Effusion studies of the thermal decomposition of magnesium and calcium sulfates

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.

On the Thermal and Thermodynamic (In)Stability of Methylammonium Lead Halide Perovskites

“…17. The resulting values of were: −688.3 ± 7.8 kJ/mol, −567.5 ± 8.7 kJ/mol, and −403.6 ± 9.7 kJ/mol for MAPbCl 3 , MAPbBr 3 and MAPbI 3 , respectively.…”

The vaporization of NH4NO3

“…Details of this method are described in Refs. [18,19]. If more than one more gas species is present in the vapor phase, then the weight-average molecular weight, M Ã , is given by the following equation:…”

Mechanistic Aspects of Metal Sulfate Decomposition Processes

“…Figure 1 shows a Whitman-Motzfeldt (7,8) plot of the observed effusion pressure data on Ca50 4 at 1200 K for several orifice sizes (1). According to the W-M model (7,8), reciprocal observed pressure P shows a linear correlation with effective orifice area, 'ifa, where f and a are orifice force factor and area, respectively; the intercept of such a plot at zero effective orifice area is the reciprocal of equilibrium pressure, P eq .…”

“…Our goal initially was to characterize these processes thermodynamically. Since they were expected to follow the equilibrium pathway MS04(s) = MO(s) + S02(g) + h 02(g) (1) information about the reaction thermodynamics and the properties of the MSOis) phases could be obtained from measured decomposition pressures and the established properties of the products. Early on, however, effusion studies on MgS04(S) showed that only S03 was observed in the effusing gas under some conditions, rather than the dominant equilibrium products S02 and O 2 (1).…”

“…Since they were expected to follow the equilibrium pathway MS04(s) = MO(s) + S02(g) + h 02(g) (1) information about the reaction thermodynamics and the properties of the MSOis) phases could be obtained from measured decomposition pressures and the established properties of the products. Early on, however, effusion studies on MgS04(S) showed that only S03 was observed in the effusing gas under some conditions, rather than the dominant equilibrium products S02 and O 2 (1). The composition of the effusate from MgS04 (s) was dependent not only on orifice size, but also on the fractional degree of decomposition (I), making the interpretation of the results somewhat complex.…”