Thermochemical property values

Table I. Summary of Ga-Sb Thermochemical Property Values Used in Equations 7 and 9...

In order to calculate polynomial coefficients, the thermodynamic and thermochemical property values must be found in tables or calculated from molecular properties by the methods of statistical thermodynamics. The available tables will be described in the following section. 

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. 

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. 

Throne, J.L., Griskey, R.G. Heating Values and Thermochemical Properties of Plastics, Modern... 

If we use B3LYP/VTZ+1 harmonics scaled by 0.985 for the Ezpv rather than the actual anharmonic values, mean absolute error at the W1 level deteriorates from 0.37 to 0.40 kcal/mol, which most users would regard as insignificant. At the W2 level, however, we see a somewhat more noticeable degradation from 0.23 to 0.30 kcal/mol - if kJ/mol accuracy is required, literally every little bit counts . If one is primarily concerned with keeping the maximum absolute error down, rather than getting sub-kJ/mol accuracy for individual molecules, the use of B3LYP/VTZ+1 harmonic values of Ezpv scaled by 0.985 is an acceptable fallback solution . The same would appear to be true for thermochemical properties to which the Ezpv contribution is smaller than for the TAE (e.g. ionization potentials, electron affinities, proton affinities, and the like). 

Values based on rule of group additivity of thermochemical properties, using an assumed value, Hf0, for O—(6)2 group of 19 2 kcal., and further assuming that in the molecular series H—OnH, DH°(H—OnH) = 90 2kcal. for n > 2. 

The principal feature of this relationship is that F values are derived solely from molecular formulae and chemical structures and require no prior knowledge of any physical, chemical or thermochemical properties other than the physical state of the explosive that is, explosive is a solid or a liquid [72]. Another parameter related to the molecular formulae of explosives is OB which has been used in some predictive schemes related to detonation velocity similar to the prediction of bri-sance, power and sensitivity of explosives [35, 73, 74]. Since OB is connected with both, energy available and potential end products, it is expected that detonation velocity is a function of OB. As a result of an exhaustive study, Martin etal. established a general relation that VOD increases as OB approaches to zero. The values of VOD calculated with the use of these equations for some explosives are given in the literature [75] and deviations between the calculated and experimental values are in the range of 0.46-4.0%. 

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 fact that reactions go to the equilibrium position was discovered empirically, and the equilibrium constant was first defined empirically. All the aforementioned applications can be accomplished with empirically determined equilibrium constants. Nonetheless, the empirical approach leaves unanswered several important fundamental questions Why should the equilibrium state exist Why does the equilibrium constant take its particular mathematical form These and related questions are answered by recognizing that the chemical equilibrium position is the thermodynamic equilibrium state of the reaction mixture. Once we have made that connection, thermodynamics explains the existence and the mathematical form of the equilibrium constant. Thermodynamics also gives procedures for calculating the value of the equilibrium constant from the thermochemical properties of the pure reactants and products, as well as procedures for predicting its dependence on experimental conditions. 

The data determined directly by Knudsen cell measurements, plus a strong correlation between the bond strengths of metal hydroxide bonds and metal halide (in particular, chloride and fluoride bonds) in the gaseous metal hydroxides and halides were developed and allow us to more reliably estimate the enthalpy of formation of many hydroxide and oxyhydroxide metal compounds whose values of thermochemical heat and formation were previously unknown. These thermochemical properties were then used to estimate volatility of various supporting oxide substrates and catal)dically-active solids that were relevant for the fabrication of catalytic combustors. 

Some empirical methods have provided useful correlations concerning the effects of substituents on thermochemical properties of phenols. Griller and coworkers developed a photoacoustic method for measuring bond dissociation energies (BDE) of phenols and showed for the first time a linear relationship between the Hammett a+ para-substituent constant and BDEs. Wayner and coworkers found a correlation between experimental ABDE values for a series of substituted phenols compared with phenol and the Hammett a+ constants (equation 58). 

Recent advances in computational chemistry have made it possible to calculate enthalpies of formation from quantum mechanical first principles for rather large unsaturated molecules, some of which are outside the practical range of combustion thermochemistry. Quantum mechanical calculations of molecular thermochemical properties are, of necessity, approximate. Composite quantum mechanical procedures may employ approximations at each of several computational steps and may have an empirical factor to correct for the cumulative error. Approximate methods are useful only insofar as the error due to the various approximations is known within narrow limits. Error due to approximation is determined by comparison with a known value, but the question of the accuracy of the known value immediately arises because the uncertainty of the comparison is determined by the combined uncertainty of the approximate quantum mechanical result and the standard to which it is compared. 

Once the validity of a quantum mechanical procedure has been established by its ability to reproduce various accurate experimental results, the way is clear to calculate unknown thermochemical values of unstable or explosive compounds, unsuited to classical thermochemical methods, or to calculate thermochemical properties of molecules, radicals, or ions of fleeting existence (e.g., Zavitsas, Matsunaga, and... 

The selection of a reference base for property tabulations is arbitrary. Whatever base is selected, the base values of the different extensive properties will cancel out of the thermodynamic property calculations--assuming, of course, that the calculations are carried out correctly. (That is, when employing thermochemical property tabulations to make property calculations, it is necessary to assure that the base values cancel out this must be done for all extensive properties--enthalpy, entropy,..., availability, etc.)... 

The relatively small differences in Ktll suggest that any differences in the observed values of 2kt must originate in the irreversible dissociation of R04R to RO, reaction (173) that is kinetic, not thermochemical, properties govern the overall rate of termination. 

There have been a number of previous reviews of various aspects of the thermochemical properties of zirconium and its compounds. The chemical behaviour of zirconium was reviewed by Blumenthal [58BLU]. The review of [58BLU] contains an extensive history of the element and its compounds. It references much of the older literature, including those on the thermochemistry of zirconium and its compounds. The data given for thermochemical parameters in [58BLU], however, were not critically reviewed and therefore do not agree with the present review. Nevertheless, the review of [58BLU] is of particular value due to the detailed historical information it contains. 

Warhus et al. [88WAR/MAI] measured the specific heat of Na2ZrSi207(s) from 100 to 800 K using adiabatic and differential scanning calorimetry. Values calculated by [88WAR/MA1] for the thermochemical properties are as given below ... 

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