Are Conventional Thermochemical Calculations a Viable Alternative to Measurements of Vaporization Enthalpy of...

The majority of the studies of vaporization enthalpy dealt with pure substances. Reports of this quantity for azeotropes were scarce despite that similar experimental methods could be applied in such measurements. Vaporization enthalpies of azeotropes were determined using classical methods in the past and with a method based on the enthalpy of solution recently. Since the reported results showed discrepancies that often exceeded the declared uncertainty limits, we calculated the vaporization enthalpies of 12 azeotropes at normal boiling temperature and 298.15 K using the conventional thermochemical cycle at several levels of approximation. We validated our calculation procedure and assessed the uncertainty of the results. The assessments were based on (i) a comparison of the calculated vaporization enthalpies with the experimental ones reported in the literature, and (ii) a Monte Carlo simulation involving 10⁶ trials with the independent variables characterized by continuous uniform distributions. The calculated vaporization enthalpies of the azeotropes proved to be correct even if they were only roughly approximated by the mole-fraction-weighted averages of the vaporization enthalpies of pure components. Thermochemical cycle calculations provided results at least as good as the experimental vaporization enthalpies, those obtained from the enthalpies of solution in particular.