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

Kinetic and spectrophotometric measurements made during the Fe 3+ ion catalyzed decomposition of H 2 O 2 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: k 3 / K M = 4.94 M −1 min −1 , k 4 = 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 O 2 evolved in the reaction using [H 2 O 2 ] ∼ 10 −2 M has been calculated and found to be nearly stoichiometric. O 2 evolution experiments in this region cannot, thus, distinguish between the “complex mechanism” predicting nearly stoichiometric evolution of O 2 and the “free radical mechanism” predicting exactly stoichiometricamounts of O 2 . There are discrepancies within the “free radical scheme” with regard to the correct values of the rate constants to fit the reactions of H 2 O 2 both with Fe 2+ and Fe 3+ ions, as well as other reactions assumed to proceed via free radicals.