Thermochemistry standard data

A new volume of Landolt-Bomstein appeared in 1961. This deals with calorimetric quantities and is concerned with elements, alloys, and compounds, and with reaction enthalpies. Subjects covered include the experimental and theoretical basis of thermochemistry, standard values of molar enthalpies, entropies, enthalpies of formation, free energies of formation, and enthalpies of phase change. Planck, Einstein, and Debye functions, anharmonicity, and internal rotation are considered. The final section presents thermodynamic data for mixtures and solutions. 

For pure organic materials, it is also possible to calculate the heating value starting from the heats of formation found in tables of thermodynamic data. The NHV is obtained using the general relation of thermochemistry applicable to standard conditions of pressure and temperature (1 bar and 25°C)) f 9j... 

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. ... 

J. A. Martinho Simoes. Organometallic Thermochemistry Data. In NIST Chemistry WebBook NIST Standard Reference Database no. 69 P. J. Linstrom, W. G. Mallard, Eds. National Institute of Standards and Technology Gaithersburg, June 2005 (webbook.nist.gov). 

Equation 7.31 was derived from a least squares fit to the data given in W.N. Hubbard, D. W. Scott, G. Waddington. Standard States Corrections for Combustions inaBomb at Constant Volume. In Experimental Thermochemistry, vol. 1 F. D. Rossini, Ed. Interscience New York, 1956 p. 93. 

In Fig. 1, various elements involved with the development of detailed chemical kinetic mechanisms are illustrated. Generally, the objective of this effort is to predict macroscopic phenomena, e.g., species concentration profiles and heat release in a chemical reactor, from the knowledge of fundamental chemical and physical parameters, together with a mathematical model of the process. Some of the fundamental chemical parameters of interest are the thermochemistry of species, i.e., standard state heats of formation (A//f(To)), and absolute entropies (S(Tq)), and temperature-dependent specific heats (Cp(7)), and the rate parameter constants A, n, and E, for the associated elementary reactions (see Eq. (1)). As noted above, evaluated compilations exist for the determination of these parameters. Fundamental physical parameters of interest may be the Lennard-Jones parameters (e/ic, c), dipole moments (fi), polarizabilities (a), and rotational relaxation numbers (z ,) that are necessary for the calculation of transport parameters such as the viscosity (fx) and the thermal conductivity (k) of the mixture and species diffusion coefficients (Dij). These data, together with their associated uncertainties, are then used in modeling the macroscopic behavior of the chemically reacting system. The model is then subjected to sensitivity analysis to identify its elements that are most important in influencing predictions. 

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. 

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. 

Pilcher73 has recently reviewed the experimental data for a variety of carbonyl and thio-carbonyl compounds. Enthalpies of formation for dithiocarbonic acids, R2N—C(=S)SH, thioamides and thioureas were determined by standard calorimetric methods. For an important, older review on the thermochemistry and thermochemical kinetics of sulfur-containing compounds, see work of Benson74. 

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 ... 

Thermodynamic data from Lias, S.G., Bartmess, J.E., Liebman, J.F. et al. (1988) Gas-phase ion and neutral thermochemistry. J. Phys. Chem. Ref. Data, 17 (Suppl. 1). Also available on laser disk from NIST (National Institute of Standards and Technology), Washington, DC, USA). 

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). 

Superoxide (02 ) and the peroxyl radical (H02) have been intensively studied, and a good account of their thermochemistry is presented in Standard Potentials (pp. 60-63). They are related by the pKa of H02, which is 4.8 0.1 (51). The reduction potential of the 02/02 couple has been determined by a variety of methods, including, for example, the equilibria with various quinone-semiquinone systems. The value cited, — 0.33 V, is taken with respect to a standard state of 1 atm 02 pressure. When expressed relative to the 1 Mstandard state of 02, the potential is —0.16 V. Standard NBS data permit calculation of AfG° = 4.4 kJ/mol and 31.8 kJ/mol for H02 and 02, respectively. Some related potentials include 0.12 V for (H+, 02)/H02,1.44 V for (H+, H02)/H202, and 0.75 V for H02/H02 . 

