Subject thermochemical data

Boron Monoxide and Dioxide. High temperature vapor phases of BO, B2O3, and BO2 have been the subject of a number of spectroscopic and mass spectrometric studies aimed at developiag theories of bonding, electronic stmctures, and thermochemical data (1,34). Values for the principal thermodynamic functions have been calculated and compiled for these gases (35). 

Boron subhaHdes are binary compounds of boron and the halogens, where the atomic ratio of halogen to boron is less than 3. The boron monohaUdes, BCl, [20583-55-5] bromoborane(l) [19961-29-6] BBr, and iodoborane(l) [13842-56-3] BI, are unstable species that have been observed spectroscopicaHy when the respective ttihaUdes were subjected to a discharge (5). Boron dihaUde radicals have been studied, and stmctural and thermochemical data for these species ( BX2) have been deduced (5). 

From the standard thermochemical data ArG° = (—371.3 — 379.9 + 733.9) kJ mol-1 = —17.3 kJmol-1, corresponding to an equilibrium constant K = 1.1 x 103 M-1. This is a worrying result because all peptides in solution at 298 K should spontaneously fall apart to the monomers and hence all proteins are subject to degradation due to spontaneous hydrolysis. Fortunately, the reaction is kinetically hindered, which means that it occurs very slowly. Kinetics always control the rate at which equilibrium is achieved, relating the ratio of the forward and backward rate constants to the equilibrium constant ... 

Kinetic studies in solution and in the gas phase have been playing an increasingly important role as a source of thermochemical data (see examples in chapter 15). Here we discuss how to relate thermochemical and kinetic information by approaching the subject as we did in the previous chapter by highlighting important practical issues and reducing to a minimum the description of theoretical models. In other words, the present chapter also relies on the material usually covered at the undergraduate level [1]. Further details can be found in more specialized books [55-59],... 

We now turn to the subject of aromatic amines. The number of classes of aromatic species vastly exceeds the number of classes of aliphatic and alicyclic amines. We somewhat arbitrarily consider mixed aromatic/ ali- species such as 7V,7V-dimethylaniline and 7V-methyldiphenylamine as aromatic and so they accompany the totally aromatic triph-enylamine instead of joining the totally ali- species trimethylamine. However more numerous the compounds of potential interest, the number of relevant aromatic species for which there are appropriate thermochemical data is also few. 

Eventually, a polymeric substance of the form -(As GaCH2)w- may be formed. Adduct formation is thought to be responsible for the observed lower decomposition temperature of Ga(CH3)3 in the presence of AsH3 relative to Ga(CH3)3 in a carrier gas (120, 121). Furthermore, with AsH3 and D2, the primary reaction product appears to be CH4 rather than CH3D, as expected on the basis of the free-radical mechanism (equations 16a-f). However, the formation of CH4 may be due to reactions of Ga(CH3)t and CH3 with adsorbed AsH v species (122). Estimates indicate that the adduct is too unstable to play a major role in the growth chemistry (129), but this conclusion is subject to uncertainties in the thermochemical data base. 

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 ... 

The uses of thermochemical data in estimating heats of detonation, detonation product compns, fire and expin hazards and critical diameters for expls were reviewed earlier (Vol 7, H38-L ff). For instances where heats of formation of organic expls are lacking, their estimation from group activities was described in Vol 7, H47-L ff A method for linking thermochemical data to the prediction of hazards was presented in Vol 7, HI 5-L to H23-L. The reader is directed to this article for a discussion of the complexity of the subject. For a further exposition of the problem, see Safety in the Energetic Materials Field in this Vol... 

It should be kept in mind that the calculation of the equilibrium constant is subject to high sensitivity to small errors in thermochemical data due to its exponential dependency to the standard Gibbs free energy variation, as expressed by the relation ... 

What defines a universal constant is subjective. It may be cogently argued that the acceptance of a ca 10 kJ mol-1 spread in <519 (g F, OH R) reflects the paucity of reliable thermochemical data of fluorinated organic compounds. 

Turning to nickel oxide, the thermochemical data of Tables II and IV show that the oxidation via the CO3 complex should be more difficult than on cuprous oxide or cobaltous oxide. The various reaction paths are shown schematically in Fig. 4. Because of the increase in the heat of formation of the complex, reaction (4) is now strongly endothermic and the reaction will be subject to poisoning. The poisoning effect has been confirmed experimentally, not only in our own studies, but also by Roginskii and Tselinskaya (SO) and by Winter (21, 22). The reaction CO(g) - - =... 

