We have seen how the rate of a reaction can
vary with temperature. Its variation with pressure can also provide
information about the mechanism of a reaction. According to absolute rate
theory, the rate of a reaction depends on the change of the Gibbs' free
energy DG‡ between the starting reactants and the activated
complex. Its variation with pressure at constant temperature is given by:
(d/dP)|TD‡
= DV‡
This is the activation volume for the
reaction. The variation with pressure of a number of equilibrium constants
and reaction rates is shown in the figure. For the group I reactions, DV
is of the order of 40 Å3. |
