Gases, heat capacities intermolecular forces

Our discussion so far has dealt with the estimation of the nonideal gas bdiavior contribution to - Lj), i.e. Li - and (Lj - Lj). For Li - L ) the required ideal gas heat capacity depends on the internal molecular structure (e.g. bond lengths, vibration frequencies, and configuration of the constituent atoms) and, unfortunately, cannot be estimated through generalized correlations, such as those used for (L - L), which reflect the intermolecular forces. 

While the intermolecular forces are absent, the internal structure of the molecule and the attendant energies remain unaltered from the real gas. The internal energy, heat capacity, and related functions retain their specific values for each substance in the ideal-gas state. 

Despite the simplicity of these expressions for the heat capacity of a monatomic ideal gas, expressions for polyatomic molecules such as H2O can become quite complex. Of course HjO is not an ideal gas the question is what is the heat capacity (and other heat capacity-dependent properties) of a gas having molecules with the properties of the H2O molecule, but with no intermolecular forces This is a nontrivial problem in statistical mechanics. For H2O, the results of Wooley (1980) are used. These were fit to an equation by Cooper (1982) which, after some rearranging, was used by Wagner and Pru6 (2002) in their equation of state for H2O (see below). This equation is a summation of simple harmonic oscillator functions, and is... 

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