JANAF tables

Table 4.3 summarizes values taken from the JANAF tables for the Gibbs free energy functions and standard enthalpies of formation for a few common substances. The JANAF tables provide a more complete tabulation. 

The changes in free energy of formation of Reaction (1) are shown in Fig. 2.1 as a function of temperature. " The values of AG were calculated using Eq. (1) above for each temperature. The Gibbs free-energy values of the reactants and products were obtained from the JANAF Tables.1 Other sources of thermodynamic data are listed inRef 6. These sources are generally accurate and satisfactory forthe thermodynamic calculations of most CVD reactions they are often revised and expanded. 

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. ... 

Table 2.1 Thermodynamic properties of AIN at selected temperatures (data are taken from NIST-JANAF tables [17]). Enthalpy reference temperature = T = 298.15 K p° = 1 bar.

Species N.B.S. Circ. 500 (1952) N.B.S. Tech. Note 270/3 (1969) JANAF Tables (1971) CATCH Tables (1972r CODATA Bulletins (1977, 1978) ... 

The column heading vZS refers to the results obtained by Van Zeggeren and Storey (1970) for the same system but using the JANAF tables of thermodynamic data. 

This useful and simple-to-use software package relies on Benson s group additivity scheme [47] to estimate thermochemical data for organic compounds in the gas phase. It also contains values from several NIST databases, including NIST Positive Ion Energetics [32] and JANAF Tables [22]. The first version of... 

This is the electronic form of the third edition of the JANAF Tables [22], As such, it is rather useful, but unfortunately the software is very primitive. 

Extensive tables of standard heats of formation are available, but they are not all at the same reference temperature. The most convenient are the compilations known as the JANAF [1] and NBS Tables [2], both of which use 298 K as the reference temperature. Table 1.1 lists some values of the heat of formation taken from the JANAF Thermochemical Tables. Actual JANAF tables are reproduced in Appendix A. These tables, which represent only a small selection from the JANAF volume, were chosen as those commonly used in combustion and to aid in solving the problem sets throughout this book. Note that, although the developments throughout this book take the reference state as 298 K, the JANAF tables also list A// for all temperatures. 

The reactants in most systems are considered to enter at the standard reference temperature 298 K. Consequently, the enthalpy terms in the braces for the reactants disappear. The JANAF tables tabulate, as a putative convenience, (Hj - H°29i) instead of (U°T - Ha(]). This type of tabulation is unfortunate since the reactants for systems using cryogenic fuels and oxidizers, such as those used in rockets, can enter the system at temperatures lower than the reference temperature. Indeed, the fuel and oxidizer individually could enter at different temperatures. Thus the summation in Eq. (1.10) is handled most conveniently by realizing that T 0 may vary with the substance j. 

Again, note that can be different for each reactant. Since the heats of formation throughout this text will always be considered as those evaluated at the reference temperature T0 = 298 K, the expression in braces becomes (Hj - /) - (//. - H° ) = (/I T - Hj), which is the value listed in the JANAF tables (see Appendix A). 

In the same context as the heat of formation, the JANAF tables have tabulated most conveniently the equilibrium constants of formation for practically every substance of concern in combustion systems. The equilibrium constant of formation (KPt[) is based on the equilibrium equation of formation of a species from its elements in their normal states. Thus by algebraic manipulation it is possible to determine the equilibrium constant of any reaction. In flame temperature calculations, by dealing only with equilibrium constants of formation, there is no chance of choosing a redundant set of equilibrium reactions. Of course, the equilibrium constant of formation for elements in their normal state is one. 

Calculate the detonation velocity in a gaseous mixture of 75% ozone (03) and 25% oxygen (02) initially at 298 K and 1 atm pressure. The only products after detonation are oxygen molecules and atoms. Take the AffjfO-,) I40kj/mol and all other thermochemical data from the JANAF tables in the appendixes. 

The JANAF tables specify a volatilization temperature of a condensed-phase material to be where the standard-state free energy A Gf approaches zero for a given equilibrium reaction, that is, M/fyl), M/)y(g). One can obtain a heat of vaporization for materials such as Li20(l), FeO(l), BeO(l), and MgO(l), which also exist in the gas phase, by the differences in the All" of the condensed and gas phases at this volatilization temperature. This type of thermodynamic calculation attempts to specify a true equilibrium thermodynamic volatilization temperature and enthalpy of volatilization at 1 atm. Values determined in this manner would not correspond to those calculated by the approach described simply because the procedure discussed takes into account the fact that some of the condensed-phase species dissociate upon volatilization. 

Examination of Fig. 9.4 for the B-N2 system reveals that BN decomposes into gaseous nitrogen and liquid boron. Since these elements are in their standard states at 1 atm and the decomposition temperature, the A/fy must equal the enthalpy of formation All" of the BN at the decomposition temperature. Indeed, the A/fy (300kJ/mol) calculated by the means described agrees with the value of AH°T (300kJ/mol) given in the JANAF tables, as it should. The same condition holds for the Si-N2 system. 

