Kinetics and mechanism of heterogeneous cadmium sulphide and homogeneous manganese(II) catalysed oxidation of sulphur(IV) by dioxygen in acetate buffered medium

Abstract

Cadmium sulphide and Mn(II) catalysed autoxidation of aqueous sulphur(IV) has been studied in detail. The CdS catalysed reaction obeys rate law (i), with <i>k</i>₃<i> </i>= 1.17×10^-12 mol L-¹s^-1<br><br>

<i>-d </i>[S(IV)] <i>/dt </i>= <i>k</i>₃<i> </i>[S(IV)]² [H⁺]^-2 + <i>k</i>₄<i> </i>[CdS] [S(IV)]² [H⁺]^-1 .....(i)<br><br>

and <i>k</i>₄<i> </i>= 4.5 × 10^-7 L².mol^-2 g^-1 s^-1 at 30°C in acetate buffered medium. For manganese(II) catalysed reaction, the overall rate law is given by Eq. (ii) with <i>k</i>_e =2×10^-4 L mol^-1s^-1 and <i>k</i>_f<i> </i>= 6.9×10^-2 L² mol^-2 s^-1<br>

<i>R</i>_obs= <i>R</i>_o + <i>k</i>_e [Mn(II)] [S(IV)] [H⁺]^-1 + <i>k</i>_f[Mn(II)] [S(IV)]² [H⁺]^-1 ....(ii)<br><br>

at 30°C in acetate buffered medium. <i>R</i>_o term represents the uncatalysed and <i>/</i>or<i> </i>trace metal ion catalysed rate. Ethanol strongly inhibits both the reactions indicating the operation of a radical chain mechanism. A study of the effect of ethanol shows that whereas the non-transition metal oxide catalysed sulphur(IV) autoxidation is severely inhibited, the transition metal oxide catalysed autoxidation is inhibited only slightly. This has been interpretated in

terms of the operation of a radical chain mechanism in the case of former and a non-radical mechanism in the case of the latter.