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Kinetics of some oxidation-reduction reactions in aqueous solutions. Harkness, Alan Chisholm

Abstract

The kinetics of the electron transfer reaction U(IV) + T1(III) → U(VI) + T1(I) were examined in aqueous perchloric acid solution. The rate law was found to be of the form [ Formula omitted ] The two rate constants were identified with reaction paths involving activated complexes of the compositions (U.OH.T1)⁶⁺ and (U.O.T1)⁵⁺ , respectively. The corresponding heats and entropies of activation, evaluated from rate measurements over the temperature range 16 to 25°, are ΔH₁± = 24.6 kcal/mole, ΔH₂± = 21.7 kcal/mole, ΔS₁±= 16 e.u. and ΔS₂±= 7 e.u. The effect of ionic strength and the specific effects of various anions and cations on the rate were examined. The results suggest, but do not prove, that the reaction occurs through a single two-equivalent step rather than through successive one electron changes. The homogeneous oxidation of carbon monoxide by metal ions in aqueous solutions was studied. At temperatures below 80° only Hg²⁺ and MnO₄¯ were found to oxidize carbon monoxide. The ions Cu²⁺ , Ag⁺ , Hg₂²⁺ , Fe³⁺ , T1³⁺ and Cr₂O₇²¯, were inactive. Kinetic measurements of the reaction 2Hg²⁺+ CO + H₂O → Hg₂²⁺+ + CO₂ + 2H⁺ in dilute perchloric acid over the temperature range 26 to 54° yielded the rate law [ Formula omitted ] The activation parameters are ΔH± = 14.6 kcal/mole and ΔS± =-13 e.u. It is believed that the reaction proceeds by a mechanism which involves the insertion of CO between Hg²⁺ and a coordinated water molecule [ Formula omitted ] The oxidation of carbon monoxide by MnO₄¯ was found to proceed readily over the temperature range 28 to 50°. The rate law was found to be [ Formula omitted ] with ΔH± = 13 kcal/mole and ΔS± = -17 e.u., both substantially constant over the pH range 1 to 13. The rate determining step is considered to be the formation of hypomanganate MnO₄¯+ CO + H₂O → MnO₄³¯ + CO₂ + 2H which then undergoes further fast reactions to yield MnO₄²¯ in basic solution and MnO₂ in acid and neutral solutions. A remarkable feature of the latter reaction is its very marked sensitivity to catalysis by Ag⁺ and Hg²⁺ (but not by Cu²⁺, Cd²⁺, Fe³⁺, or T1³⁺ ). The rate law of the catalyzed path is, in each case, [ Formula omitted ] where M = Ag⁺ or Hg²⁺. For Ag⁺, k at 0° is 1.10 x 10⁵ M¯² sec¯¹ with ΔH± = 1.3 kcal/mole and ΔS± = -30 e.u. For Hg²⁺, k at 0° is 1.09 x 10³ M¯² sec¯¹ with ΔH± = 6.5 kcal/mole and ΔS± = -21 e.u. It is suggested that the remarkably high reactivities exhibited by carbon monoxide in these catalytic reactions are related to favourable oxidation paths involving intermediates such as [ Formula omitted ]

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