KINETICS OF REACTION BETWEEN VANADIUM(2) AND NEPTUNIUM(4) IN AQUEOUS PERCHLORATE SOLUTIONS

The reaction Np(IV) + V(II) = Np(III) + V(III) was studied in acid perchlorate solutions from 0.016 to 20 M HClO4, from 0.9 to 36.7° at an ionic strength of 2.0 M (LiClO4). The rate law at 25° is -d[Np(IV)]/dt = {2.53 + (3.8 × 10-2/[H+]) + (6 × 10-4/[H+]2)} [Np4+][V2+]. The temperature dependence of the pre-dominant, hydrogen ion independent term leads to ΔH* = 9.72 ± 0.05 kcal/mol and ΔS* = -24.0 ± 0.2 cal/ mol deg for Np4+ + V2+ = activated complex. Chloride ion causes a moderate increase in rate. For the most important chloride term the net activation process is Np4+ + V2+ + Cl- = activated complex, for which ΔH* = 13.45 ± 0.15 kcal/mol and ΔS* = -5.5 ± 0.4 cal/mol deg. The effect of ionic strength is in accord with an extended form of the Debye-Hückel equation from 0.275 to 4.0 M. The hydrogen ion, chloride ion, and temperature-dependence data are consistent with an outer-sphere mechanism.