Enthalpy change calculation from bond dissociation

The primary signihcance of bond enthalpies lies in the calculation of the enthalpy of a reaction involving a compound for which no enthalpy data are available. For example, if the enthalpy of formation of Se2Cl2(g) were not known, it could be calculated from bond enthalpies by the following steps. As the bond enthalpy refers to the dissociation of Cl-Se-Se-Cl gas into gaseous atoms, the enthalpy change for the formation of this gaseous molecule from the atoms should... 

Calculate enthalpy changes from bond-dissociation enthalpies. 

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. 

Thermodynamic aspects of the insertion reactions have been considered, especially with regard to reactions (b), Y = O. The enthalpy change of the reaction is related to the disruption of the metal-carbon bond, the formation of the new metal-carbon bond, the formation of the new carbon-carbon bond, and the reduction of the CO bond order. From the estimated metal-carbon bond dissociation enthalpies (BDE) of reagent and product and from the energetics of the other quantities involved in the process, the AC of reaction (g) has been calculated to be negative for R = Me or Ph and positive for R = CF3. This is in agreement with the observation that the trifluoroacetyl derivative... 

The bond dissociation enthalpies of bonds such as C-H, C-Cl, C=0, N=N and O-H are approximately the same in different molecules. If the values of bond dissociation enthalpy for a bond between two atoms (A and B) in several different molecules are averaged, the resulting value is called the mean bond enthalpy (Table 13.4). Mean bond enthalpies are useful in estimating enthalpy changes for reactions for which standard enthalpies of formation are unavailable, but the fact that such values may have been obtained by averaging bond dissociation enthalpies from different types of molecule may lead to substantial errors in the calculated value of AH. ... 

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. 

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. ... 

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