Oxidations by uranium Vl

Since reaction (13.5) is more rapid than (13.2), (13.3), or (13.4), reactions (13.1) and (13.5) are predominant under these conditions, and the overall process corresponds to the (J(Vr)-catalysed reaction of V(n) and V(fV). In the absence of U(V1) this reaction is relatively slow . Kinetic data were obtained spectro-photometrically at 760 m/i (where V(IV) is the principal absorbing species) in 0.05 to 2.0 M perchloric acid between 0.6 and 36.8 °C. The rate law is 

U(VI) oxidises V(III) slowly in acid perchlorate solutions, the first step being 

In equation (13.11), the first term corresponds to the catalysed part of the reaction and the remaining terms, which make a relatively small contribution, apply to the uncatalysed part. Kinetic data at constant acidity were in good agreement with the integrated form of the calculated rate expression. The rate coefficients k2,k, k, and the ratio k. jk were evaluated. Almost linear plots of log 2 versus log [ ] were obtained at four temperatures with slopes close to —1.8. This result suggests that the dominant activated complex is that formed by loss of two ions, viz. 

This contrasts with the Pu(VI) +V(III) reaction which occurs by parallel paths. The activation parameters corresponding to the net activation processes (13.12) and (13.13) are A// = 22.1 and 17.7 kcal.mole and A5 = 12.9 + 0.5 and 3.8 + 0.9 cal.deg . mole respectively. Table 36 contains similar data for the analogous reactions of Np02 (ref. 253) and Pu02 (ref. 254) with The positive AS value for the 1102 +V reaction has led Newton and Baker to suppose that the U(VI) reaction is inner-sphere, whereas the Np(Vl) and Pu(VI) reactions are outer-sphere. 

On mixing acid solutions of U(VI) and Cr(II) at 0 °C, a rapid reaction takes place and a bright green solution is produced. A slower reaction then occurs to yield a solution having the darker green colour characteristic of U(IY) and Cr(lII). The appearance of the absorption spectrum of the principal intermediate leads to the conclusion that it is a complex formed between U(V) and Cr(Ill), viz. 

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