Kinetics and mechanism of the oxidation of uranium(III) by aqueous acidic solutions of iodine and bromine

Abstract

The rates of oxidation of U³⁺ by I₂ and Br₂ in aqueous acidic solutions have been investigated. For the iodine reaction the rate equation is (i) where k₁=(3.92 ± 0.78)× 10⁵ dm³ mol^–1 s^–1 and k₂=(0.31 ± 0.02)× 10⁵ dm³ Rate =–d[I₂]_T/dt=k₁[U³⁺][I₂]+k₂[U³⁺][I₃^–](i) mol^–1 s^–1 at 25 °C and I= 1.0 mol dm^–3 Li[ClO₄]. The corresponding activation parameters are ΔH₁^‡= 3.2 ± 0.4 kcal mol^–1, ΔS₁^‡=–11.0 ± 1.0 cal K^–1 mol^–1, ΔH₂^‡= 3.9 ± 0.4 kcal mol^–1, and ΔS₂^‡=–12.6 ± 1.5 cal K^–1 mol^–1. The rate is unaffected by acid for the range [H+]= 0.05–0.5 mol dm^–3, but added Cl^– increases the rate. For the bromine reaction the rate equation is (ii) where k_Br=(7.2 ± 0.8)× 10⁵ dm³ mol^–1 s^–1 at Rate =–d[Br₂]/dt=k_Br[U³⁺][Br₂](ii)I= 1.0 mol dm^–3, 25 °C, and [H+]= 0.1 mol dm^–3. Activation parameters are ΔH*= 3.1 ± 0.3 kcal mol^–1 and ΔS*=–21.2 ± 2.3 cal K^–1 mol^–1. The rate is unaffected by acid in the range [H+]= 0.1–0.5 mol dm^–3 or by bromide in the range [Br^–]= 0.004–0.2 mol dm^–3. On employing Marcus theory, an excellent correlation has been obtained between the rates of uranium(III) reduction of some oxidants, including iodine and bromine, and the free energies of these reactions. Since these other non-halogen reactions go via the outer-sphere mechanism, it is concluded that at least the first step in the two-step oxidation of U³⁺ by Br₂, I₂, or [I₃]^– is outer sphere. The homonuclear exchange rate constant k_ex for U³⁺+ U⁴⁺ is deduced to be 1.66 ± 0.16 dm³ mol^–1 s^–1.