The isobaric heat capacity of liquid H20, 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 D20. 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 - a)Cp.0.Aa + caCp.o.A + DCp,app. The term DCp.app depends explicitly on the reaction enthalpy. In H20, the enthalpy DH = -1.84kJ mol-1 for the dissociation reaction and the heat capacity Cp. B = 47.8J K- 1 mol- 1 for free water molecules are calculated. Analogous calculations performed for D20 yield the enthalpy, DH = - 1.64 kJ mol 1 and the heat capacity, Cp.a = 49.18 J K- 1 mol- 1.
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