The oxidation-reduction kinetics involved in the Np(V) + Fe(II) ⇌ Np(IV) + Fe(III) system in nitric acid solutions

Abstract

The rate law for the overall reaction Np(V)+Fe(II)⇌Np(IV)+Fe(III) may be expressed as d[Np(V)]/d t = k 2[Np(IV)][Fe(III)][H +] −3 − k 1[Np(V)][Fe(II)] [H +] 1.38 over the acidity range of 0.8−1.5 M. Values of the forward rate constant, k 1, and the reverse rate constant, k 2, are 15.7 M −2.38 min −1 and 1.71 M 2 min −1, respectively, at 25°C in 1.0 M nitrate solutions with ionic strength equal to 1.5. The proposed reaction mechanism for the forward reaction involves two activated complexes, one of which contains at least two protons. The reverse reaction involves either an activated complex containing three hydroxyl groups or one of its hydroxyoxygenated equivalents. The apparent activation energy of the forward reaction is 10 kcal mole −1, while that of the reverse reaction is 35 kcal mole −1. The effects of ionic strength on the specific rate constants are consistent with a Debye-Hückel treatment of the chemical equilibria involved in the formation of activated complexes. The concentration influence of nitrate ion on the specific rate constants can be quantitatively explained by a parallel pathway mechanism after assuming that one of the activated complexes of both the forward and reverse reactions contains a nitrate ion.