Question

Discuss the difference between Order and Molecularity of a reaction.

Hint:

Order = experiment-based; Molecularity = collision-based concept.

Answer 1

Concept: Order and molecularity are important kinetic concepts that describe reaction behavior, but they differ in definition and applicability.
Answer:
Order of Reaction:

• It is the sum of powers of concentration terms in the rate law.
• It is determined experimentally.
• It can be zero, fractional, or integer.
• It applies to the overall reaction.

Molecularity of Reaction:

• It is the number of molecules colliding in an elementary step.
• It is a theoretical concept.
• It is always a whole number (never zero or fractional).
• It is defined only for elementary reactions.

Differences:

• Order is experimental; Molecularity is theoretical.
• Order can be fractional; Molecularity is always an integer.
• Order applies to overall reaction; Molecularity applies to elementary steps only.