Effusion studies of the thermal decomposition of magnesium and calcium sulfates

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Abstract
The torsion‐effusion method, with simultaneous torque angle and weight loss measurements, was used to study the thermal decomposition of MgSO4 and CaSO4. The initial step in the decomposition/vaporization process appears to be the evolution of an SO3 molecule from the lattice, followed by conversion to SO2 and O2 on the surface. Measurements of decomposition pressure as a function of orifice area show that both steps are kinetically hindered. The experimental results can be correlated very satisfactorily by the Whitman–Motzfeldt model, so that reliable equilibrium data can be derived despite large degrees of undersaturation of the vapors. With MgSO4 it was possible to evaluate both the SO3 and SO2+O2 equilibrium pressures, the latter by addition of a small amount of Fe2O3 catalyst to the sample. Analysis of the derived equilibrium pressures yields values of −310.3±0.5 and −344.8±0.5 kcal/mol for the standard enthalpies of formation of MgSO4(s) and CaSO4(s), respectively, at 298 K.