Bond enthalpy definition

We must be careful to distinguish between bond enthalpy and the dissociation enthalpy of a given bond. The latter is a definite quantity that refers to the enthalpy absorbed when a given bond of some specific compound is broken. However, bond enthalpy is an average value of the dissociation enthalpies of a given bond in a series of different dissociating species. 

Concomitant with these developments in spectroscopy, thermochemists were finding that, to a reasonable approximation, molecular enthalpies could be determined as a sum of bond enthalpies. Thus, assuming transferability, if two different molecules were to be composed of identical bonds (i.e., they were to be isomers of one kind or another), the sum of the differences in the strains of diose bonds from one molecule to the other (which would arise from different bond lengths in the two molecules - the definition of strain in this instance is the positive deviation from the zero of energy) would allow one to predict the difference in enthalpies. Such prediction was a major goal of the emerging area of organic conformational analysis. 

Before any discussion about the application of bond enthalpy terms, it is important to check the definitions of bond dissociation energies and bond enthalpy terms... 

Bond enthalpy values derived from the aforementioned calorimetric approaches and all other D(M-R) quantities determined in solution (vide infra) are not strictly gas phase quantities as specified in the formal definitions (Equations 1,3,4). However,... 

Analyze For part (a), we need to recall what we have learned about p, sp, and sp hybridization and geometry. > (Section 9.5) For part (b), we need to write a balanced equation. For part (c), we need to use the ideal-gas equation. (Section 10.4) For part (d), we need to recall the definitions of endothermic and exothermic and how bond enthalpies can be used to predict overall reaction enthalpies. (Section 8.8)... 

A/i the dissociation or bond energy of hydrogen (it is also, by definition, twice the enthalpy of atomisation two gram atoms being produced). 

As a result of the small, but apparent single bond character of the triafulvene C3/C4 bond and the good stabilization of the transition state of the rotation established earlier, rotation around this bond should be lower in energy in comparison to simple ethylene derivatives183. In fact, 1H-NMR spectra of several types of asymmetrically substituted triafulvenes 219-224 proved to be temperature-dependent and showed reversible coalescence phenomena at definite temperatures diagnostic for internal rotation processes. These were characterized by the free enthalpy of activation AG at the coalescence point of appropriate substituent signals61. ... 

According to the definition of the A-B bond dissociation enthalpy, reactants and products in reaction 5.1 must be in the gas phase under standard conditions. That is to say that those species are in the ideal gas phase, implying that inter-molecular interactions do not exist. DH (A-B) refers, therefore, to the isolated molecule AB, and it does not contain any contribution from intermolecular forces. Though this is obviously the correct way of defining the energetics of any bond, there are many literature examples where bond dissociation enthalpies have been reported in solution. 

We conclude this section by giving, for the sake of clarity, the most general definition of mean bond dissociation enthalpy. For any molecule AYmXn, where A is a central atom and X and Y are any mono- or polyatomic groups, the A-X... 

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