Discussion of Some Basic Questions Concerning Path and Rate-Determining Step

Abstract

The general nature of this has been outlined,^92,94 and the essence is that of tunneling of electrons to an activated state produced by stretching of the ion-solvent bonds. The majority of the heat of activation is contributed by the latter.⁹⁴ The often raised question as to why the symmetry factor has the value 1/2 can be answered as follows: It is not exactly 0.5 but the factors which determine β do make it in practice between about 0.4 and 0.6.* Thus, in the usual interpretation of the symmetry factor,⁶² a variation in the applied potential produces a relative shift of the potential energy curves with respect to each other; this shift results in a change in the free energies of activation for the forward and backward reactions which is related to the slopes of the potential energy curves at the point of intersection. More precisely,

$$\beta = \frac{{{m_i}}}{{{m_i} + {m_f}}}$$

where m _i and m _f are the slopes of the potential energy curves of initial and final state, respectively, at the point of intersection.