Kinetics of the oxidation of uranium(IV) ions by halogens in aqueous solution. Part I. Iodine and bromine

Abstract

The reaction U⁴⁺+ 2H₂O + I₂→ UO₂²⁺+ 2I^–+ 4H⁺ obeys the rate expression –d[I₂]/dt=k₂′[U^IV][I₂][H⁺]^–2, with k₂′= 8·9 × 10^–4 l mol^–1 s^–1 at 40 °C and I= 1·0M. The rate of reaction decreases with increasing [I^–] and the results fit the expression –d[I₂]/dt=k₈[U^IV][I₂]+k₉[U^IV][l₃^–], where k₈= 9·9 × 10^–2 and k₉= 1·9 × 10^–2 l mol^–1 s^–1 at 40 °C, [H⁺]= 0·10M, and I= 1·0M. Activation energies and entropies of activation for the two paths are: for k₈, E_a= 17·3 ± 2·6 kcal mol^–1, ΔS^‡=–9·1 ± 1·5 cal K^–1 mol^–1; and for k₉, E_a= 19·9 ± 1·2 kcal mol^–1, ΔS^‡=–4·6 ± 0·3 cal K^–1 mol^–1. The effect on the rate of added SO₄^2–, F^–, Cl^–, Br^–, N₃^–, and NCO^– ions is also reported. The reaction U⁴⁺+ Br₂+ 2H₂O → UO₂²⁺+ 2Br^–+ 4H⁺ has been confirmed as obeying the rate expression –d[U^IV]/dt=k₃[U^IV][Br₂], with k₃= 1·16 × 10^–3 l mol^–1s^–1 at 25 °C, [H⁺]= 0·70M, and I= 3·0M. The activation energy and entropy for this reaction are E_a= 23·3 ± 5·0 kcal mol^–1 and ΔS^‡= 1·99 ± 0·05 cal K^–1 mol^–1.