Equation of State and Thermodynamic Properties of Gases at High Temperatures. I. Diatomic Molecules

Methods for evaluating the thermodynamic properties of assemblies of chemically reacting unionized atoms are discussed. The desirability of using the virial coefficients at high temperatures instead of the customary use of the molecular partition functions with anharmonicity corrections is emphasized. The most realistic three‐parameter diatomic potential energy function that is available at present, i.e., the Rydberg potential, U/Ue=−(1+b′ξ) exp(−b′ξ) with ξ=(r/re)−1 is selected for the evaluation of the classical second virial coefficient. B(T), T(dB/dT) and T2(d2B/dT2) are obtained as linear combinations involving the five functions: Ak(θ)= ∑ n=1∞[(θe) n/nn+k], [k= —1, 0, 1, 2, and 3, and θ=(Ue/kT)] with only the coefficients that multiply Ak depending on b′. Ak is tabulated for 0.05≤θ≤10. A simple expression for estimating the quantum correction to B(T) is given. The inclusion of the contribution of the higher diatomic electronic states to B(T) is considered. The treatment is applied to sodium (includi...