Some Kinetic and Thermodynamic Aspects of the Electrochemical Reduction of Sulfur Dioxide

摘要

The heterogeneous rate constant for the one-electron electroreduction of SO sub 2 in a LiBr/acetonitrile solution has been determined from -11 exp 0 to 45 exp 0 C. Over this range it increases from 0.12 to 0.19 cm.s exp -1 which is nearly fast enough to be classified as reversible. This electron transfer is quickly followed by a dimerization of the SO sub 2- reduction product which produces S sub 2 O sub 42- (dithionite). From cyclic chronocoulometric data, the rate constant for this dimerization was measured in acetonitrile solutions of LiBr and tetraethylammonium bromide (TEABr) from -11 exp 0 to 40 exp 0 C. In the presence of LiBr, the dimerization is much faster. (At 0 exp 0 C it is 1.1 x 10 exp 5 L.mol exp -1 S exp -1 in a LiBr solution and 2.5 x 10 exp 4 L.mol exp -1 S exp -1 in a TEABr solution.) For either supporting electrolyte the rate of the follow-on reaction clearly decreases with increasing temperature. The onset of an alternative second order follow-on reaction at higher temperatures has been postulated. It is believed to be the disproportionation of SO sub 2- to SO and SO sub 32- . From linear sweep voltammetric data the standard reduction potential for the electroreduction was determined at several temperatures. At 25 exp 0 C it is calculated to be -0.622 V (vs N.H.E.). This reduction potential is quite temperature-sensitive (dE/dT = 3.7 mV/K exp 0 ). These results can be converted to give: delta G sub 298 = 14.4 kcal/mol, delta H = 40.1 kcal/mol, and delta S = 86.4 cal/K exp 0 mol. From the equilibrium constant for the dimerization reaction at 25 exp 0 C (k/sub eq/ = 2.33 x 10 exp 7 L/mol) the free enthalpy of dimerization, delta G sub 298 , is calculated to be -10.1 kcal/mol. (ERA citation 09:016623)