15. The Rates of Chemical Reactions   Previous PageNext Page
       Molecular Collisions and Reaction

The activated complex will decompose when molecular vibrations rupture either one of these bonds, and it could just as well result in reactant molecules as product molecules.

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 energy 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 Afe-3.61/RT[NO2] [CO]
rater = Are-85.7/RT[NO] [C02]

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.


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