Various collision theories, dealing with the frequency of collision between
reactant molecules, have been put forward. In the earliest theories reactant molecules were
regarded as hard spheres, and a collision was considered to occur when the distance
between the centres of two molecules was equal to the sum of their radii. For a gas
containing only one type of molecule,
A, the
collision density is given by simple collision theory as:
Here
is the
number density of molecules and
is the mean molecular speed, given by kinetic theory to be
, where
is the molecular mass, and
. Thus:
The corresponding expression for the
collision density
for two unlike molecules
A and
B, of masses
and
is:
where
is the
reduced mass
, and
.
For the
collision frequency factor these formulations lead to the following expression:
where
is the
Avogadro constant. More advanced collision theories, not involving the assumption that molecules behave
as hard spheres, are known as generalized kinetic theories.
Source:
PAC, 1996, 68, 149
(A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC
Recommendations 1996))
on page 160
Cite as:
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by
A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997).
XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic,
J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8.
https://doi.org/10.1351/goldbook.
