Thermochemistry and Thermodynamics of Substances

Abstract

In recent years interest in the thermodynamic properties of substances at very high temperatures and pressures has increased. For our present pur­ poses, we may define high temperatures and high pressures to be conditions under which any material of which an experimental apparatus might be made cannot long maintain its integrity, i.e., T >3000°C. and P > 2 X 105 atm. Such temperatures and pressures have long been of interest in geophysi­ cal (19, 50, 114 to 116) and astrophysical (7, 117 to 119) applications. More recently, these conditions have been attained and measured in transient ex­ periments. Since the experimental techniques are so difficult, a large role has necessarily been played by theoretical calculations. The present article will be divided into three main sections. The first will be devoted to the so-called Thomas-Fermi method which is most appropriate to matter at high densities, the second to a discussion of recent experimental developments, and the third to theoretical methods, most appropriate at low densities, which treat matter as a mixture of chemically reacting ideal gases. In a still very illuminating review of this subject written in 1936 Hund (124) has shown how the variation of the density of matter over a very wide range of temperature and pressure can be sketched from simple quantum and statistical mechanical ideas. At sufficiently low temperatures and laboratory pressures, the thermo­ dynamically stable state for all substances is a condensed phase, either liquid or solid, with a density which is nearly independent of temperature and pres­ sure, but highly dependent on position in the periodic table. The binding energy per atom corresponds to temperatures varying from tens and hun­ dreds of degrees for molecular crystals to several thousands of degrees for ionic or valence crystals; the atomic volumes of elements vary tenfold from transition elements to the alkali metals (and crystalline rare gases) which are the largest. As the temperature is increased at a fixed pressure of say one atm. the condensed phase becomes unstable against a gaseous phase which may consist entirely of atoms or more frequently atoms bound together by chemical valence forces into molecules. At temperatures of perhaps 2 X 1040K. all effects of chemical binding have disappeared. Above 2 X 104°K., ionization becomes increasingly important up to about 1060K. when all of the electrons