Kinetics, Thermodynamics and Mechanisms of Reduction of Au(III) Complex Ion by Methanol in Acidic Medium

Abstract

Kinetics, thermodynamics and mechanisms of the reduction of gold(III) complex ion, [AuCl₃(OH)]^- , by methanol (MeOH) was studied  spectrophotometrically in perchloric acid medium at ionic strength (µ ) = 0.05 mol dm^–3 (NaClO₄) and T = 30.0 ± 1 °C. Stoichiometry of the  reaction showed that one mole of [AuCl₃(OH)]^- was reduced by one mole of methanol. The rate of reaction was first order in oxidant  concentration as well as in [CH₃OH] with a second order overall. Addition of acid within the range 5.0 x10^-4 to 1.2 x 10^-2 mol dm^-3   decreased the rate of the reaction. Increasing µ from 2.0 x10^-2 to 1.2 x10^-1 mol dm^-3 (NaClO₄) had no effect on the rate of the reaction.  The same trend was observed on varying dielectric constant from 78.40 to 73.59. Michaelis – Menten – type plot was linear and with  negligible intercept. Entropy of activation was found to be - 196.82 JK^-1 mol^-1 while activation enthalpy was also found to be 6.101 kJ mol^-1  . AuI was found to be the product of [AuCl₃(OH)]- reduction while aldehyde was obtained for MeOH oxidation. FTIR spectral  showed band for aldehyde formation (C=O) at 1764 cm^-1 . Based on negligible intercept from Michaelis – Menten – type plot and the  absence of spectroscopically determinable intermediate complex, the reaction was proposed to have proceeded through the outer –  sphere pathway.