Lecture 32 : Theories Of Reaction Rates : Collision Theory
Most collisions are in random directions and only a small fraction of collisions with the correct relative alignments lead to the products. This requirement of "proper relative" alignment between molecules is called the steric effect and we can define the steric factor P as
=
P
(32.11)
For the four reactions in Table 32., the values of P are 0.16, 2.5 * 10 -3, 1.7 * 10 - 6 and 4.8 respectively. Before we consider being larger than , we will consider the dependence of on energy or velocity of molecules. When the steric factor is small, very few collisions occur with suitable relative orientations of reactants.
The above discussion brings out the central role of reaction cross sections ( ) in chemical dynamics. In addition to the dependence of the reaction cross section on the relative orientations of reactants, depends on the relative velocity (or relative kinetic energy) as well. For smaller relative velocities, the reactants often have more time to feel the presence of one another.
Figure 32.3 Dependence of (reaction cross section) on relative velocity (
) .
=
P 
) . 
