The kinetics of decomposition of H 2 O 2 catalyzed by Fe 3+ in aqueous solution in the presence of HNO 3 has been followed by permanganate titration and studied by the initial-rate method. In the experimental range [H 2 O 2 ] 0 /[Fe 3+ ] ) 41-2028 used in this study, the initial rate follows the lawBoth 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 Fe 2+ , HO • , HO 2 • , and O 2 •-as propagating intermediates. At high values of the [H 2 O 2 ] 0 /[Fe 3+ ] ratio two different reactions compete as chaintermination steps (dismutation of HO 2 • radicals and reduction of HO 2 • by Fe 2+ ), whereas at lower values of that ratio a third chain-termination step (reduction of HO • by Fe 2+ ) also contributes. Thus, the kinetics of this reaction can be considered as an indirect proof of the participation of hydroxyl radicals in the mechanism.

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It is a common practice among authors of chemical kinetics textbooks to include a series of rate laws applying to different kinds of chemical reactions (I). However, the case of autocatalytic processes is often ignored despite the extensive occurrence

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