Define collision frequency.

Definition: Collision frequency is the number of collisions that occur per unit time per unit volume of a system. It is usually denoted by Z.

In gases, it represents how often molecules collide with each other in a given time and space.

Expression

For a gas, the collision frequency between two molecules can be expressed as:

Z=2 π σ2 cˉ n2

Where:

• σ\sigma = collision diameter of molecules
• cˉ\bar{c} = average speed of molecules
• nn = number of molecules per unit volume

Importance

• Determines the rate of chemical reactions (higher collision frequency → more chances of effective collisions).
• Helps explain gas behavior and kinetic theory of gases.