Chromium(II) and vanadium(II) reductions of hexa-aquocobalt(III) in aqueous perchlorate media

Abstract

A full kinetic study has been made of the reaction of Cr²⁺ with hexa-aquocobalt(III), [H⁺]= 0·10–3·0M, µ= 3·0M(LiClO₄). The rate law is of the form –d[Co^III]/dt=(k₁+k₂[H⁺]^–1)[Cr²⁺][Co^III] and at 25 °C : k₁= 1·25 × 10⁴| mol^–1 s^–1, ΔH₁^‡= 9·5 ± 0·4 kcal mol^–1, ΔS₁^‡=–7·8 ± 1·5 e.u.; k₂= 6·59 × 10³ s^–1, ΔH₂^‡= 12·7 ± 0·6 kcal mol^–1, ΔS₂^‡= 1·7 ± 2·0 e.u. Chloride ions catalyse the reaction and the products CrCl²⁺ and Cr³⁺ were determined quantitatively after ion-exchange separation. The formation of CoCl²⁺ proceeds relatively slowly and it was possible to identify two chloride-dependent terms, k₃[Cr²⁺][CoCl²⁺] and k₄[Cr²⁺][Co³⁺][Cl^–]. The reaction of V²⁺ with hexa-aquocobalt(III) was too fast to study directly. Competition studies indicated a rate constant k_vca. 8·8 × 10⁵| mol^–1 s^–1 at 25 °C and µ= 3·0M(HClO₄).