Usually, when A and B form a complex (AB), more ordering is created and entropy is reduced. S is thus negative. This is based on a collision complex model for the reaction. In addition to this decrease, there is a further orientational or steric decrease due to the preference of only certain configurations for the activated complex (as illustrated in lecture 32 for the reaction between C2H4 and H2. The steric factor of collision theory may be associated with e
, i.e.,
P = e where
(33.42)
s = scollisional + sorientational or steric
(33.43)
Similar to activation enthalps and entropy, activation volume can also be defined through
G /
p = V. For reactions in solution, these V values are small, but for gas phase reactions, they may be approximated by the ideal gas values
, more ordering is created and entropy is reduced. 
S
, i.e., 
p = 
