Molecularity and Order of Reaction
Molecularity
Molecularity is the number of reacting species (atoms, ions, or molecules) taking part in an elementary chemical reaction, which must collide simultaneously in order to bring about a chemical reaction.
- It is a theoretical value derived from the reaction mechanism.
- It is always a whole number (1, 2, or 3). It cannot be zero, negative, or fractional.
- Molecularity higher than three is rare because the probability of more than three molecules colliding simultaneously is very low.
Example: Decomposition of Ozone (Complex Reaction)
2O3 → 3O2 (Overall)
Step 1: O3 → O2 + O (Slow, Unimolecular)
Step 2: O3 + O → 2O2 (Fast, Bimolecular)
2O3 → 3O2 (Overall)
Step 1: O3 → O2 + O (Slow, Unimolecular)
Step 2: O3 + O → 2O2 (Fast, Bimolecular)
Order of Reaction
The sum of the powers of the concentration of the reactants in the rate law expression is called the order of that chemical reaction.
For a general reaction: nA + mB → Product
Rate = k[A]x[B]y
Order = x + y
Rate = k[A]x[B]y
Order = x + y
The values of x and y are determined experimentally and may or may not be equal to the stoichiometric coefficients n and m. If a reaction is complex (multi-step), the slowest step is known as the Rate Determining Step (RDS).
Comparison Table
| Feature | Molecularity | Order of Reaction |
|---|---|---|
| Definition | Number of molecules colliding in an elementary step. | Sum of exponents in the rate law equation. |
| Type of Value | Theoretical. | Experimental. |
| Possible Values | Whole numbers only (1, 2, 3). Never zero or fraction. | Can be zero, fractional, negative, or an integer. |
| Application | Only for elementary reactions; meaningless for complex reactions. | Applies to both elementary and complex reactions. |
Note: For elementary (single-step) reactions, molecularity and order are usually the same. For complex reactions, the order is determined by the slowest step, while molecularity is defined separately for each individual step.