DISSOCIATION ENERGIES OF THE MONOCHLORIDES AND DICHLORIDES OF CR, MN, FE, CO, AND NI

摘要

The gaseous equilibria MCl + Ag = M + AgCl and M + MCl(2) = 2 MCl, where M = Cr, Mn, Fe, Co, and Ni were studied by mass spectrometry in the region around 1500 K, and thermochemical data were derived by second and third law methods. Dissociation energies of the metal monochlorides were evaluated by reference to AgCl, leading to the values at 298 K in kcal/mol(-1): CrCl, 90.3; MnCl, 80.9; FeCl, 78.8; CoCl, 80.7; NiCl, 90.1; all +/-1.6 kcal mol(-1). Entropies and enthalpies of sublimation of the solid dichlorides were derived from sublimation pressures measured by the torsion-effusion method. Sublimation enthalpies lead to MCl(2) dissociation energies that are in close agreement with values derived from the mass spectrometric results; D(MCl(2)) values at 298 K are CrCl2, 179.3; MnCl2, 188.3; FeCl2, 190.1; CoCl2, 182.6; and NiCl2, 178.2; all +/-1.0 kcal mol(-1). Vapor entropies evaluated from the sublimation data show that the electronic energy levels of the doubly-charged free metal ions provide a reasonable representation of the electronic entropies of the metal dichlorides; in addition, the entropy data are consistent with a bent structure for CrCl2, but with linear structures for the other dichlorides. Judging from the second and third law agreement for the monochloride exchange reactions, the M(+) free ion electronic levels also seem appropriate for the transition metal MCl species. (C) 1995 American Institute of Physics. [References: 34]