The identification of one- and two-equivalent paths in the reaction of vanadium(II) with mercury(II)

Abstract

The reaction of vanadium(II) with mercury(II) has been studied by the stopped-flow method over the temperature range 10–25° with µ= 2·0M (LiClO₄). One- and two-equivalent reactions occur concurrently, and rate constants and activation parameters have been obtained for both paths. The one-equivalent path, rate-determining step V^II+ Hg^II→ V^III+ Hg^I, is independent of [H⁺]. At 25°, a(=k₁) is 1·32 l mol^–1 s^–1, ΔH₁^‡= 15·2 ± 0·8 kcal mol^–1 and ΔS₁^‡=––7·2 ± 2·9 e.u. An inverse [H⁺] dependence is observed for the two-equivalent path, V^II+ Hg^II→ V^IV+ Hg⁰. With the assumption that the latter is for the reaction of HgOH⁺ ions (where the acid dissociation constant for Hg²⁺ is K_a), then b=k₂+k₃K_a[H⁺]^–1, and at 25°k₂= 1·85 l mol^–1 s^–1, ΔH₂^‡= 10·9 ± 0·7 kcal mol^–1 and ΔS₂^‡=–20·6 ± 2·3 e.u.; and k₃= 3·2 × 10⁴ l mol^–1 s^–1, ΔH₃^‡= 7·9 ± 0·4 kcal mol^–1, and ΔS₃^‡=-11·5 ± 1·4 e.u. The addition of chloride ions retards both thxe one- and two-equivalent paths.