The different homo- and hetero-complexes observed and their thermochemical data are summarized in various review articles by McPhail et al. [424], Schafer [425, 426], 0ye and Gruen [427], Hastie [428, 429], Novikov and Gavryuchenkov [430], Biichler and Berkowitz-Mattuk [268], as well as Bauer and Porter [59]. Gaseous metal halide species is the only subject of these articles. Rules were developed to predict many properties of complexes (see Refs. 426, 428, 430). Brooker and Papatheodorou [431] as well as Papatheodorou [432] give an account of the vibrational properties and spectroscopic studies of the complexes. Theoretical and experimental investigations of alkali halide clusters are reviewed by Martin [433]. 

If thermochemical data are the flesh and blood of chemical thermodynamics, then it is true to say that the bone structure is made up of a small number of defining and operating equations. While to make such a selection is necessarily arbitrary, it is felt that to present it is the best way of demonstrating the underlying structure of the subject. 

The consistency criterion is not only employed within a data set for a particular substance, but also between the data of various substances the thermochemical data of a substance can be determined on the basis of several reactions [1]. Because of the large amount of both available and periodically appearing material the establishment of self-consistent data for all substances that have been the subject of thermochemical investigation is a considerable and demanding task. This task was and still is undertaken by experts in large institutes. 

A similar idea has been developed using chemical activation techniques hot H atoms, formed from the photolysis of HBr or H2S at certain wavelengths, are allowed to react with 1-butene to form vib-rationally hot n-butyl radicals with the wavelengths used in these experiments [80.G1 81.G], the n-butyl radicals, subject to the normal uncertainties in the assumed thermochemical data, were formed with excess internal energies of about 22, 28, 30, and 42 kcal mol" above the reaction threshold. ... 

Van t Hoff s work in stereochemistry will be described later (see p. 755). His book on organic chemistry develops the subject on general lines, lays great stress on physical properties (given in detail), and tries to connect stability and reactivity with thermochemical data, reaction velocities, and chemical equilibria. It emphasises the inert character of most carbon compounds, but makes little use of stereochemical ideas. It suggests that the chemical attraction between two atoms is gravitational and shows that, if the atom is non-spherical, the intensity of attraction on its surface has a certain number of... 

This chapter is intended to provide a unified view of selected aspects of the physical, chemical, and biological properties of the actinide elements. The f block elements have many unique features, and a comparison of the lanthanide and actinide transition series provides valuable insights into the properties of both. Comparative data are presented on the electronic configurations, oxidation states, redox potentials, thermochemical data, crystal structures, and ionic radii of the actinide elements, together with a miscellany of topics related to their environmental and health aspects. Much of this material is assembled in tabular and graphical form to facilitate rapid access. Many of the topics covered in this chapter, and some that are not discussed here, are the subjects of subsequent chapters of this work, and these may be consulted for more comprehensive treatments. This chapter provides a welcome opportunity to discuss the biological and environmental aspects of the actinide elements, subjects that were barely mentioned in the first edition of this work but have assumed great importance in recent times. 

The properties of the hydrogen molecule and molecule-ion which are the most accurately determined and which have also been the subject of theoretical investigation are ionization potentials, heats of dissociation, frequencies of nuclear oscillation, and moments of inertia. The experimental values of all of these quantities are usually obtained from spectroscopic data substantiation is in some cases provided by other experiments, such as thermochemical measurements, specific heats, etc. A review of the experimental values and comparison with some theoretical... 

In summary, computational quantum mechanics has reached such a state that its use in chemical kinetics is possible. However, since these methods still are at various stages of development, their routine and direct use without carefully evaluating the reasonableness of predictions must be avoided. Since ab initio methods presently are far too expensive from the computational point of view, and still require the application of empirical corrections, semiempirical quantum chemical methods represent the most accessible option in chemical reaction engineering today. One productive approach is to use semiempirical methods to build systematically the necessary thermochemical and kinetic-parameter data bases for mechanism development. Following this, the mechanism would be subjected to sensitivity and reaction path analyses for the determination of the rank-order of importance of reactions. Important reactions and species can then be studied with greatest scrutiny using rigorous ab initio calculations, as well as by experiments. 

While it is straightforward to obtain theoretical heats of formation from processes which greatly disrupt bonding, e.g., the G3 recipe, it is also possible to make use of isodesmic reactions together with limited experimental data, or alternatively data from high-level quantum chemical calculations, to estimate heats of formation. Once in hand, these can be used for whatever thermochemical comparisons are desired. The key is to find an isodesmic reaction which is both uniquely defined, and which leads to products with known heats of formation. This is the subject of the present chapter. 

This points to one of the great weaknesses in our understanding of this subject, namely our lack of knowledge of the thermodynamics of many of these processes. This is difficult to come by because many of these redox steps are not reversible hence we cannot obtain thermodynamic data in the usual way from equilibrium studies and must instead resort to thermochemical measurements of which we need more. 

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