Evaluation of Departure Functions

Examination of Eqs 9.3.2,9.3.10,9.3.15, and 9.3.19 indicates that evaluation of isothermd changes respectively for U, H, A, and G in terms of PVT data - directly or through an equation of state - can be accomplished readily for the last two only. Thus  

Since most equations of state (EoS) are in the form P = f(V,T), rather than V =f P,T), we choose Eq.9.7.3 and, to eliminate the problem of integrating between an ideal gas and a real gas state, we break the integral into two parts  

Finally, to avoid the difficulties involved with the infinity limit, we add to and subtract from the right-hand side the integral  

A°-A = - p- JdV + l rinz The other departure fimctions can be now easily developed  

Evaluation of d arture functions from experimental PVT data can be carried out graphically using these expressions. This t proach, however, involves evaluation of derivatives from plots of experimental data which introduces substantial uncertainties. In the typical case, therefore, the PVT data are fitted to an accurate EoS to facilitate the calculations. Or, in the absence of such data, some generalized EoS can be used. 

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