Collision frequency

Collision frequency is defined in chemical kinetics and collision theory, in the background of theoretical kinetics. If a system contains n_a type-a molecules per unit volume and n_b type-b molecules per unit volume, then the number of a-b collisions per unit time in that volume will be proportional to the product n_an_b.

or, in terms of molar volumes ([x]=n_x /N_A where N_A is the Avogadro constant)

By kinetic theory it can be shown that:

where

The collision frequency (Z) is usually expressed in units of "moles of collisions" per unit time per unit volume:

or equivalently:

where R is the universal gas constant and μ_ab is measured in atomic mass units.

If each collision resulted in the a and b molecules combining to form a type c molecule (a+b->c), then Z[a][b] would equal the rate of production of c (in moles of type-c particles created per unit time per unit volume).

...
Wikipedia