Basic thermochemical data

The temperature function of the heat capacity (Cp(T ) belongs to the category of basic thermochemical data. These must be known to describe quantitatively the thermochemical properties of a substance in the form of temperature functions. 

Almost all of the directly measured thermochemical data for the sulfoxides, sulfones, sulfites and sulfates are due to the work of Busfield and Mackle and their coworkers at the University of Leeds and The Queens University, Belfast1-14. This work involved measurement of enthalpies of combustion, fusion and vaporization. It is the basis of the subsequent compilations of Benson and coworkers15, Cox and Pilcher16 and Pedley, Naylor and Kirby11. The data given by the latter are used as the basic data set in the present work. Corrections and omissions are noted in the next section. Data on additional compounds were sought by searching the IUPAC Bulletin of Thermochemistry and Thermodynamics for the years 1980 198318, and by searches of Chemical Abstracts. 

Proton Transfer and Electron Transfer Equilibria. The experimental determination used for the data discussed in the above subsections of Section IV.B were obtained from ion-molecule association (clustering) equilibria, for example equation 9. A vast amount of thermochemical data such as gas-phase acidities and basicities have been obtained by conventional gas-phase techniques from proton transfer equilibria,3,7-12-87d 87g while electron affinities88 and ionization energies89 have been obtained from electron transfer equilibria. 

Where do the thermochemical data that are used to determine the energetics of a reaction come from For closed-shell species that can be generated chemically via proton transfer, gas phase acidities (reaction [2]) and basicities (reaction [3]) are the principal sources. If the acidity or basicity for a reaction leading to a given ion is known, then the heat of formation for that ion can be calculated via Equations (4) and (5). This latter point is important, because this is the source for much of the ionic thermochemical data that are used for application of the no endothermic reactions tool. 

Tn 1965 Benson (3) discussed the basic thermochemistry and kinetics of combustion reactions. Since then additional thermochemical data have become available and the Benson and Buss Group Additivity Tables (7) have been revised (8). 

The catalytic cracking of four major classes of hydrocarbons is surveyed in terms of gas composition to provide a basic pattern of mode of decomposition. This pattern is correlated with the acid-catalyzed low temperature reverse reactions of olefin polymerization and aromatic alkylation. The Whitmore carbonium ion mechanism is introduced and supported by thermochemical data, and is then applied to provide a common basis for the primary and secondary reactions encountered in catalytic cracking and for acid-catalyzed polymerization and alkylation reactions. Experimental work on the acidity of the cracking catalyst and the nature of carbonium ions is cited. The formation of liquid products in catalytic cracking is reviewed briefly and the properties of the gasoline are correlated with the over-all reaction mechanics. 

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. 

An alternative approach to the experimental estimation of REs utilizes equilibrium (protonation) data rather than thermochemical data, the idea being that comparisons of the basicities of pyrrole and its benzo fused analogues with those of non-aromatic systems which form cations of 7r-electron structure similar to the aromatic compounds should furnish a measure of the loss of RE accompanying protonation of the aromatic system (76T1767, 72CI(L)335). Thus, for the a-protonation of N-methylpyrrole, the model non-aromatic system was chosen as (20). Combination of pKa values for the protonation of the aromatic and non-aromatic molecules, taking into account the intrinsic resonance stabilization of the... 

Good linear correlations of BE with gas-phase basicities of the anionic Lewis bases (the analysis included not only the halides F, CL, Br and F, but also other anions such as CN , N02, H , HCT, CH3 and CF3) were observed for a few Lewis acids (C02, S02, Et3B)305. The predictive power of these plots of anion BE vs anion basicity for obtaining estimates of new thermochemical data for novel anions has been pointed out305. 

This chapter describes basic physico-chemical relations between the gas phase transport of atoms and molecules and their thermochemical properties, which are related to the adsorption-desorption equilibrium. These methods can either be used to predict the behavior of the adsorbates in the chromatographic processes, in order to design experiments, or to characterize the absorbate from its experimentally observed behavior in a process. While Part I of this chapter is devoted to basic principles of the process, the derivation of thermochemical data is discussed in Part n. Symbols used in the following sections of Part I are described in Section 5. For results, which were obtained applying the described evaluation methods in gas-adsorption chromatography, see Chapters 4 and 7 of this book. 

Gas-phase methods also constitute a source of important information on basic physical properties of silylenium ions. In particular, the thermochemical behavior is well characterized (30,33,34,47,61). Thermochemical data are applied for the evaluation of relative thermodynamic reactivities of silylenium ions in some systems. For example, affinities of R3Si+ and R3C+ toward various bases may be compared as the heterolytic dissociation energies of corresponding bonds [Eq. (12)] (47,61). It was shown that... 

