The kinetics of the oxidation of iodide ions by octacyanomolybdate(V) ions in aqueous solution

Abstract

The kinetics of the reaction 2Mo(CN)₈^3–+ 3I^–→ 2Mo(CN)₈^4–+ I₃^– have been studied as a function of temperature and ionic strength. The reaction is first-order with respect to both the octacyanomolybdate(V) and iodide concentrations and there is evidence of retardation by product octacyanomolybdate(IV). The second-order rate constant is k_obs= 3·52 ± 0·13M^–1 sec.^–1 in the expression –d[Mo(CN)₈^3–]/dt=k_obs[Mo(CN)₈^3–][I^–] and the activation energy and entropy of activation have the values E_a= 5·68 ± 2·57 kcal./mole and ΔS*=–39 ± 8·5 e.u. at 25·7°C and I= 0·1M. The plot of log k_obs against √I is linear with a slope of + 3·12, which suggests that ion-pairs may not play an important role in this reaction.