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), 2CrII+ 2HgII→ 2CrIII+(HgI)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[CrIII]/dt= 2kobs.[CrII][HgII], and the hydrogen-ion dependence of kobs. is of the form kobs.=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 CrII+ HgII→CrIII+ HgI is followed by the fast reactions CrII+ HgI [graphic omitted] CrIII+ Hg0 Hg0+ HgII [graphic omitted] (HgI)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+.