Question

Differentiate between the Order of Reaction and Molecularity.

Hint:

For elementary reactions, order and molecularity may be the same, but for complex reactions they usually differ.

Answer 1

Step 1: Meaning of Order of Reaction.
The order of reaction is defined as the sum of the powers of the concentration terms in the rate law expression of a reaction. It is determined experimentally from the rate equation and may or may not be equal to the stoichiometric coefficients of the balanced chemical equation.

For example, if the rate law is:
Rate = k [A]² [B]
Then, order of reaction = 2 + 1 = 3.

Step 2: Meaning of Molecularity.
The molecularity of a reaction is the number of reacting species (atoms, ions, or molecules) that collide simultaneously in an elementary step to bring about a chemical reaction. It is always a whole number and is based on the mechanism of the reaction.

For example:
If two molecules collide in an elementary reaction, the molecularity is 2 (bimolecular reaction).

Step 3: Key Differences Between Order and Molecularity.
1. Definition:
Order of reaction is the sum of powers of concentration terms in the rate law.
Molecularity is the number of reacting species involved in a single elementary step.

2. Determination:
Order is determined experimentally.
Molecularity is determined from the reaction mechanism.

3. Possible Values:
Order can be zero, fractional, or even negative.
Molecularity is always a positive whole number (1, 2, 3, etc.).

4. Applicability:
Order applies to the overall reaction.
Molecularity applies only to elementary reactions.

5. Relation to Stoichiometry:
Order may or may not be equal to stoichiometric coefficients.
Molecularity is directly related to the number of species in an elementary step.

Conclusion.
Thus, order of reaction is an experimentally determined quantity related to the rate law, whereas molecularity is a theoretical concept based on the mechanism and represents the number of species colliding in an elementary reaction step.