Collision Probabilities and Rate of Chemical Reaction In discussions of the dependence of reaction rate on the concentrations of the reactants, the relation of collision probabilities to reaction rate is usually pointed out. The six-fold increase in rate when the amount of A is increased from 2 to 4 particles and B is increased from 2 to 6 particles is accounted for by the increase from the 4 different collision possibilities for 2 particles of A and 2 of B to 24 different camhinztitionsof 4 particles of A with 6 particles of B. It is then usually mentioned, without using such simple numbers, that if only one chemical species is involved, as in B, the rate is proportional to [A]¶ because of the increase in collision probability as the 2A number of particles of A increases. As a matter of fact, the increase in collision probability with the doubling of the number of particles of A only approaches the value 4 as the total number of particles of Aincreases. This cenreadily beshown by s. set of figures snchas the following
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Number of Particles of A
Ratio ojNumber of Particles of A
Number of Collision Possibilities
Ratio of Number $ Collision Posszbililies
I t is obvious that with any ordinary chemical system the number of particles involved is so large that the doubling of the number will result in 4 times the number of collision possibilities. However, it isinteresting to note that thisis thelimitingvalue, not anabsoluteone.
PAULK. GLASOE
390
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Journol o f Chemical Educofion
