Question

Explain with example the difference between molecularity and order of reaction.

Hint:

A simple way to remember: Molecularity = "how many molecules collide" in a step, Order = "how rate depends on concentration" in experiment.

Answer 1

Molecularity:
- Molecularity of a reaction is the number of reactant species (atoms, ions or molecules) that collide simultaneously in a single step to bring about the reaction.
- It is always a whole number (1, 2, or 3).
- It is a theoretical concept based on reaction mechanism.
Example:
Unimolecular reaction: Decomposition of N$_2$O$_5$ in a single step has molecularity = 1.
Order of reaction:
- The order of a reaction is the sum of the powers of the concentrations of reactants in the experimentally determined rate law expression.
- It can be zero, fractional, or whole number.
- Order is determined experimentally.
Example:
For decomposition of H$_2$O$_2$: Rate = k[H$_2$O$_2$], order = 1.
Key Differences:
1. Molecularity is theoretical and cannot be zero or fractional, while order can be zero or fractional.
2. Molecularity refers to a single step, whereas order is obtained from the overall rate law.
\[ \boxed{\text{Molecularity is mechanism-based, order is experiment-based.}} \]