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 kBr=(7.2 ± 0.8)× 105 dm3 mol–1 s–1 at Rate =–d[Br2]/dt=kBr[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 kex for U3++ U4+ is deduced to be 1.66 ± 0.16 dm3 mol–1 s–1.