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.