Collision Theory of Reaction Rates

This theory makes the assumption that, for a reaction to occur, there must be collisions between the reacting species. This means that the rate of reaction depends on two factors: the number of collisions per unit time, and the fraction of these collisions that are successful because enough energy is involved. There is a definite relationship between the concentrations of the reactants and the number of collisions. The reaction of

A + B ---> AB

can be examined as the concentrations are changed.

A+B

(Single) - One possible collision

A+B+A+B

(Double Concentration) - Four possible collisions

A+B+A+B+A

(Triple Concentration of A and Double Concentration of B) - Six Possible Collisions

This shows that the number of collisions, and consequently the rate of reaction, are proportional to the product of the concentrations. Simply stated, then, the rate of reaction is directly proportional to the concentrations.