Estimation of Thermochemistry by Conventional Methods

These techniques include methods that are highly empirical in nature, such as those based on various forms of additivity principles, as well as those that are based on the use of statistical mechanical calculations. Although the latter methods represent precise tools to determine S and Cp from molecular properties, they are of little utility for the prediction of AHj. 

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 

Bond Contributions to AH, S, and Cp (from Benson, Copyright 1976 John Wiley  

This is within 5% of the experimentally measured value of —19.6 kcal/mol, and no corrections are needed for the AHf in this example. Similarly, the intrinsic entropy for CH3CI can be determined as follows  

Symmetry number (u) is the total number of independent permutations of identical atoms or groups in a molecule that can be arrived by simple rotations. Both the internal ( 7j ,) and external (Ue,) symmetries must be considered in establishing the symmetry number, where 

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