Kinetics of the Reaction of Molecular Hydrogen with Mercuric and Mercurous Perchlorates in Aqueous Solution

THE PRINCIPLE OF EQUIVALENCE CHANGE IN OXIDATION–REDUCTION REACTIONS

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.

EFFECTS OF COMPLEXING ON THE HOMOGENEOUS REDUCTION OF MERCURIC SALTS IN AQUEOUS SOLUTION BY MOLECULAR HYDROGEN

The effects of various complexing agents on the homogeneous reduction of mercuric salts by molecular hydrogen in aqueous solution were determined. In all cases the kinetics suggest that the rate-determining step is a bimolecular reaction between a mercuric ion or complex and a hydrogen molecule, probably leading to the formation of an intermediate mercury atom. The reactivity of various mercuric complexes was found to decrease in the following order: HgSO4 > Hg++ > HgAc2, HgPr2 > HgCl2 > HgBr2 > Hg(EDA)2++. Addition of anions such as OH−, CO3=, Ac−, Pr−, and Cl−, in excess of the amounts required to form stable mercuric complexes, was found to increase the rate. An interpretation of these effects is given.