Specific Approaches to AHf

The most direct route to calculating the heat of formation of a compound is to simply apply the definition of AHf it is AH for the reaction (often hypothetical) whereby the compound is formed from its elements. The latter should be in whatever are their most stable states at the temperature in question tabulated AHf are generally at 25°C (298.15 K). (The pressure is typically one bar, i.e. standard state, although that is normally not a factor in computing AHf.) Thus AHf for 3-iodo-5-bromonitrobenzene, 1, is AH for the process, 

The first step then is to compute the energy at 0 K for the carbon atom and for the N2, 02, Br2, I2 and 1 molecules. Next the zero-point vibrational contributions are added to the molecular energies, plus the thermal corrections to convert each E(0 K) to H(T), whatever is the temperature of interest. (We will asssume it to be 298 K.) Finally the heat of vaporization of Br2 and the heats of sublimation of graphite, I2 and 1 must be included 

Hrot =— RT (non-linear molecule), Erot = RT (linear molecule) (3) 

In eqs. (2) - (5), v, is the vibration frequency of the, h normal mode and Na is Avogadro s number. The other constants have their usual meanings. 

A modified version of this approach to computing heats of formation is to calculate AH not for the formation of the compound but rather for its atomization, i.e. separation of the gas phase molecule into the individual atoms. For 1, this is the process, 

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