Kinetics and mechanisms of redox reactions in aqueous solution. Part 5. Oxidation of hypophosphorous acid by cerium(IV) in aqueous nitric acid

Abstract

The rate of title reaction has a first-order dependence both on [Ce^IV] and [H⁺], and an order less than unity on [H₃PO₂]. However, the observed first-order rate constant k_obs. is dependent on the initial [Ce^IV], decreasing with increasing [Ce^IV]. The rate-limiting step is considered to be attack of a proton on the intermediate complex formed between Ce^IV and the active tautomer of hypophosphorous acid. The energy and entropy of activation for the rate-limiting reaction have values of 66 ± 2 kJ mol^–1 and –88 ± 8 J K^–1 mol^–1 respectively. The reaction is catalysed by Ag^I and there is no change in the order of the reaction with respect to [Ce^IV]. The relation between k_obs. and [Ag^I] is consistent with the equation (i) where a and b are constants. K_obs.=a+b[Ag^I](i)