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

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 H2O20/Fe3+ = 41−2028 used in this study, the initial rate follows the law v0 = k (Fe3+3/2H2O201/2) / (H+3/2) / (1 + k' (Fe3+/ (H2O20H+) ) ) 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 H2O20/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 participation of hydroxyl radicals in the mechanism.