A shock-tube study of the kinetics of decomposition of sulphur dioxide

Abstract

The rate of increase in strength of absorption bands of SO has been measured in shock-heated mixtures of sulphur dioxide and argon. Arrhenius-type plots indicate a unimolecular first step of the order d [SO]/d t = k [SO 2 ] [ M ], where [SO], [SO 2 ] and [ M ] are concentrations of [SO], [SO 2 ] and total gas. The apparent activation energy at around 3500 °K is 56 kcal/mole. It is shown that on unimolecular reaction theory, if four harmonic modes of oscillation in the SO 2 molecules contribute to the energy available for transformation, the true activation energy is 74 kcal/mole. This agrees with the energy of excitation to a known triplet state of SO 2 , and on this basis it is suggested that the first steps in the decomposition are SO 2 + M = SO* 2 + M — 73.6 kcal/mole (1) and SO* 2 + SO 2 = SO 3 + SO + 25.6 kcal/mole. (2) Step (2) is spin-allowed, whereas the more direct reaction SO 2 + SO 2 = SO 3 + SO —48 kcal/ mole is spin-forbidden. This is an unusual type of decomposition mechanism and occurs because of the high dissociation energy of SO 2 , because the direct step of low-energy is spinforbidden, and because there is a favourably situated triplet state of the molecule.