Collision frequency

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Collision frequency is defined in chemical kinetics and collision theory, in the background of theoretical kinetics.

For a reaction between A and B, the collision frequency (Z) is the number of collisions between A and B occurring in the container per unit volume per unit time.[1]

SI unit of Z is number of collision , where is Avogadro constant.


  • is the number density of A in the gas (unit ),
  • is the number density of B in the gas (unit ),
  • is the reaction cross section (unit ), the area when two molecules collide with each other, simplified to , where the radius of A and the radius of B.
  • is the Boltzmann's constant unit ,
  • is the temperature (unit ),
  • is the reduced mass of the reactants A and B, (unit )

Collision in diluted solution[edit]

Collision in diluted gas or liquid solution is regulated by diffusion instead of direct collisions, which can be calculated from Fick's laws of diffusion.