Enthalpy of the isotopic exchange reaction H₂O + D₂O → 2HDO

Abstract

The enthalpy of the reaction

                         H₂O(l)+D₂O() → 2HDO(l)

at 298 K is calculated from the statistical-mechanical enthalpy of the corresponding gas-phase reaction and recent vapour-pressure isotope effect results. Enthalpies of the same reaction obtained from calorimetric results by assuming that the H₂O + HDO + D₂O liquid mixture is an ideal solution agree, to within the estimated uncertainities, with the calculated value. It is concluded that any 'physical' contributions are less than 20% of the measured enthalpy changes. Molar excess enthalpies of D₂O(l)+H₂O(l) at 298.15 K are reported and compared with earlier studies. Enthalpies of reaction obtained from our excess enthalpies show a systematic variation with mole fraction.