Collision frequency is:

Number of collisions per second per unit volume of the reaction mixture

The correct answer is option 2. Number of collisions per second per unit volume of the reaction mixture.

Collision frequency refers to the number of collisions between reacting particles that occur per unit time and per unit volume in a reaction mixture. It is a key factor in understanding reaction rates and kinetics.

Let us look the options in detail:

1. Number of collisions per second:

This term describes how many collisions occur per second, but it does not provide information about the volume in which these collisions are occurring. It gives a rate of collisions but lacks context regarding the spatial dimension (volume).

2. Number of collisions per second per unit volume of the reaction mixture:

This is the precise definition of collision frequency. It accounts for both:

Time Factor: The number of collisions occurring each second.

Volume Factor: The number of collisions per unit volume of the reaction mixture.

Importance: By including both time and volume, this measure provides a comprehensive view of how frequently particles collide in a given space. This is essential for calculating reaction rates and understanding how changes in concentration and volume affect the rate of reaction.

3. Number of collisions per unit volume of the reaction mixture:

This term describes the density of collisions within a given volume but does not specify the time aspect. It tells us how many collisions happen within a specific volume but lacks the detail on how often these collisions occur per second.

4. Number of collisions of sufficient energy:

This refers to the concept of effective collisions, where only collisions with enough energy (greater than the activation energy) lead to a chemical reaction. However, this does not describe the collision frequency, which is concerned with how often collisions happen, not their effectiveness.

Thus, the most accurate and comprehensive definition of collision frequency is: 2. Number of collisions per second per unit volume of the reaction mixture.