Transpiration Study of Magnesium Oxide

Abstract

Vapor densities of magnesium-bearing species over magnesium oxide were determined by the transpiration method. The MgO reacted with water vapor present in the carrier gas to form gaseous Mg(OH)2. The standard free energy of formation in calories per mole is given over the temperature range from 1660° to 2010°K by the equation ΔGf° = —168 600+49.8T cal mole—1. Under conditions where water vapor is minimized, a second vaporization process becomes predominant. It is concluded that this process is the congruent sublimation of MgO. A second-law treatment of these data indicates ΔH298 = 142.5+3.3 kcal mole—1 while a third law treatment yields 145.1±0.1 kcal mole—1 using a 1Σ ground state for MgO. The ground state for gaseous MgO would appear to be Σ and most likely 1Σ. A value of 93 kcal mole—1 was obtained for D0 for MgO based upon the second- and third-law treatment of the data.