Relation between the thermodynamic parameters at the critical point and determination of the effective volumes of molecules (Q735713)

The expression for the heat of evaporation is derived based on the first law of thermodynamics. The calculated values of the heat of evaporation are presented for various atoms and molecules. The results of calculation using the free volume agree better with the experimental data as compared with the calculation based on the geometrical volume. There is a significant error in the behavior of heavy atoms like Xenon, Krypton as well as the methane molecules. Expressions for the geometric radii of the inert gases, alkali metals, methane and ammonia using the spherical model and for the symmetric diatomic molecules using the ellipsoidal model. The results of calculations based on the geometrical models of the heat of evaporation shows better agreement with the experimental data. However, the atoms like Krypton and ammonia molecules show no improvement.