Thermochemistry pertains to changes in energy or enthalpy that accompany chemical reactions generally one deals with the heat of reaction which refers to the quantity of heat Q that must be absorbed or released at the end of a process in order that the temperature at the conclusion of the reaction shall be the same as at the outset. As follows from the discussion of Section 1.19, at constant volume Qy — AEd, whereas at constant pressure QP - AHd. Here, AEd - Sd E and AHd - Sd A. wherein, as before, the vt are the stoichiometry coefficients in the chemical reaction S(1) iAi - 0, and < 0 or > 0 according to whether one deals with reagents or products. It is customary to provide all information normalized to 25°C and P - 1 atm. Where experimental data are taken under other conditions the data are corrected for standard conditions as discussed in Section 1.18 see also Exercise 3.8.1. 

NIST also maintains a website called the NIST Chemistry WebBook (http //webbook. nist.gov), which provides access to a broad array of data compiled under the Standard Reference Data Program. This site allows a search for thermochemical data for more than 7000 organic and small inorganic compounds, reaction thermochemistry data for over 8000 reactions, IR spectra for over 16,000 compounds, mass spectra for over 15,000 compounds, UV/VIS spectra for over 1600 compounds, electronic and vibrational spectra for over 5000 compounds, spectroscopic constants of over 600 diatomic molecules, ion energetics data for over 16,000 compounds, and thermophysical properties data for 74 selected fluids. The site allows general searches by formula, name, CAS registry number, author, and stracture and also a few specialized searches by properties like molar mass and vibrational energies. 

The enthalpy change for a chemical reaction in which all reactants and products are in their standard states and at a specified temperature is called the standard enthalpy (written AFf°) for that reaction. The standard enthalpy is the central tool in thermochemistry because it provides a systematic means for comparing the energy changes due to bond rearrangements in different reactions. Standard enthalpies can be calculated from tables of reference data. For this purpose, we need one additional concept. The standard enthalpy of formation AH° of a compound is defined to be the enthalpy change for the reaction that produces 1 mol of the compound from its elements in their stable states, all at 25°C and 1 atm pressure. For example, the standard enthalpy of formation of liquid water is the enthalpy change for the reaction... 

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.

The heat associated with a chemical reaction depends on the pressure and temperature at which the reaction is carried out. All thermochemical data presented here are for reactions carried out under standard conditions, which are a temperatnre of 298 K (24.85°C) and an applied pressure of one bar. The quantity of heat released in a reaction depends on the amount of material undergoing reaction. The chemical formulas that appear in a reaction each represent 1 mole (see article on Mole Concept ) of material for example, the symbol CH4 stands for 1 mole of methane having a mass of 16 grams (0.56 ounces), and the 2 02(g) tells us that 2 moles of oxygen are required. Thermochemistry also depends on the physical state of the reactants and products. For example, the heat liberated in equation (1) is 890... 

Many substance data have been determined repeatedly in the past. In many cases the deviations may be regarded as small from the standpoint of experimental thermochemistry. CODATA [13] have set up key values by choosing from all the available data, and these can be employed as standard values, par-... 

To provide as complete a model of the thermochemistry of zirconium as possible, the reviewers adopted the following approach each experiment is evaluated, reviewed, and, if necessary and possible, the results are recalculated to be consistent with other experimental conclusions and the SIT model (Appendix A). Uncertainties are assigned at this point, subjectively, if necessary. These results, with their associated uncertainties, are accepted if there is no clear reason to reject them and such data are reported in Chapter V. Accepted results are used in the determination of the relevant thermodynamic parameter or to confirm a parameter derived by other methods. Uncertainties assigned at the review stage and associated with the extrapolation to the 7=0 standard state are propagated (Appendix C), to the extent possible, throughout the procedure and the final recommended results with the associated uncertainties are given in Chapter III. In some cases, uncertainties are derived from a sensitivity analysis of the... 

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