The kinetics and mechanism of the reaction of chromium(II) with mercury(II) in aqueous perchloric acid solutions

Abstract

The reduction of mercury(II) with chromium(II), 2Cr^II+ 2Hg^II→ 2Cr^III+(Hg^I)₂, in aqueous perchloric acid solutions, µ= 2·0M, has been studied over the temperature range 5–20° by conventional spectrophotometry. The rate equation is first-order in both reactants, d[Cr^III]/dt= 2k_obs.[Cr^II][Hg^II], and the hydrogen-ion dependence of k_obs. is of the form k_obs.=k₁+k₂′[H⁺]^–1, where k₁ makes only a small contribution (0·04 I. mole^–1 sec.^–1 at 20°). Activation parameters for k₂′(0·89 sec.^–1 at 20·) are ΔH₂′‡= 17·8 (+0·5) kcal. mole^–1 and ΔS₂′‡= 2·4 (±1·7) e.u. The form of the rate equation suggests a mechanism in which the rate-determining step Cr^II+ Hg^II→Cr^III+ Hg^I is followed by the fast reactions Cr^II+ Hg^I [graphic omitted] Cr^III+ Hg⁰ Hg⁰+ Hg^II [graphic omitted] (Hg^I)₂ The mercury(II) reactant must be in excess of the chromium(II) otherwise a stage is reached when the last reaction is no longer possible and mercury is precipitated. Chloride ions catalyse the reaction, but much less strongly than hydroxide, and at 10° the true rate constants for the reactions of Cr²⁺ with Hg²⁺, HgCl⁺, and HgOH⁺ are 1·75 × 10^–2, 1·5 (±0·1), and ca. 2·3 × 10³ I. mole^–1 sec.^–1, respectively. The dichloro-complex HgCl₂ appears to be much less reactive than HgCl⁺.