Molecularity

Definition

Molecularity describes the number of reactant molecules involved in an elementary reaction. It is crucial for understanding the step-by-step mechanism of a chemical reaction.

5 Must Know Facts For Your Next Test

1. Molecularity can be unimolecular, bimolecular, or termolecular, corresponding to one, two, or three reactant molecules respectively.
2. An elementary reaction with molecularity greater than three is extremely rare due to the improbability of four or more molecules simultaneously colliding.
3. Molecularity is always a whole number since it represents discrete entities (molecules).
4. The rate law for an elementary reaction can be directly derived from its molecularity.
5. Molecularity differs from order of reaction; molecularity applies only to elementary reactions while the order can apply to complex reactions.

Review Questions

• What is the difference between molecularity and the order of a reaction?
• Why are termolecular reactions less common compared to unimolecular and bimolecular reactions?
• How does the molecularity of an elementary reaction relate to its rate law?

Related terms

Elementary Reaction:

A single-step process with a specific molecularity that describes how reactant molecules interact at the most fundamental level.

Reaction Mechanism:

A detailed sequence of elementary steps that describe how reactants are converted into products in a complex reaction.

Rate Law:

$$ \text{An expression that shows how the rate of a chemical reaction depends on the concentration of its reactants.} $$