Once the thermodynamic functions were obtained in the standard state for each temperature, the formation functions were calculated. Enthalpies of formation include the enthalpies of the reference elements:
The Gibbs energy of formation was calculated with the enthalpy of formation and the entropies of the reference elements:
The enthalpies and entropies of the elements were obtained from Chase and are shown in Table 1. Table 2 presents the Gibbs energy of formation for each xylene species with the corresponding uncertainty; error associated to pressure and molar volume was not taken into account since it was negligible compared to that of enthalpy and entropy.