Kinetics of reaction of vanadium(IV) with chlorine in aqueous solutions

Abstract

The kinetics of the reaction 2[VO]²⁺+ Cl₂+ 2H₂O → 2[VO₂]⁺+ 2Cl^–+ 4H⁺ have been studied in aqueous solutions at 25 °C, with hydrogen-ion concentration in the range 0.10–1.0M and an ionic strength of 1·0M. In the initial stages of the reaction, and for conditions in which [V^IV]₀[Cl₂]₀, without any initial addition of V^V, the reaction follows the rate law –½d[V^IV]/dt=–d[Cl₂]/dt=k₀′[V^IV][Cl₂]/[H⁺] where k₀′= 5·32 ± 0·39 s^–1 at 25 °C. The reaction is strongly inhibited by vanadium(V), and hence marked deviation from the rate law is observed as the reaction progresses. Chloride ion also decreases the rate. The results are consistent with a mechanism involving [Cl₂]^–˙ as a transient intermediate. From the analysis of the Cl^– dependence of the apparent second-order rate constant obtained under conditions in which the rate law is valid, the rate constant for the V^IV–HOCl reaction is estimated as 19·60 ± 0·03 l mol^–1 s^–1 at 25 °C. Rate constants for the reaction of [Cl₂]^–˙ with V^IV and V^V are estimated to be 5·1 × 10⁷⩽k_–1⩽ 1·3 × 10¹¹ and 4·1 × 10⁵⩽k₂⩽ 1·2 × 10⁹ l mol^–1 s^–1, respectively.