Equilibrium state thermodynamic properties of rare gas dimers and trimers obtained from equations of state and statistical thermodynamics: Application to neon, argon, krypton and xenon

Abstract

The equilibrium thermodynamic properties of gas phase dimerization and trimerization of Ne, Ar, Kr, and Xe are calculated from equations of state (EOS). The equilibrium constants, changes in enthalpy, Gibbs energy, entropy and heat capacity are determined over 10-kelvin ranges are reported. These quantities are calculated from accurate and precise equations of state available from the NIST Thermophysical Tables. In addition, the molar entropies and heat capacities are calculated for the dimers, and molar entropies for the trimers. Statistical thermodynamic (ST) methods are used to complement these calculations and employ dimer partition functions obtained from a sum-over-state approach that includes both bound and metastable rovibrational states. For the trimers, such calculations are carried out only for neon, for which the rovibrational energies of the bound states are available. The results of the EOS and ST calculations for the Ne – Xe dimers are in good agreement with each other, while those for the neon trimer are less so. The effect of excluding the metastable states from the dimer partition functions is demonstrated. The results indicate that the thermodynamic properties of dimerization/trimerization for polyatomic molecules may be obtained if accurate EOS data is available, suggesting that the method may be applied to polyatomic molecules. Relationships between the enthalpies of dissociation of the dimers and trimers and the enthalpies of vaporization of the respective liquids are demonstrated.