Enthalpy of sublimation AHs

The magnitudes of the three attractive components of the intermolecular energy per mole of gas of some simple molecules are compared in Table 4.4.2, along with the bond energy of the polyatomic species and the enthalpy of sublimation (AH°). The calculation is based on the assumption that the molecules interact in pairs at a separation 8 where the total potential energy U(8) = 0. 

The measured value for the enthalpy of sublimation, AH, of the ethyl complex, and the values estimated for the remaining molecules are also included in Table I. While it is obvious that this exercise involved speculation, its effect on the bond enthalpy data will (hopefullyl) be covered by the assigned uncertainties. Anyhow, it is stressed that the experimental values for the enthalpies of formation of the crystalline complexes enable to make reliable estimates of reaction enthalpies in solution or in standard reference states. 

In Uhllg s earlier view, one of the factors determining passivation (and chemisorption) is the ratio of the work function to the enthalpy of sublimation AH. If this ratio is less than unity, conditions are favorafle to passivation because the electron would escape more readily than the atom, favoring the chemisorption of substances like oxygen. A passive film is composed, then, from chemisorbed atomic and molecular oxygen (supplemented perhaps by OH and H 0). The formation of chemical bonds satisfies the surface affinities of the metal without metal atoms leaving their lattice site. 

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