Collision frequency is:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Number of collisions per second per unit volume of the reaction mixture
In collision theory the reacting molecules are treated as hard spheres, and a reaction can occur only when they collide. The collision frequency (Z) is defined as the number of collisions per second per unit volume of the reaction mixture. It enters the rate expression as Rate = ZABe−Ea/RT.
Why the other options are incorrect
Option (1) Number of collisions per second — Incorrect. Without “per unit volume” the number would depend on how much reaction mixture is taken.
Option (3) Number of collisions per unit volume of the reaction mixture — Incorrect. It leaves out time; a frequency is a number per second.
Option (4) Number of collisions of sufficient energy — Incorrect. These are the effective collisions, only a fraction (e−Ea/RT) of all collisions.
NCERT (2026-27 edition):
Collision Theory of Chemical Reactions – “… It is based on kinetic theory of gases. According to this theory, the reactant molecules are assumed to be hard spheres and reaction is postulated to occur when molecules collide with each other. The number of collisions per second per unit volume of the reaction mixture is known as collision frequency (Z).”