Thermochemistry data sources

Tables of AH° for compounds are the most important data source for thermochemistry. From them it is easy to calculate AH° for reactions of the compounds, and thereby systematically compare the energy changes due to bond rearrangements in different reactions. Appendix D gives a short table of standard enthalpies of formation at 25°C. The following example shows how they can be used to determine enthalpy changes for reactions performed at 25°C and 1 atm pressure.

We discuss the basic thermochemistry and thermodynamic fundamentals, describe important data sources and experimental techniques, criticize the reliability of the database, and discuss in detail methods for calculating or estimating thermochemical... 

Other major sources of data on the thermochemistry of sulfur-containing compounds are the review by Benson19, which is of particular value in evaluating data on radicals and other labile species, and the review of the present author on the thermochemistry of the inorganic S—O—F compounds20. 

The needed thermochemistry for many thousands of molecules is available from standard sources such as the JANAF tables. " Polynomial fits of this data in the form required by our kinetics software are also available. However, experimental thermochemical data is often lacking for many of the intermediate species that should be included in a detailed kinetics mechanism. Standard methods have been developed for estimating these properties, discussed in detail by Benson. ... 

F. D. Bichowsky and F. D. Rossini, The Thermochemistry of Chemical Substances, Reinhold New York, 1936. The authors report results at 18°C instead of the already more accepted 25°C because a major source of the experimental data they use is the classic work of J. Thomsen, Thermochemische Untersuchungen, Vol. I-IV, J. A. Barth, Leipzig, 1882-86. 

Our intention here is certainly not to survey either ion thermochemistry or methods for its study, but to describe and illustrate some new approaches particularly related to the ion-trapping FTICR instrument. The NIST tabulation describes many of the standard techniques and collects most of the existing data. Reference 3 is a useful source of bond strength data and techniques for smaller ions and molecules, and Ref. 4 tabulates bond strength information for many cluster ions. 

G3 is a recipe involving a variety of different models with the purpose of providing accurate thermochemical data. Original reference (a) L. A. Curtiss, K. Raghavachari, PC. Redfem, V. Rassolov and J.A. Pople, J. Chem. Phys., 109, 7764 (1998). For an up-to-date, on-line source of G3 data see (b) L A. Curtiss, Computational Thermochemistry, chemistry.anl.gov/compmat/ comptherm.htm ... 

The sources of thermodynamic data and reaction-specific data are increasing rapidly, and this overview is by no means comprehensive. Still, important information on thermochemistry for important species or rate constants for key reactions may be unavailable. In such situations data estimation procedures may be employed. A number of simple as well as more advanced methods can be used for this purpose. For an overview the reader may consult the review by Senkan [356]. 

The sources and magnitudes of thermochemical data have been the subject of many entries in this Encycl. The use of the data presupposes a general acquantance with chemical thermodynamics (next article) and with detonation theory (Vol 4, D268-L to D298-R). The principle difference between classic thermodynamics and the thermochemistry of reactive systems is that expins and deflagrations do not represent equilibrium processes. In principle, the heat of reaction is obtained by ... 

In some cases, a literature source of thermodynamic data may exist, allowing one to perform the conversion. Fortunately, standard references (such as Refs. 168 and 180) frequently tabulate both the condensed and gas phase thermochemistry values. When that is not the case, the following relations may be used, where the enthalpies and entropies of vaporization and fusion must be at 25 °C ... 

Source Data are largely from Metallurgical Thermochemistry, O. Kubaschewski and C.B. Alcock, 5th ed. Pergamon Press, 1979. 

This review makes extensive use of ancillary thermodynamic data. The source of such data, if not specified, is the NBS tables (323). The potentials in Table A-I, in most cases, have not been measured directly, and so there is considerable uncertainty in their magnitudes. Only in one case, the C102/C102 system, has the potential been corrected for activity coefficients to obtain a standard potential. A common approach in estimating the thermochemistry of aqueous free radicals is to use gas-phase data with appropriate guesses of solvation energies an important source of data for the gas-phase species is the JANAF tables (80). 

The thermal and photolytic decomposition of hydrazine and substituted hydrazines and azines provide a source of many nitrogen containing radicals . For this reason a considerable amount of thermochemical data is now available relating to the bond energies of the bonds involved in these reactions. Friswell and Gowen-lock" have recently reviewed the chemistry and thermochemistry of nitrogen containing radicals, and the salient features of their article will serve as an introduction to this section. 

In addition to the estimated properties, we measured the thermochemistry of several important vapor species. These measurements were conducted in a Knudsen effusion cell using special line-of-sight vaporization under subambient pressures with flowing O2 and H2O vapor mixtures [4]. The gaseous species over silica [5], manganese oxide [6], lanthana, alumina, and palladium metal were detected and relative partial pressures measured as a function of temperature. These vapor pressure measurements were calibrated by using the known metal atom or binary metal oxide volatility as a calibration source. Oxide species concentrations were measured relative to that of a reference compound, e.g., metal atom. The identification of oxide and hydroxide compounds was facilitated by Ae technique of threshold electron ionization [7]. These data were then evaluated using estimated entropy functions and the third law temperatures. 

Data mainly from F. R. Bichowsky and F. D. Rossini, The Thermochemistry of the Chemical Substances, 1936 (heats of formation) W. M. Latimer, The Oxidation States of the Elements and their Potentials in Aqueous Solutions, 1938 (free energies and entropies) Kelley, U. S. Bur. Mines Bull., 434 (1941) (entropies) see also, Chapter XIII, ref. 7. Because of temperature differences and the variety of sources, the data are not always completely consistent the deviations are, however, usually not greater than the experimental errors. 

Computer-assisted thermochemistry is a tool that can be applied in many fields today. In particular, with the aid of reliable thermochemical source data and appropriate application software, optimum operating tempera tures, reacting amounts and/or gas pressures necessary to obtain a product of the required purity can be calculated. Costly and time-consuming experimental work can thereby be reduced considerably [1]. 

This chapter presented a short noncomprehensive overview of some of the aspects of thermochemistry that are relevant to scientific and engineering applications. It explained the calculation methods used to obtain thermodynamic data and showed how to evaluate the soundness of different methods and some of their pitfalls. In addition, it provided the sources of accurate thermodynamic data and explanation of the formats used. This makes the use of these data in engineering applications such as computational fluid dynamic (CFD) simulation relatively easy. 

An excellent source of much thermochemical data is S. J. Ashcroft and C. T. Mortimer, Thermochemistry of transition metal complexes. Academic Press, London and New York 1979, although, today, computer databases are better if access can be gained to one. 

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