Abstract:
The kinetics and mechanisms of the redox reactions of µ-oxo-bridged iron(III) complex ion Na4[(FeEDTA)2 O].12H2O denoted as Fe2 O4+, with the thiols-2-mercaptobenzothiazole(MBSH), 2- mercaptophenol(PhSH), 2-mercaptoacetic acid (MSH), and l-cysteine (RSH) have been investigated in aqueous perchloric acid medium at [H+]=1×10-4 mol dm-,3,I=0.05mol dm-3(NaClO4) and at T =27.0±0.1oC. The reactions were monitored under pseudo-first order condition .The rate of reaction was first-order in reductant and oxidant for all the systems giving overall second –order reactions. The inorganic and organic products of the reaction between Fe2O4+ -MBSH, PhSH, MSH and RSH and oxidants were found to be Fe(II) ions and disulphides respectively. The stoichiometries of Fe2O4+ -MBSH, PhSH, MSH, and RSH was determined by mole ratio method and was found to be 1:2 for all the systems. The reactions of the thiols (MBSH.PhSH,MSH and RSH) had an inverse dependence on hydrogen ion concentration ,and so the general rate law can be given as follows
dt
d[Fe2O4+] = (a + b) [H+]-1) [Fe2O4+] [reductant]
Changes in ionic strength of the reaction medium had a negative effect on the rate of reaction of Fe2O4+ - MBSH and RSH and positive effect in the reaction of Fe2O4+ - PhSH and MSH. Reduction of Fe2O4+ by MBSH, PhSH, MSh and RSH showed no dependence on dielectric constant because decrease of dielectric constant did not change kobs. CH3COO-,/NO3-/Cl-/SO42-/K+ and Mg2+,were used to determine the effect of catalysis on Fe2O4+-MBSH,PhSH,MSH and RSH reactions and there was decrease in catalysis. The effect of temperature on the rate of reduction of Fe2O4+ with reductants was studied and was found to have negative entropy which confirmed the formation of binuclear complexes at the activated complex. The results of the study indicate that the reactions of Fe2O4+ and thiols probably occur by the outer-sphere mechanism.
