Enthalpy changes determination using bond enthalpie

Because bond enthalpies are often average values it means that enthalpy changes calculated using bond enthalpies will not be exactly equal to an accurate experimentally determined value. 

Tables of AH° for compounds are the most important data source for thermochemistry. From them it is easy to calculate AH° for reactions of the compounds, and thereby systematically compare the energy changes due to bond rearrangements in different reactions. Appendix D gives a short table of standard enthalpies of formation at 25°C. The following example shows how they can be used to determine enthalpy changes for reactions performed at 25°C and 1 atm pressure.

The standard enthalpy change for formation of KrF2(g) is +60.20 kJ/mol. Using atomization data from Appendix III, determine the bond dissociation energy for the Kr—F bond. 

Reaction enthalpies can be estimated by using mean bond enthalpies to determine the total energy required to break the reactant bonds and form the product bonds. In practice, only the bonds that change are treated. Because bond enthalpies refer to gaseous substances, to use the tabulated values, all substances must be gases or converted into the gas phase. 

Ervin et al. [27] have determined the electron affinity of the acetylide radical, HC = C-, to be equal to 2.969 + 0.010 eV and the enthalpy of the acid dissociation of acetylene in the gas phase to be equal to 377.8 + 0.6 Kcal mol Use these data, together with the ionization potential of the hydrogen atom, 13.595 eV, to calculate the enthalpy for the dissociation of the CH bond in acetylene. The ionization potentials are properly applied at 0 K, but a good approximation is to assume that they are equal to enthalpy changes at 298.15 K, the temperature at which the enthalpy of the acid dissociation was measured. 

Head et al. developed a PLS-based model VALIDATE [47] to scale the relative contributions of entropy and enthalpy to binding affinity for a variety of complexes whose crystal structures had been determined. Molecular mechanics were used to calculate several parameters most correlated with enthalpy of binding, while changes in surface area, number of rotatable bonds fixed upon binding and other parameters more related to the entropy of binding were also included in the model. Of interest was that the principal components of the model were dominated by two terms (AH and AS,... 

Since the nature of the hydride chemical shifts, particularly in transition metal hydride complexes, is not simple [32], there is no reliable correlation between Sh and the enthalpy of dihydrogen bonding. Nevertheless, the chemical shifts of hydride resonances and their changes with temperature and the concentration of proton-donor components, for example, can be used to obtain the energy parameters for dihydrogen bonding in solution. As earlier, the enthalpy (A/f°) and entropy (AS°) values can be obtained on the basis of equilibrium constants determined at different temperatures. Let us demonstrate some examples of such determinations. 

A spectral band is characterised by its frequency range, its intensity, and its shape and breadth. The frequency change in the XH stretching tend (Av) is often used as an approximate measure of the strength of the H-bond. Enthalpies of H-bond formation are usually determined from the temperature variation of the free-associated intensity ratio of the same bands. Of even greater interest are, however, the geometries and the potential surfaces of H-bonded species and the distribution of the electronic charge therein. For this the spectra of the isolated H-bond complexes that is, gas phase spectra are needed. The fine structure of the bands has to be examined. Key questions are how do the new vibrational motions introduced by the formation of a H-bond interact with the internal motions of the components X and Y How could this be inferred from the observed breadth and fine structure of the bands ... 

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