Question

Differentiate between order and molecularity of a reaction.

Hint:

Order is experimental and can be fractional or zero; molecularity is theoretical and always a small whole number from an elementary step.

Answer 1

Step 1: Order of a reaction.
1) It is the sum of the powers to which the concentration terms are raised in the experimentally determined rate law.
2) It is an experimental quantity, found from the rate expression, not from the balanced equation.
3) It can be zero, a whole number, or even a fraction (e.g. \(\tfrac{1}{2}\), \(1.5\)).
4) It applies to the overall reaction as well as to elementary steps.
Step 2: Molecularity of a reaction.
1) It is the number of reacting species (atoms, ions or molecules) that collide simultaneously in an elementary step to bring about the reaction.
2) It is a theoretical quantity, obtained from the mechanism of the reaction.
3) It is always a whole number (1, 2 or 3) and can never be zero or fractional.
4) It is defined only for a single elementary step, not for a complex overall reaction.
Step 3: Key point. For an elementary reaction, order and molecularity are usually the same; for a complex (multi-step) reaction the order is decided by the slowest (rate-determining) step, so order and molecularity may differ.