The kinetics of the oxidation of europium(II) with vanadium(III) and chromium(III) in aqueous perchloric acid solutions

Abstract

The reactions of europium(II) with vanadium(III) and chromium(III) in aqueous perchloric acid solutions, µ= 2·0M have been studied using conventional spectrophotometric techniques. In preliminary experiments it was shown that europium(II) and chromium(II) do not react with perchlorate ions over periods of at least ten days. The slow reaction between vanadium(II) and perchlorate ions was confirmed at 25°. The reaction Eu^II+ V^III→ Eu^III+ V^II is first order in both the reactants d[V^II]/dt=k_obs[Eu^II][V^III] where k_obs is dependent on the hydrogen-ion concentration k_obs=k₁+k′[H⁺]^–1. The constant k₁(9·0 × 10^–3 l.mole^–1 sec.^–1 at 25°) corresponds to the reaction of Eu²⁺ with V³⁺ and it is assumed that k′ is for the reaction of Eu²⁺ with VOH²⁺. Thus k′ may be written k₂K_H where k₂(2·0 l. mole^–1 sec.^–1 at 25°) is the true rate constant and K_H the hydrolysis constant for V³⁺. From the temperature dependence ΔH₁ ^‡= 11·4 kcal. mole^–1, ΔS₁ ^‡=–30·1 e.u., ΔH₂ ^‡= 6·2 kcal. mole^–1, and ΔS₂ ^‡∼–35 e.u. The forward and reverse reactions Eu^II+ Cr^III [graphic omitted] Eu^III+ Cr^II are both slow at 25°, and with [H⁺]= 0·5Nk₃∼ 1·7 × 10^–5 l. mole^–1 sec.^–1 and k_–3∼ 1·4 × 10^–5 l. mole^–1 sec.^–1. The addition of free chloride ions does not effect the forward reaction, but in the reverse the rate is much faster and substitution-inert CrCl⁺ is formed. With CrCl²⁺ as the chromium(III) reactant the forward reaction Eu²⁺+ CrCl²⁺→ Eu³⁺+ Cr²⁺+ Cl^– is also much faster. At 25°k₄= 2·23 × 10^–3 l. mole^–1 sec.^–1 and the activation parameters for this path are ΔH₄ ^‡= 17·0 kcal. mole^–1 and ΔS₄ ^‡=–13·9 e.u. A transition complex [CrCl Eu]⁴⁺ seems likely, but alternatives [CrCl H₂O Eu]⁴⁺ and [Cl Cr H₂O Eu]⁴⁺ cannot as yet be entirely excluded.