Hydrogen bonding enthalpy changes

A new procedure is implemented for an accurate and fast determination of the hydrogen-bond enthalpy. In this method, a single solution of cyclopropylamine and 4-fluorophenol is studied by measuring its absorbance change as a function of temperature. 

We must also consider the changes in solvation of the ligands which occur upon coordination. If we consider an amine in water, we would anticipate strong hydrogen-bonding. If we compare 1,2-diaminoethane with ammonia, we would expect the latter to be more highly solvated. This corresponds to a more unfavourable enthalpy associated with the desolvation. 

Energy is needed to break the ionic bonds in the solid salt and energy is liberated forming hydration complexes like VI. We also break some of the natural hydrogen bonds in the water. The overall change in enthalpy is termed the enthalpy of solution, A// olutioni. Typical values are —207 kJmol-1 for nitric acid 34 kJmol-1 for potassium nitrate and —65.5 kJmol-1 for silver chloride. 

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. 

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