Thermochemical Values

The enthalpy of formation, AfHJ=231 1.3 kJ/mol, was derived from the adiabatic ionization potential of N2H3 and the enthalpies of formation of N2H4 and H [1]. Ab initio calculations gave AfH = 238.5 [2], 226.0 [3], 222.6 [4], and 227.7 kJ/mol [5]. A much higher value of 364 kJ/ mol [6] was based on the erroneous assumption that the radical has the same planar structure as the N2H3 ion. 

---

Eormulae and Calculated Thermochemical Values for Propellants, Picatinny Arsenal (PTA), Dover, N.J., 1956. 

W. F. Egelhoff Jr., Thermochemical Values for Cu-Ni Surface and Interface Segregation Deduced from Core-Level Binding Energy Shifts, Phys. Rev. Lett. 50 587 (1983)... 

Values in kJ/mol calculated from primary data, using updated reference thermochemical values from... 

K enthalpy of formation in kJ/mol calculated using updated auxiliary thermochemical values see reference 95. 

Table 3. Standard enthalpy of formation of the gaseous metal atom, A/ff (M, g), coordina-tion number n of the atom in the bulk metal at room temperature and values of M and T calculated by equations 1 and 3. Thermochemical values in kj mol-1...

Unless your instructor has told you to memorize specific thermochemical values, you will be provided the numbers you need. Most textbooks have a table of values in the chapter and an additional table in Appendix. 

We have attempted to collect in appendix B a list of the most used thermochemical databases. Each one has been built with a particular class of substances and a specific set of properties in mind. We can find compilations of thermochemical values for gas-phase ions, for condensed and gas-phase pure organic compounds, for organometallic molecules, for gas-phase organic free radicals, for inorganic substances, and so on. Most are available in printed form, some are distributed in a software package, and a few can be used online, through the World Wide Web. 

It is instructive to compare the thermochemical values with those obtained by the same authors when they assumed the ideal gas model (i.c.,/co = pco) and neglected the CO solubility change with temperature ArH%12 = 45.5 kJ mol-1 and ATS 72 = 267 J K-1 mol-1. 

What main conclusions can we draw from the three examples discussed here First, although van t Hoff plots should involve Km rather than Kc data, the use of the latter may afford sensible and possibly accurate thermochemical values (always under the assumption of ideal solutions ), particularly if the density term of equation 14.5 is considered in the calculation of the reaction entropy. Second, due to the lack of gas solubility data, the second law method is much... 

The Active Thermochemical Tables (ATcT) are the result of an effort to derive accurate, reliable, and internally consistent thermochemical values based on a thermochemical network approach. They contain data for over 350 compounds. Recommended values for key compounds such as CC>2(g), H20(g), and CH4(g) are slightly different from those given by CODATA. 

This is, in our opinion, the primary source of thermochemical values and therefore it should be the starting point of all the other databases. The selections have been made by the Task Group on Key Values for Thermodynamics appointed in 1968 by the Committee on Data for Science and Technology (CODATA) of the International Council of Scientific Unions. Unfortunately, the number of species for which data are recommended in the final report is rather small ( 150). 

The reader should refer to the original tables for the reference material on which the thermochemical data are based. The reference state used in Chapter 1 was chosen as 298 K consequently, the thermochemical values at this temperature are identified from this listing. The logarithm of the equilibrium constant is to the base 10. The unit notation (J/K/mol) is equivalent to (JK mol ). Supplemental thermochemical data for species included in the reaction listing of Appendix C, and not given in Table A2, are listed in Table A3. These data, in combination with those of Table A2, may be used to calculate heats of reaction and reverse reaction rate constants as described in Chapter 2. References for the thermochemical data cited in Table A3 may be found in the respective references for the chemical mechanisms of Appendix C. 

Quantum-chemical calculations now can provide values of enthalpies of formation with a precision and accuracy comparable with thermochemical values and those calculated from statistical thermodynamics. The basis for these calculations is beyond the scope of this text, but it is interesting to observe some values calculated in this way for comparison with other values in Tables 4.3-4.5. The data in Table 4.6 were obtained by a method called Gaussian-3 (G3) [5]. 

TRPD has been applied to quite a number of ion dissociations, yielding new thermochemical values as well as insight into the dissociation mechanisms of nonsimple systems. In keeping with the theme of this chapter, we will emphasize... 

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. 

Flame temperatures can be measured directly, using special high-temperature optical methods. They can also be calculated (estimated) using heat of reaction data and thermochemical values for heat of fusion and vaporization, heat capacity, and transition temperatures. Calculated values tend to be higher than the actual experimental results, due to heat loss to the surroundings as well as the endothermic decomposition of some of the reaction products. Details regarding these calculations, with several examples, have been published [5]. 

The thermochemical values used in this work have been taken for the most part from the compilation Selected Values of Chemical Thermo-... 

Resonance of the carbonate type occurs in urea, CO(NHs), and guanidine, CNH(NH )2. For urea the thermochemical value of the resonance energy is 37 kcal/mole, and for guanidine 47 kcal/mole, the latter being calculated with use of the estimated value 24 kcal/mole for the heat of sublimation. 

Where it is expedient and necessary, each unusual method of calculation used in the text is explained. The ordinary and simple methods of calculating thermochemical values are assumed to be known to the reader, and the advanced methods which are used but not described may be found explained in one of the many current books on advanced physical chemistry or thermodynamics. 

Good thermochemical values for halogenated compounds have lagged behind those for hydrocarbons. Until recent years only the crudest estimates were available, even for some of the most common compounds. Now, however, reliable thermodynamic data have become available for some species, though there is still uncertainty for many other species, especially those containing oxygen. 

Using the dihydrogen salt, NaH2As03, von Zawidzki2 determined the equivalent conductivity in the presence of N/32 arsenious acid in order to diminish hydrolytic dissociation, and concluded that it resembled the salt of a monobasic acid. At extreme dilutions the equivalent conductivity increased, apparently owing to hydrolysis and oxidation. This view that arsenious acid is essentially a feeble monobasic acid is supported by Thomsen s thermochemical values for the heats of neutralisation of the acid (see p. 140). 

Heterocyclic Compounds with Indole and Carbazole Systems , Interscience, NY(1954), 70-109 17)P.Tavernier M.Lamouroux, MP 39, 355(1957)(Thermochem values) 18)Sax(1957), 435(Toxicity) 19)CondChemDict(l96l), 215 20)... 

In this article both unsensitized and mercury vapor sensitized reactions are discussed. The plan has been to proceed from the chemically simplest system, oxygen molecule, to the chemically most complex, ozone plus hydrogen peroxide. Unless otherwise stated, thermochemical values have been taken from compilations of the National Bureau of Standards (68) and are reported in kcal. for the reaction represented in mole amounts. Conversion factors for energy units, spectroscopic notation, and unless noted, spectroscopically based values have been taken from... 

Gaviola and Wood have also shown that the fluorescence spectrum of both HgH and OH may be obtained from mixtures of mercury and water vapor irradiated with mercury resonance radiation, 2537 A. Beutler and Rabinowitch (15) considered this and the thermochemical values... 

---