The Dissociation Energy of NiO and Vaporization and Sublimation Enthalpies of Ni

Abstract

The vaporization thermodynamics of nickel oxide have been controversial for a number of years. Johnston and Marshall (1) determined the vaporization of NiO by heating a nickel ring coated with the oxide in a high vacuum. They assumed that the only species which transported oxygen was NiO(g). From these weight loss data in the temperature range 1438 to 1566 K, Johnston and Marshall calculated a ΔH_o of 117.05 ± 1 kcal/mole for the heat of sublimation of NiO(s). Subsequently, Brewer and Mastick (2) re-examined the data of Johnston and Marshall and concluded that the O₂ pressures calculated would agree with a treatment of their data of the vaporization of NiO(g) to O₂ instead of NiO(g). Thus Brewer and Mastick determined that the dissociation mechanism is the only hypothesis which fits the data of Johnston and Marshall. Brewer and Mastick also performed three effusion experiments of NiO in a beryllium crucible at temperatures of 1816 K and 1782 K. From an analysis of the effused material they found that vaporization by dissociation is the chief method by which NiO vaporizes. They therefore concluded that the vapor pressure data of Johnston and Marshall would be an upper limit; they calculated a D_o of NiO(g) of ≤ 99 kcal/mole.