Kinetics and thermodynamics of decomposition of barium sulphate

Abstract

Equilibrium vapour pressures of barium sulphate and free surface sublimation pressures of single crystals were both measured for the first time, using torsion–effusion and torsion–Langmuir techniques, respectively. Mass spectrometer measurements showed the principal decomposition reaction to be BaSO₄(s)→ BaO(s)+ SO₂(g)+½O₂(g) for both equilibrium and free surface conditions.

The flux in the Langmuir experiments remained constant at about 10^–2 times that for effusion of equilibrium gaseous products over the temperature range 1422–1540 K. Effusion of the product gases through the porous product layer does not reduce the reaction rate even when the product layer thickness is as great as 1 mm. The simplest model consistent with the data assumes that all steps of the reaction except desorption to the gaseous products are at equilibrium, but that desorption occurs from only 1% of the surface sites. The equilibrium studies yielded, for the crystal modification stable above 1422 K, ΔH^°_D= 588.3 ± 6.7 kJ mol^–1 and ΔS^°_D= 257.3 ± 4.6 J K^–1(mol BaSO₄)^–1. The kinetic studies yielded ΔH^*_D= 575.3 ± 12.6 kJ mol^–1 and ΔS^*_D= 200.4 ± 8.4 J K^–1(mol BaSO₄)^–1 as the apparent enthalpy and entropy of activation for the decomposition reaction of the high temperature form of BaSO₄ when the activated state is assumed to consist of SO₂(g)+½O₂(g) per mole of barium sulphate.