Thermodynamic approach to catalysis

Abstract

The most important attempt to explain catalytic reactions and bimolecular reactions in general was made by Arrhenius in 1889. Arrhenius showed that the relation between the reaction velocity constant and the temperature could be expressed by the equation

$$ \ln {\kern 1pt} k = B - \frac{A}{{RT'}} $$

((1))

in which A indicates the difference in energy between the active molecules and the average energy of all the molecules. The meaning of A is essentially the same as that held by Arrhenius but the meaning of B has undergone considerable change. Scheffer and Kohnstamm ¹ assumed that in order to react the molecules had to go through an intermediate state characterized by a definite difference of energy from the average energy and a definite difference of entropy from the average entropy. On the basis of thermodynamic as well as kinetic considerations, they arrived at the following equation

$$ \ln {\kern 1pt} k = \frac{{{\varepsilon _{{i - }}}{\varepsilon _{m}}}}{{RT}} + \frac{{{\eta _{{i - }}}{\eta _{m}}}}{R} + C $$

((2))

where ε _i -ε _m and η _i -η _m represent the differences in energy and entropy in question. This formula was applied to experimental data and discussed in somewhat altered form in a later paper.