Iron(III)−Hydrogen Peroxide Reaction: Kinetic Evidence of a Hydroxyl-Mediated Chain Mechanism

Abstract

The kinetics of decomposition of H2O2 catalyzed by Fe3+ in aqueous solution in the presence of HNO3 has been followed by permanganate titration and studied by the initial-rate method. In the experimental range [H2O2]0/[Fe3+] = 41−2028 used in this study, the initial rate follows the law v0 = {k([Fe3+]3/2[H2O2]01/2)/([H+]3/2)}/{(1 + k‘([Fe3+]/([H2O2]0[H+])))1/2}. Both rate constants k and k‘ decrease with increasing ionic strength, and the corresponding apparent activation energies are 146 ± 10 and 88 ± 21 kJ mol-1. The experimental rate law strongly suggests a radical-chain mechanism, with Fe2+, HO•, HO2•, and O2•- as propagating intermediates. At high values of the [H2O2]0/[Fe3+] ratio two different reactions compete as chain-termination steps (dismutation of HO2• radicals and reduction of HO2• by Fe2+), whereas at lower values of that ratio a third chain-termination step (reduction of HO• by Fe2+) also contributes. Thus, the kinetics of this reaction can be considered as an indirect proof of the partici...