The three laws of thermodynamics provide the theoretical basis required to master nearly all the concepts that are relevant in discussions of molecular energetics. We shall not dwell on those laws, because they are mandatory in any general physical chemistry course, but we will ponder some of their outcomes. It is also necessary to agree on basic matters, such as units, nomenclature, standard states, thermochemical consistency, uncertainties, and the definition of the most common thermochemical quantities. The International Union of Pure and Applied Chemistry (IUPAC) recommends the use of the International System of Units (SI) in all scientific and technical publications. Appendix A list the names and symbols adopted for the seven SI base units, together with several SI derived units, which have special names and are relevant in molecular energetics. Among the base units, the kelvin (symbol K) and the mole (mol), representing thermodynamic temperature and amount of substance, respectively, are of particular importance. Derived units include the SI unit of energy, the joule (J), and the SI unit of pressure, the pascal (Pa). It is generally acknowledged that the International System has brought order out of the previous multisystem chaos. The IUPAC recommendations regarding units will therefore be followed in the present book. In some countries, like the United States, units like the calorie, the torr, and the atmosphere, for example, are still common, but they have gradually been replaced by their SI equivalents. However, non-SI units, such as the electronvolt (eV) and the hartree (Eh) are more convenient to use in many cases. These units, particularly the eV, are prevalent in a large number of recent publications on molecular energetics. A list of some non-SI units, together with their SI values, and a table containing the “best” values of some fundamental physical constants are given in appendix A. The names and symbols of physical chemical quantities have also been recommended by the IUPAC. It would be tedious to list even a minor fraction of the suggestions on symbols, subscripts, and superscripts, in italic, Roman, or Greek fonts. But these matters have importance, and a few common symbols will be described here.
A critical review of the proposed definitions of fundamental chemical quantities and their impact on chemical communities (IUPAC Technical Report)
