THE PRINCIPLE OF EQUIVALENCE CHANGE IN OXIDATION–REDUCTION REACTIONS

Abstract

The principle of equivalence change predicts that oxidation–reduction reactions between 1-equivalent oxidants and 2-equivalent reductants (or vice versa) will, in general, be slow, since they must proceed either through termolecular paths or through the formation of unstable intermediates. In this paper, the kinetics and mechanisms of a number of reactions of this type are examined and an attempt is made to assess the validity of the considerations on which this principle is based. Among the reactions considered are (1) electron transfer between metal ions; (2) oxidation of metal ions by oxygen; and (3) reduction of metal ions by hydrogen. In each of these cases it is found that the principle of equivalence change has only limited validity and that a number of other factors are important in determining the relative rates and mechanisms of reactions of different equivalence type. Among these are the formation of stabilized intermediate complexes between oxidant and reductant and the possibility of unstable intermediates acting as carriers in chain reactions. In reactions of thallium(I) or thallium(III) with 1-equivalent metal ions, thallium(II) is formed as an intermediate. Some of these reactions are not as slow as expected, apparently because of favorable entropies of activation. Several of the reactions examined proceed simultaneously through bimolecular and termolecular paths, the latter being favored because of lower activation energies.