Estimating thermochemical properties using group additivity

2 Estimating thermochemical properties using group additivity 

If thermochemical data for a given substance are not available in the compilations cited above or in easily accessible literature, then the next best approach is to use an experimentally based empirical estimation technique. The most common approach of this type is group additivity, 

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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. 

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. 

This study reports bond energies, enthalpy, entropy, heat capacity, internal rotation potential, and structure data for a series of unsaturated peroxides. Thermochemical property groups are developed as well for future use in group additivity estimation methods. 

Stein, S. S. NIST Structures and Properties (Version 2.0) NIST Standard Reference Database 25 National Institute of Standards and Technology Gaithersburg, 1994. This useful and simple to use software package relies on Benson s group additivity scheme to estimate thermochemical data for organic compounds in the gas phase. It also contains values from several NIST databases, including NIST Positive Ion Energetics and JANAF Tables. The first version of NIST S P is from 1991. The database is superseded by the NIST Chemistry. WebBook. 

Benson [15] has created and developed a general method of calculation of the values of thermochemical and kinetic parameters based on a systematic use of the additivity of group properties on the one hand and the activated complex theory on the other. Other methods for estimating a priori kinetic parameters have recourse to structural analogies or semi-empirical correlations. 

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