Bond dissociation energy enthalpy change calculation from

A more useful quantity for comparison with experiment is the heat of formation, which is defined as the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states. The heat of formation can thus be calculated by subtracting the heats of atomisation of the elements and the atomic ionisation energies from the total energy. Unfortunately, ab initio calculations that do not include electron correlation (which we will discuss in Chapter 3) provide uniformly poor estimates of heats of formation w ith errors in bond dissociation energies of 25-40 kcal/mol, even at the Hartree-Fock limit for diatomic molecules. 

A standard bond dissociation energy is different from an average bond dissociation energy. The latter is just the value obtained by calculating the heat of atomization of a compound (the enthalpy change on converting the molecule to individual atoms) divided by the number of bonds from one atom to another in the molecule. For more details on this distinction, see reference 67. Blanksby, S. J. Ellison, G. B. Acc. Chem. Res., 2003, 36, 255. This reference provides the uncertainties for the values in Tables 1.10 and 1.11. 

It is evident from Table 16.3 that quinones react rapidly with the hydroperoxyl radicals. Calculation shows that the change in the enthalpy of this reaction is relatively small. The dissociation energy of the —H bond in the semiquinone radical 4- 40 is 228.1 kJ mol 1 and the enthalpy of the reaction Q + H02 is A// 220.0 228.1 8.1 kJ mol. ... 

We can understand why bromination is more selective than chlorination by using bond dissociation enthalpies (Table 4.3) to calculate the energy changes for the propagation step in which each halogen atom abstracts a hydrogen from ethane. 

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