Kinetics and mechanism of the oxidation of europium(II) by aqueous solution of iodine and triiodide

Abstract

The rates of reduction of I 2 and I − 3 by Eu 2+ have been measured in acqueous solutions. The data are consistent with the rate equation: Rate = −d/dt [I 2] Total = K 1[Eu 2+] [I 2 + K 2[Eu 2+][I 3 3] with k 1 = (6.14 ± 0.19) × 10 3M −1sec −1, k 2 = (0.37 ± 0.05) × 10 3M −1 sec −1 at I = 1.0M (LiClO 4), [H +] = 0.1M, t = 25°C. The corresponding activation parameters are ΔH* 1 = (3.4 ± 0.6)kcal mol −1, ΔS* 1 = −(29.8 ± 5.4) cal mol −1 deg −1, ΔH * 2 = (5.0 ± 0.6) kcal mol −1, ΔS * 2 = −(30.0 ± 3.1) cal mol −1 deg −1. The proposed mechanism is a series of univalent changes with ·I − 2 as a reaction intermediate. The activation parameters are compared with those obtained for other halogen reactions. The data give no conclusive distinction between the inner sphere or the outer sphere mechanism for these reactions.