ON THE THERMODYNAMICS OF VAPORIZATION AND THE ENTHALPIES OF FORMATION OF CASE, SRSE AND BASE

The congruent vaporization of the solid compounds CaSe, SrSe and BaSe of stoichiometric composition was studied over the temperature ranges 1832-2138 K, 1862-2122 K and 1860-2158 K, respectively, by the Knudsen effusion weight-loss method. Using enthalpy and entropy data from the literature for gaseous M, MSe, Se, and Se (M = Ca, Sr, Ba) and estimated data for the standard entropies and enthalpy functions of solid MSe, it can be shown that within the given temperature ranges CaSe and SrSe vaporize predominantly to the atomic species, while in case of BaSe the mode of vaporization to the atoms and to the molecular species BaSe are of about equal importance. The Se2-content of the gas phase is very small in all cases. The following second and third law enthalpies and entropies (indices II and III respectively) were derived for the vaporization to the gaseous elements: see "Inhaltsubersicht". The following standard enthalpies of formation of MSe (s) were derived from the third law enthalpies (in kJ.mol-1): CaSe: -445 +/- 44; SrSe: -451 +/- 42; BaSe: -467 +/- 44.