Enthalpies of formation and computational chemistry

Self-test 1.6 ) Use standard enthalpies of formation to calculate the enthalpy of combustion of solid glycine to C02(g), H20(l), and N2(g). 

The reference states of the elements define a thermochemical sea level , and enthalpies of formation can be regarded as thermo chemical altitudes above or below sea level (Fig. 1.25). Compounds that have negative standard enthalpies of formation (such as water) are classified as exothermic compounds, for they lie at a lower enthalpy than their component elements (they lie below thermo chemical sea level). Compounds that have positive standard enthalpies of formation (such as carbon disulfide) are classified as endothermic compounds and possess a higher enthalpy than their component elements (they lie above sea level). 

The computational approach also makes it possible to gain insight into the effect of solvation on the enthalpy of formation without conducting experiments. A calculation performed in the absence of solvent molecules estimates the properties of the molecule of interest in the gas phase. Computational methods are available that allow for the inclusion of several solvent molecules around a solute molecule, thereby taking into account the effect of molecular interactions with the solvent on the enthalpy of formation of the solute. Again, the numerical results are only estimates, and the primary purpose of the calculation is to predict whether interactions with the solvent increase or decrease the enthalpy of formation. As an example, consider the amino acid glycine, which can exist in a neutral (5) or zwitterionic (6) form, in which the amino group is protonated and the 

25 The enthalpy of formation acts as a kind of thermochemical altitude of a compoimd with respect to the sea level defined by the elements from which it is made. Endothermic compoimds have positive enthalpies of formation exothermic compoimds have negative energies of formation. 

---