“Complex” versus “free radical” mechanism for the catalytic decomposition of H₂O₂ by ferric ions

Kinetic and spectrophotometric measurements made during the Fe³⁺ ion catalyzed decomposition of H₂O₂ have been analyzed using the computer simulation method. Improved values of the rate constants of the “complex scheme” and of the molar absorptivities ofthe intermediates were obtained: k₃/K_M = 4.94 M⁻¹ min⁻¹, k₄ = 193 M⁻¹ min⁻¹, ε_I/K_M = 52.3 M⁻² cm⁻¹, ε_II = 25.7 M⁻¹ cm⁻¹. The simulation revealed details of the reaction which were unavailable by other means and which explained several features of its kinetics. The total amount of O₂ evolved in the reaction using [H₂O₂] ∼ 10⁻² M has been calculated and found to be nearly stoichiometric. O₂ evolution experiments in this region cannot, thus, distinguish between the “complex mechanism” predicting nearly stoichiometric evolution of O₂ and the “free radical mechanism” predicting exactly stoichiometricamounts of O₂. There are discrepancies within the “free radical scheme” with regard to the correct values of the rate constants to fit the reactions of H₂O₂ both with Fe²⁺ and Fe³⁺ ions, as well as other reactions assumed to proceed via free radicals.