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The activation parameters of the V(II)--Fe(III) and U(III)--Fe(III) reactions were determined as a function of ionic strength in perchlorate media. Contrary to expectations, the values of ..delta..S/sup + +/ for these reactions were found to be independent of ionic strength, the changes in rate constant being reflected exclusively in the values of ..delta..H/sup + +/. Plausible explanations for these observations are discussed.

The kinetics and mechanisms of the reaction of U(III) with Tl(III) and Cr(III) have been examined in perchlorate media. The Tl(III) reaction was found to obey the rate law -d(U(III))/ dt = (U/sup 3 +/)(Tl/sup 3 +/) (k/sub -1/K/(H + K)) in which k/sub -1/ has a value of (3.6 +- 0.2) x 10/sup 5/ M/sup -1/ s/sup -1/ at 20/sup 0/C and an ionic strength of 2.0 M, whereas the Cr(III) reaction was found to exhibit a mechanism dominated by a reaction path having an inverse second-order concentration dependence. The rate constants are compared with those obtained for other similar systems; it is suggested, that the acid concentration independent and the inverse acid concentration dependent reaction paths of many electron-transfer reactions involving metal ions exhibit a limiting rate of 10/sup 6/ to 10/sup 7/ M/sup -1/ s/sup -1/. Plausible reasons for this limiting value are discussed.

The kinetics of the oxidation of U(III) by Co(III) was studied in perchlorate media. The rate law of the reaction was found to be given by the expression -d[Co(III)]/dt = k1 [Co(III)][U(III)]. The value of k1 was determined as (1.12 ± 0.1) x 106 M-1 sec-1 at 20°C and ionic strength of 2.00 and was found to be independent of acid concentration at constant ionic strength. The temperature dependence of the reaction was studied in the range 11 to 31°C, and the values of ΔH2+ and ΔS2+ were determined as 2.00 ± 0.25 kcal mole-1 and -25.6 ± 0.8 cal deg-1 mole-1, respectively. In common with many other reactions of Co(III), the rate constant of this reaction is much lower than expected from the very high ΔG0 value and plausible reasons for this observation are discussed.