Standard states thermochemical databases

The values included in thermochemical databases (see appendix B) are normally referred to the substances in their standard states. The standard state notion, which is a consequence of the mathematical formalism used to describe the thermodynamics of reaction and phase equilibria [1], greatly simplifies the calculation of thermochemical quantities for the infinite variety of real processes, that is, those where one or more substances are not in their standard states. This situation will be exemplified in several chapters of the present book, but several case studies are discussed here. 

It must be stressed that the temperature is not included in the definition of standard state. Nevertheless, all modern thermochemical databases list the values at 298.15 K, so this is now regarded as a reference temperature. As shown in appendix B, very few data compilations give values at any other temperature. 

The case of liquid solutions is more complicated because the conventions vary. These are always stated in introductory chapters of the thermochemical databases and deserve a careful reading. In most tables and in the present book, it is agreed that the standard state for the solvent is the pure solvent under the pressure of 1 bar (which corresponds to unit activity). For the solute, the standard state may refer to the substance in a hypothetical ideal solution at unit molality (the amount of substance of solute per kilogram of solvent) or at mole fraction x = 1. 

K. The standard state pressure is taken as 1 bar (100,000 Pa). The tables are presented in the JAN AF Thermochemical Tables format (Reference 2). The numerical data are extracted from IVTANTHERMO databases except for C2H4O, CjH O, CjH, QH O, CjoHg, and CH5N, which are based upon TRC Tables. See the references for information on standard states and other details. 

The database contains thermochemical property data at standard state conditions for over 15 000 inorganic substances and small organic molecules having up to two carbon atoms. NISTTHERMO corresponds to the Tables of Chemical Thermodynamic Properties. Standard state properties at 298.15 K and 1 bar stored are enthalpy of formation, Gibbs energy of formation for the elements, enthalpy, heat capacity at constant pressure, and entropy. 

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