complex will decompose when molecular vibrations rupture either
one of these bonds, and it could just as well result in reactant
molecules as product
In the transition-state theory, the rate
of the forward reaction depends on the concentration of activated
complex, and the probability that it will break down to form products
and not reactants.
We will find the picture of an activated
complex as being a reaction intermediate useful, even though we
will not go into the mathematics of the transtion state theory.
The activated complex is the state at the top of the "mountain"
in the drawing opposite.
In the N02
+ CO reaction, which we have been using as an illustration, the
of activation of the reverse reaction (Ea
= 85.7 kcal) is greater than that of the forward reaction (Ea
= 31.6 kcal), as shown.
By the simple collision theory, the rates
of the forward and reverse reactions are
ratef = 1
rater = Are-85.7/RT[NO]
The factors Af
and Ar are
approximately equal, thus most of the difference in the two rate
constants lies in the exponentials that contain Ea.