Thermal determination of the kinetics of the iron(III)–tin(II) redox reaction in chloride solution

From a temperature against time curve measured when a chemical reaction occurs in a batch reactor immersed in a constant temperature environment, it is possible to calculate both the rate constant and the enthalpy of a single reaction. This technique has been applied to the iron(III)–tin(II) redox reaction in chloride solution, studied over a twenty-fold variation in non-complexed chloride concentration and over a 50 K temperature range. Over this range of conditions the pseudo-second order rate constant for the reaction varied by a thousand-fold. Heats of reaction were also calculated. The experimental results obtained at 298 K agree with those of Duke and Pinkerton and support the finding that the rate-determining step involves chloro-complexes of the stannous and ferric ions. However, results at different temperatures indicate that rate-determining electron transfer occurs in precursor complexes of the tin(II), iron(III) and chloride ions rather than between iron(III) and tin(II) chloride complexes as suggested by Duke and Pinkerton. There is a common precursor complex for a given number of chloride ions. Furthermore, the enthalpies of activation of the electron transfer reaction in the precursor complexes are all the same and the different rate constants for electron transfer in each precursor complex is determined by the charge on the precursor complex