The enthalpy of the isotope-exchange reaction: H 2O + D 2O = 2HDO at temperatures to 673 K and at pressures to 40 MPa

Abstract

The enthalpy of mixing of {(1 − x)H 2O + xD 2O} was measured at temperatures from 310 K to 673 K at pressures from 7 MPa to 37 MPa with a flow calorimeter. Thermodynamic quantities for the isotope-exchange reaction: H 2O + D 2O = 2HDO were calculated from these results, assuming ideal mixing of the products and reactants. A slight asymmetry with x of the mixing enthalpies, observed at higher temperatures, is not considered sufficient to assign values of excess enthalpies of mixing. The equilibrium constant K increased from 3.82 at 298.15 K and 0.1 MPa to 3.93 at 673.15 K at 37 MPa, while Δ r H m o decreased from 129.4 J·mol −1 to 67.5 J·mol −1. The statistical limit K = 4.0 was not attained in the high-temperature limit of the present study.