Thermochemical properties, estimation

The above analyses of species concentrations and net reaction rates clearly indicate which reactions and which chemical species are most important in this reaction mechanism, under the particular conditions considered. However, for purposes of refining a reaction mechanism by eliminating unimportant reactions and species and by improving rate parameter estimates and thermochemical property estimates for the most important reactions and species, it would be helpful to have a quantitative measure of how important each reaction is in determining the concentration of each species. This measure is obtained by sensitivity analysis. In this approach, we define sensitivity coefficients as the partial derivative of each of the concentrations with respect to each of the rate parameters. We can write an initial value problem like that given by equation (35) in the general form... 

The next higher order of approximation, the first-order approximation, is obtained by estimating molecular properties by the additivity of bond contributions. In the following, we will concentrate on thermochemical properties only. 

Two standard estimation methods for heat of reaction and CART are Chetah 7.2 and NASA CET 89. Chetah Version 7.2 is a computer program capable of predicting both thermochemical properties and certain reactive chemical hazards of pure chemicals, mixtures or reactions. Available from ASTM, Chetah 7.2 uses Benson s method of group additivity to estimate ideal gas heat of formation and heat of decomposition. NASA CET 89 is a computer program that calculates the adiabatic decomposition temperature (maximum attainable temperature in a chemical system) and the equilibrium decomposition products formed at that temperature. It is capable of calculating CART values for any combination of materials, including reactants, products, solvents, etc. Melhem and Shanley (1997) describe the use of CART values in thermal hazard analysis. 

A powerful and readily applied method for the estimation of thermochemical properties for gas phase species is that of group additivity developed by Benson and his coworkers15 21-22. The method is based on the observation that the thermochemical properties of a molecule can be represented as a sum of contributions from the individual groups which make up the molecule. The method of defining groups and arriving at group... 

These results thus show that whereas the flashpoint was only moderately influenced by the compound structure (their chemical functionality but especially their atomic composition and vapour), autoignition temperatures seem to be closely linked to the structural factors that affect the chain. So additivity rules for estimation of AIT should be sought. Every time a chemical or physical property is highly influenced by the structure, chemists tried to establish rules that enable one to reduce a molecule to characteristic groups for which the contribution to the value of this property is known. This was done for instance by Kinney for boiling points and Benson2 for thermochemical properties. 

This equation suggests that the Fe content of tetrahedrite-tennantite positively correlates with that of sphalerite at constant temperature and pressure, indicating Fe and Zn contends of tetrahedrite-tennantite are useful to estimate physicochemical parameters (/sj, /02 etc.) as well as Fe content of sphalerite, although detailed study on thermochemical properties of tennantite-tetrahedrite solid solution is still needed. 

In this book, the experts who have developed and tested many of the currently used electronic structure procedures present an authoritative overview of the theoretical tools for the computation of thermochemical properties of atoms and molecules. The first two chapters describe the highly accurate, computationally expensive approaches that combine high-level calculations with sophisticated extrapolation schemes. In chapters 3 and 4, the widely used G3 and CBS families of composite methods are discussed. The applications of the electron propagator theory to the estimation of energy changes that accompany electron detachment and attachment processes follow in chapter 5. The next two sections of the book focus on practical applications of the aforedescribed... 

The bond additivity rule is the first-order formalism to estimate thermochemistry of stable molecules. Bond contributions to thermochemical properties for some select bonds are given in Table II. The application of this... 

The application of group additivity is also straightforward. For example. Table V illustrates the application of this method for the estimation of the thermochemical properties of I-C4H8. Computerized versions of this approach are also available (Martinez, 1973 Seaton et al., 1974). 

In summary, bond and group additivity rules, as well as the model compound approach, in conjunction with statistical mechanics, represent useful tools for the estimation of thermochemical properties. However, their utility for the determination of thermochemistry of new classes of compounds is limited, especially with regard to the determination of Aiff. For new classes of compounds, we must resort to experiments, as well as to computational quantum mechanical methods. 

In summary, computational quantum mechanics methods represent powerful new tools for the estimation of thermochemistry. However, their routine use clearly must be avoided, as there still are unresolved limitations of these methods. Consequently, we must continue to rely on conventional methods, experiments, and chemical intuition for the estimation of thermochemical properties. 

The thermochemical properties of tetraperchloratoborates have been studied in detail.136 The estimated heats of dissociation for BX (g) BX3(g) + X ) decrease in the order F>H> Cl > C104. The sulfatoborates are probably ionic compounds with highly polymeric anions and are sensitive to hydrolysis. The chlorosulfatoborates are stable up to 80-150 °C their decomposition seems to give, as a first step, M[BCU] and SO3. 

Solid-Solution Models. Compared with the liquid phase, very few direct experimental determinations of the thermochemical properties of compound-semiconductor solid solutions have been reported. Rather, procedures for calculating phase diagrams have relied on two methods for estimating solid-solution model parameters. The first method uses semiem-pirical relationships to describe the enthalpy of mixing on the basis of the known physical properties of the binary compounds (202,203). This approach does not provide an estimate for the excess entropy of mixing and thus... 

A great number of studies related to thermochemical properties of QDO and PDO derivatives have been recently described by Ribeiro da Silva et al. [98-103]. These studies, which have involved experimental and theoretical determinations, have reported standard molar enthalpies of formation in the gaseous state, enthalpies of combustion of the crystalline solids, enthalpies of sublimation, and molar (N - O) bond dissociation enthalpies. Table 5 shows the most relevant determined parameters. These researchers have employed, with excellent results, calculations based in density functional theory in order to estimate gas-phase enthalpies of formation and first and second N - O dissociation enthalpies [103]. 

Recently, there has been considerable interest in determining thermochemical properties, such as the AH°( and EA values of carbenes, notably the halo- and dihalomethylenes, and both experimental and computational methods were applied to this end. One thorough ICR investigation produced heats of formation for CF2, CC12, CC1F, CFH and CC1H, from estimates of the thermochemistry of the proton transfer reaction of equation 44 where X and Y are F and/or Cl, and B is a base of known gas-phase basicity323. 

In Section I, Chemistry of Explosives, methods were described that enable one to estimate detonation properties (detonation velocity D and detonation pressure Pcj) from the molecular structure of an explosive. This section gives an alternate method that utilizes the thermochemical properties of an explosive in order to estimate the values of these two output properties. This method was developed by M. J. Kamlet and S. J. Jacobs of the Naval Ordnance Laboratory in White Oak, MD (Ref 9) and is referred to in this text as the KJ method. 

The Kamlet-Jacobs method, using the above product hierarchy, along with the thermochemical property, A// , the heat of detonation, estimates detonation velocity by ... 

Thermodynamic properties of molecular species that are used in reactor design problems can be readily estimated from thermodynamic data tabulated in standard reference sources such as Perry s Handbook or the JANAF Tables. Thermochemical properties of molecular species not tabulated can usually be estimated using group contribution methods. Estimation of activation energies is, however, much more difficult due to the lack of reliable information on transition state structures, and the data required to cany out these calculations is not readily available. 

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