Heat effects of hydrogen fluoride from two thermodynamic models

We examine two published thermodynamic models for their ability to describe the excess enthalpy and heat capacity of hydrogen fluoride (HF) over a range of temperatures. Emphasis is placed on the behavior away from the saturation curve and, thus, away from the conditions used to fit the models. The first model is the association + equation of state (AEOS) treatment of Anderko, which utilizes a biased yet unconstrained association scheme. The second model is due to Kao and co-workers and treats HF as a mixture of only monomers and hexamers. Both models are found to be effective in capturing important qualitative features of the HF heat effects. The AEOS describes well the location but not the magnitude of associated-induced maxima in the vapor-phase heat capacities, while the Kao model describes the magnitude well but is slightly off in characterizing the locations of these maxima.