The thermochemical data for the chemical compounds that follow in this appendix are extracted directly from the JANAF tables [ JANAF thermochemical tables, 3rd Ed., Chase, M. W., Jr., Davies, C. A., Davies, J. R., Jr., Fulrip, D. J., McDonald, R. A., and Syverud, A. N.,./. Phys. Chem. Ref. Data 14, Suppl. 1 (1985)]. The compounds chosen from the numerous ones given are those believed to be most frequently used and those required to solve some of the problem sets given in Chapter 1. Since SI units have been used in the JANAF tables, these units were chosen as the standard throughout. Conversion to cgs units is readily accomplished by use of the conversion factors in this appendix (Table Al). Table A2 contains the thermochemical data. 

The ordered listing of the chemical compounds in Table A2 is the same as that in the JANAF tables and is alphabetical according to the chemical formula with the lowest order letter in the formula determining the position. The thermochemical tables have the following order ... 

Thermochemical data were originally developed by the US Joint Army, Navy, Air Force (JANAF). Today this information is simply known as the JANAF tables. 

Standard entropies for many substances are available in tables such as Tables 11.2 through 11.6. Generally, the values listed are for 298.15 K, but many of the original sources, such as the tables of the Thermodynamics Research Center, the JANAF tables, or the Geological Survey tables, give values for other temperatures also. If heat capacity data are available, entropy values for one temperature can be converted to those for another temperature by the methods discussed in Section 11.4. 

Computations of the thermochemical values of various combinations of oxidizers and fuels can be found in the JANAF tables.FI Practical computations are carried out by the use of computer programs such as the cited NASA program.PI Table 3.2 shows an example of a computation comparing the detonation and deflagration characteristics of the gaseous mixture 2H2 -1- O2. 

Values for formation reactions can be fonnd in the JANAF tables [10], Another way in which a molten metal can attack an oxide is throngh compound formation, snch as the formation of spinel ... 

AU data, unless otherwise noted, are from the JANAF tables, see Chase, M. W., Jr. 1998. J. Phys. 

We will now consider a practical example of calculating thermochemical properties for the species CH3. Actually a lot is known about the CH3 radical, and we choose it as example in order to compare the calculated results with experimental data. The NIST-JANAF Thermochemical Tables [62] are a standard source for experimental thermochemical data, as well as moments of inertia, vibrational frequencies, and the like. The NIST-JANAF Tables use the same basic approach outlined here to calculate the temperature dependence for their thermodynamic data, based on species vibrational frequencies and moments of inertia. 

The heat capacity at constant volume Cv from the translational and rotational degrees of freedom are determined via Eqs. 8.124 and 8.128, the vibrational contributions to Cv are calculated by Eq. 8.129, and the electronic contribution to Cv is from Eq. 8.123. For an ideal gas, Cp = Cu + R, so Cp=41.418 J/mole/K. The experimental value is Cp=38.693 J/mole/K. Agreement with experiment gets better at higher temperature. At 1000 K, Cp from our calculation is 59.775 J/mole/K, compared to a value of 58.954 from the NIST-JANAF Tables. The difference between theory and experiment is due entirely to our use of the vibrational frequencies obtained from the ab initio results, rather than using the experimental frequencies. 

We compare our calculated values of Cp, S, and H — H(Tref) with data from the NIST-JANAF Tables over a wide range of temperatures in Fig. 8.8. Several comments are in order. Overall, we see excellent agreement with the standard reference values. The curves and data points in Fig. 8.8 would have been indistinguishable had we used the experimental vibrational frequencies (which were extracted from the NIST-JANAF Tables) instead of the frequencies from the ab initio results. 

Fortunately, much experimental thermochemical information exists in the literature and in data compilations. The NIST-JANAF Thermochemical Tables [62] is a particularly useful source of data. In the following, we discuss some of the data conventions adopted by the NIST-JANAF Tables in reporting this information. These compilations adopt sound and useful conventions based on fundamental thermodynamic considerations. This brief discussion offers a simple explanation of the quantities reported there. 

The values adopted here for the enthalpies of the above compounds, as well as those for the pertinent gaseous atoms (from JANAF tables), are summarized in Table I. 

The JANAF Tables (March 31, 1965) estimated A//298 CC13 = 35.0 kcal/mole, but this value is surely incorrect. The earliest reliable data which permits calculation of A77298 CC13 is that of Farmer et al.,57 who obtained D CC13—Cl = 67.9 + 3 and D CC13—Br = 49.5 3 kcal/mole from appearance potential measurements. With A//298 CC]4 = -25.94 and A 298 CCl3Br = -8.7 1 kcal/mole,23 A 298 CC13 becomes, respectively, either 13.0 or 14.1 kcal/mole. 

Ashmore and Burnett8 examined the thermal decomposition of nitrogen dioxide between 200 and 434°C. They found that log k 5 = 8.59 — 23.9/0. Using the values of 16.8 kcal/mole and 60 eu for the heat of formation and entropy of N03 (averaged from the results of Schott and Davidson371 and Ray and Ogg357), they found the equilibrium constant log J 5> 5 = 1.03 + 22.2/0, whence log k5 = 9.62 - 1.7/0. This leads to a value of 8.38 at room temperature. If thermodynamic values listed in JANAF tables are used, K5t 5 is larger at room temperature by a factor of 1.8 log k5 would be about 8.65. This paper is discussed further in Section IV-B-3. 

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