Vapor pressure measurements over calcium, magnesium and their alloys and the thermodynamics of formation of CaMg 2

Abstract

Vapor pressures have been measured by the Knudsen effusion method over magnesium, calcium and a series of calcium—magnesium alloys. Magnesium vapor pressures are in good agreement with previous work. Calcium vapor pressures are in reasonable agreement with previous measurements over liquid calcium but do not reproduce any of the past measurements over solid calcium. Vapor pressure measurements over the alloys were used to determine the free energy of formation of the compound, CaMg 2. The free energy of formation in kcal/mol can be expressed as ΔF o = −8.3 + (1.1 × 10 −3) T. The small values for the enthalpy and entropy of formation are consistent with the view that the compound forms because of atomic size considerations. An independent check on the enthalpy of formation was made by bomb calorimetry.