Benson’s group additivity method

THERM Thermodynamic Property Estimation for Radicals and Molecules, Edward R. Ritter and Joseph Bozzelli, Int. J. Chem. Kinet. 23, 767-778, (1991). A computer program for IBM PC and compatibles for estimating, editing, and entering thermodynamic property data for gas-phase radicals and molecules using Benson s group additivity method. 

If a reaction is sufficiently well balanced, its enthalpy change will be close to zero. For reactions (7)-(10), the experimental enthalpies of formation imply gas-phase reaction enthalpies (298.15 K) of 5674 + 7, —518 + 7, 52 + 7, and 30 + 7kJmol respectively. All quantum calculations can be avoided by assuming the reaction enthalpy to be zero for a well-balanced reaction. This is the basis of empirical methods such as Benson s group-additivity scheme [5,6]. Where adequate auxiliary data are available, a group-balanced reaction is an excellent choice for quantum chemistry calculations [121]. 

As stated above, the thermochemistry of free radicals can also be estimated by the group additivity method, if group values are available. With the exception of a few cases reported in Benson (1976), however, such information presently does not exist. Therefore, we rely on the model compound approach (for S and Cp) and bond dissociation energy (BDE) considerations and computational quantum mechanics for the determination of the heats of formation of radicals. 

Batch reactor, typical, DACSL kinetic and reactor modeling, 94 Benefits, HAZOP study, 37 Benson s second-order group additivity method, CHETAH program, 82 Benzoyl peroxide (BPO), effect of dilution with xylene, CHETAH program, 87... 

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. 

Where no data exist, one wishes to be able to estimate thermochemical quantities. A simple and convenient method to do that is through the use of the method of group additivity developed by Benson and coworkers15,21 22. The earlier group values are revised here, and new group values calculated to allow extension of the method to sulfites and sulfates. In addition, a method based on the constancy of S—O bond dissociation energies is applied. 

Using Benson s method of group additivity the program performs several calculations which are useful in evaluating the hazards from a pure chemical, a chemical mixture, or a chemical reaction ... 

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