Isobaric heat capacity and structure of water and heavy water in the liquid state

Abstract

The isobaric heat capacity of liquid H2O, Cp, as a function of temperature, decreases between 0° and about 35°C and then increases up to 100°C. Analogous behaviour is shown by liquid D2O. A statistical thermodynamic model has been applied to the experimental heat capacity data. The behaviour is explained by assuming that an equilibrium A + B = AB is established between clusters A and AB of water of different composition. The total heat capacity is considered as the sum of three terms Cp = (1 − α)Cp,0,AB + αCp,0,A + ΔCp,app. The term ΔCp,app depends explicitly on the reaction enthalpy. In H2O, the enthalpy ΔH = − 18.4 kJ mol−1 for the dissociation reaction and the heat capacity Cp,B = 47.8 J K−1 mol−1 for free water molecules are calculated. Analogous calculations performed for D2O yield the enthalpy, ΔH = − 1.64 kJ mol−1 and the heat capacity, Cp,B = 49.18 J K−1 mol−1.