Kinetics of oxidation of transition-metal ions by halogen radical anions. Part IV. The oxidation of vanadium(II) and chromium(II) by di-iodide, dibromide, and dichloride ions generated by pulse radiolysis

Abstract

The radical ions I₂^–, Br₂^–, and Cl₂^–, generated by nanosecond pulse radiolysis of solutions containing the halide ions, oxidise vanadium(II) to vanadium(III) and chromium(II) to chromium(III). At 22 ± 3 °C and I= 0·5 mol l^–1 rate constants for the reactions X₂^–+ V²⁺_aq→ V³⁺_aq+ 2X^– are (1·43 ± 0·2)× 10⁸, (1·48 ± 0·2)× 10⁹, and (1·95 ± 0·2)× 10⁹ l mol^–1 s^–1(X = I, Br, and Cl) and the reaction mechanism is outer sphere in all cases. Corresponding rate constants for the reactions X₂^–+ Cr²⁺_aq→[Cr(OH₂)₅X]²⁺+ X^– are (1·5 ± 0·2)× 10⁹, (1·9 ± 0·2)× 10⁹, and (2·4 ± 0·3)× 10⁹ l mol^–1 s^–1. The reaction mechanisms were established by gamma-radiolysis and flash-photolysis experiments. The reaction between the ions Cl₂^– and Cr²⁺_aq proceeds by parallel inner- and outer-sphere paths with approximately equal probability. The radical ions I₂^– and Br₂^– react with Cr²⁺_aq ions entirely by inner-sphere paths.