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Enthalpy, entropy, and temperature of the phase change liquid-gas – An analysis of data of 719 materials regarding systematic correlations
Harald Mehling

In this paper, literature data of 719 materials on the enthalpy of evaporation hv and boiling temperature Tb, including calculated entropy of evaporation sv, have been analyzed regarding systematic correlations in phase change liquid-gas. It has been observed that several groups of materials exist that form clusters with an almost linear correlation in an hv – sv/R graph. One such cluster is formed by the metallic elements. A second cluster is formed by the halogens, the noble gases, and most of the diatomic molecules. A third cluster is formed by the linear alcanes. The knowledge about these clusters can be used to improve rules to predict certain values like in group-contribution methods, and maybe also to identify a common microscopic origin. Additionally, the analysis has shown that many materials have sv/R deviating significantly from the Trouton constant. Deviations are more frequent to higher values, and deviations are more pronounced in compounds with few atoms. A chemical reaction upon evaporation has been identified in 12 cases; the formation of larger compounds upon condensation seems to be the main reason for exceptionally high values of sv/R. The analysis thus led to a deeper understanding of the physical and chemical origins on the atomic and molecular level.

Keywords: enthalpy, entropy, temperature, evaporation, phase change, materials, elements, correlation, Trouton’s rule, bond type

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