Standard Thermodynamic Values for Selected Substances at

APPENDIX B Standard Thermodynamic Values for Selected Substances at 298 K... 

Values of the standard enthalpy of formation at 25°C of some substances from the elements in their standard states are given in Tables VI-1 to VI-10. These values for the most part have been taken from the reference book Selected Values of Chemical Thermodynamic Properties, circular of the U.S. Bureau of Standards No. 500 (1952). Values are also given in Handbook of Chemistiy and Physics and other reference books. A selection of values for compounds of the transition metals, which are not included in the following tables, can be found in General Chemistty, L. Pauling, W. H. Freeman and Company, San Francisco, 3rd edition, 1970. 

The values given in the following table for the heats and free energies of formation of inorganic compounds are derived from a) Bichowsky and Rossini, Thermochemistry of the Chemical Substances, Reinhold, New York, 1936 (h) Latimer, Oxidation States of the Elements and Their Potentials in Aqueous Solution, Prentice-Hall, New York, 1938 (c) the tables of the American Petroleum Institute Research Project 44 at the National Bureau of Standards and (d) the tables of Selected Values of Chemical Thermodynamic Properties of the National Bureau of Standards. The reader is referred to the preceding books and tables for additional details as to methods of calculation, standard states, and so on. 

The standard temperature selected for the values given in this book is 18° Centigrade, following the procedure of the thermochemistry section (Bichowsky1) of the International Critical Tables. The authors have been reluctant not to use the almost universally accepted standard temperature of 25° Centigrade for thermodynamic calculations but the selection of 18° as the standard temperature is practically necessary in this case because all of the monumental work of Julius Thomsen and of Marcellin Berthelot was done at or near 18° and there are not now available sufficient heat capacity data with which to make accurate conversion to 25° (this is especially important for reactions involving substances in aqueous solution where the temperature coefficient is usually very large). In later years, as the data on heat capacities become available, or as the heats of many of the reactions, which have until the present time been measured only by Thomsen or Berthelot or both, are redetermined, it will be quite feasible to use 25° as the standard temperature. 

In a preceding paragraph we have described the heat of a reaction as the amount of heat evolved or absorbed when the reaction takes place at constant temperature and pressure. Two mutually contradictory definitions of heat of reactions are used at the present time in textbooks and reference books. For over a century it has been customary to define the heat of a reaction (heat of combustion, heat of formation, heat of solution) as the heat evolved in the process that is, as —AH°. On the other hand, heats of fusion and vaporization have been defined as the heat absorbed during fusion or vaporization. During the last few years many chemists have adopted the definition of heat of reaction as the heat absorbed in the process. This usage is to be found, for example, in the valuable reference book Selected Values of Chemical Thermodynamic Properties, Circular of the U.S. Bureau of Standards No. 500 (1952), in which values of heats of formation of compounds from elements in their standard states and some other properties of substances are given. 

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