Electrostriction and counter ion effects in an outer-sphere electron transfer reaction. Kinetics of the reduction of hexachloroiridate(IV) by iodide ion
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Abstract
The kinetics of the reaction between the hexachloroiridate(IV) ion and iodide ion in aqueous acidic media has been monitored spectrophotometrically in the temperature range 20 to 40 °C, 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 × 103 M−1 s−1 at 25 °C and atmospheric pressure. This compares favourably with a value of 1.38 × 103 M−1 s−1 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: ΔH‡ ranges from 18 to 34 kJ mol−1 depending on the reaction medium. All ΔS
‡ 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