This section will use gas-phase thermochemical data from Appendices 6 for molecules and 7 for radicals. These data include ionization energy (IE), electron affinity (EA), proton affinity (PA), gas-phase basicity (GB) and gas-phase acidity. Definitions of these parameters are given in Table 1.5. Some values of gas-phase basicities are given in Table 1.6. 

The kinetic method provides an alternative to equilibrium measurements for the determination of gas-phase thermochemical properties. It has been applied more and more in thermochemical data determination mainly because of its ability to measure very small energy differences and its simplicity. Indeed, it can be executed easily on any tandem mass spectrometer. Furthermore, this method is sensitive and is applicable with impure compounds. Its applications are broad, covering thermochemical properties in the gas phase such as proton affinity [46], electron affinity [47], metal ion affinity [48], ionization energy [49], acidity [50] or basicity [51], In addition to the determination of thermochemical data, the kinetic method has also been applied in structural and chemical analysis such as chiral distinctions. This method is able to distinguish enantiomers and to measure precisely enantiomeric ratios [52],... 

We will review the basic quantities of thermodynamics energy, temperature, heat, work, and the ideal gas law. These quantities will be used to explain the principles of thermophysics and thermochemistry, which will be applied to the specific reactions of combustion and detonation. Using the thermochemical data of heats of detonation or explosion, we will see how to calculate adiabatic reaction temperatures. These data in turn will be used to analyze or predict pressures of explosions in closed vessels. We shall also see how, using thermochemical data, to predict detonation velocities and detonation pressures. 

As is the case in the applied sciences, chemical thermodynamics work with a number of empirical definitions, conventions, standardizations and simplifications some of which may be chosen arbitrarily in accordance with basic principles. This is one of the main reasons why users of thermochemical data who are not specialized in thermodynamics unknowingly make mistakes. 

The polynomial coefficients of Cp(T) are obtained from numerical evaluations to the best possible approximation, and in many cases have no physical meaning. For certain substance phases several polynomials with up to 6 coefficients are necessary, which would require the use of up to 10 characters to present as figures. All basic thermochemical values including alpha-numerical information are stored in databases. For personal use only, they can be requested from the publisher or the author. These data are also included in the thermodynamic software system equiTherm, which is a very useful supplement to this book. 

The calculation of the thermochemical functions of a pure substance over a given temperature range demands a particular set of basic thermodynamic data. These basic data are unequivocally defined in terms of standard states and reference phases and temperatures. They comprise the following quantities ... 

Lide, D. R., and Kehiaian, H. V., CRC Handbook of Thermophysical and Thermochemical Data, CRC Press, Boca Raton, FL, 1994 Bruno, T. J., and Svoronos, P. D. N., CRC Handbook of Basic Tables for Chemical Analysis, CRC Press, Boca Raton, FL, 1989. 

It is difficult to determine these thermochemical parameters from experiment, because it is hard to monitor the precursor hydrocarbon radical and the formed peroxy radical. The experiment is further complicated by the presence of reactions to new products by the energized peroxy radicals which can prevent the monitoring of equilibrium. Experiments on ion methods using proton affinity or basicity, often with mass spectrometric analysis, are also utilized to determine enthalpies of formation of radicals. Our methods rely heavily on experimentally determined thermochemical data and we would like to point out that this data is very valuable to validate the computational methods. 

Da SilvaFilho, E.C. De Melo, J.C.P. Airoldi, C. Preparation of ethylenediamine-anchored cellulose and determination of thermochemical data for the interaction between cations and basic centers at the soUd/ liquid interface. Carbohydr. Res. 2006, 341 (17), 2842-2850. 

Calculation of Thermodynamic Functions from Molecular Properties The calculation methods for thermodynamic functions (entropy S, heat capacities Cp and Cy, enthalpy H, and therefore Gibbs free energy G) for polyatomic systems from molecular and spectroscopic data with statistical methods through calculation of partition functions and its derivative toward temperature are well established and described in reference books such as Herzberg s Molecular Spectra and Molecular Structure [59] or in the earlier work from Mayer and Mayer [7], who showed, probably for the first time in a comprehensive way, that all basic thermochemical properties can be calculated from the partition function Q and the Avagadro s number N. The calculation details are well described by Irikura [60] and are summarized here. Emphasis will be placed on calculations of internal rotations. 

There are basically a few databases that include thermochemical data in polynomial form ... 

Chapter 5 introduced basic thermodynamic ideas largely in qualitative terms. Electrochemistry offers the opportunity to develop these ideas in quantitative terms, reinforcing the application of mathematics in chemistry. For example, students could use thermochemical data (such as values of standard enthalpies of formation, available from any chemical data book or the internet) to calculate the standard enthalpy change during chemical reactions such as the hydrogen-oxygen reaction used in fuel cells. This is not, however, a normal task for most secondary students, so it could be used for extension work only. 

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