The kinetics of the reaction between the hexachloroiridate() ion and iodide ion in aqueous acidic media has been monitored spectrophotometrically in the temperature range 20 to 40 ЊC, at ambient and at elevated pressures (up to 125 MPa). With suitable selection of reactant concentrations the reaction can be studied as simple pseudo first-order (iodide ion in excess) yielding a second-order rate constant of 1.42 × 10 3 M Ϫ1 s Ϫ1 at 25 ЊC and atmospheric pressure. This compares favourably with a value of 1.38 × 10 3 M Ϫ1 s Ϫ1 for the same parameter, determined previously under the same conditions. The reaction rate varies to a limited degree with variation in the conjugate base of the acids employed, which may be the result of a secondary medium effect. Potassium ions exert a significant catalytic effect, a finding that parallels those reported for other anion-anion redox reactions. The reaction is moderately sensitive to temperature: ∆H ‡ ranges from 18 to 34 kJ mol Ϫ1 depending on the reaction medium. All ∆S ‡ values are distinctly negative, suggesting a degree of molecular ordering and/or increasing electrostriction in the rate limiting step. Under all conditions employed pressure accelerates the reaction rate, resulting in large negative volumes of activation, typically Ϫ20 to Ϫ25 cm 3 mol Ϫ1 . How the thermal and activation parameters relate to the mechanism is discussed.
The important role of metal ion catalyzed autoxidation reactions of sulfur(1V)-oxides in aqueous solution has been confirmed by numerous investigations. In general, transition metals such as Co, Fe and Mn in the 3 + oxidation state can rapidly oxidize HSO,/SO$to produce the reduced metal ions in the 2 + oxidation state and the SO1 radical, which can subsequently initiate a series of free radical propagation reactions. Experimental results are reported to demonstrate how low concentration levels of sulfite can effectively induce the autoxidation of the reduced metal ions in order to complete the catalytic cycle. In this way the mentioned metal ions exhibit redox cycling in which the nature of the redox process is determined by the sulfite and oxygen concentration levels in solution. A general mechanism to account for these observations, as well as for the synergistic effect usually observed for such reactions, is presented.
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