular reactions are more likely than bimolecular, and bimolecular are more likely than termolecular.

3. Why Molecularity Matters

Molecularity provides insight into:

1. a) Reaction Mechanism

It helps identify how many particles are involved in each step and how the reaction progresses.

1. b) Speed of Reaction

More molecules needed in a single collision means the reaction step is usually slower.

1. c) Predicting Difficult Steps

If molecularity is 3, the step is very slow and may become the rate-determining step.

1. d) Teaching Basic Collision Concepts

It helps students understand that reactions depend on particle interactions.

4. Differences Between Molecularity and Order

Although sometimes confused, molecularity and reaction order are different:

• Molecularity applies only to elementary steps and is always a whole number.
• Reaction order applies to the overall reaction and is found experimentally; it may be fractional or zero.

This distinction is important because understanding molecularity provides microscopic detail, while reaction order gives macroscopic behaviour.

5. Examples to Understand Molecularity Better

Unimolecular Example

N₂O₅ → NO₂ + NO₃
Only one molecule decomposes, so molecularity = 1.

Bimolecular Example

NO + O₃ → NO₂ + O₂
Two molecules collide, so molecularity = 2.

Termolecular Example

A + B + M → AB + M
Three particles collide in one step, so molecularity = 3 (very rare).

Conclusion

Molecularity of a reaction describes the number of reactant particles participating in a single elementary step. It can be unimolecular, bimolecular, or termolecular, with unimolecular and bimolecular being the most common. Molecularity is always a whole number, applies only to elementary reactions, and provides deep understanding of reaction mechanisms and collision behaviour. By knowing the molecularity, chemists can better understand how reactions occur at the molecular level.