Heat Capacities and Thermodynamic Functions of the Aqueous Li2B4O7 Solution in the Temperature Range from 80 K to 355 K

Abstract

The molar heat capacities of the aqueous Li2B4O7 solution at the concentration of 0.0187 mol·kg-1 have been measured using a precision automated adiabatic calorimeter in the temperature range from 80 K to 355 K. The dependence of the molar heat capacity on temperature was given as a function of the reduced temperature X by polynomial equations Cp,m (J·K-1·mol-1) = 25.44 + 11.99X + 0.454X2 + 1.671X3 for the solid phase [80 K∼258 K, X = (T − 169)/89] and Cp,m (J·K-1·mol-1) = 77.33 for the liquid phase [274 K∼355 K], respectively. The phase transition of the solution was determined on the basis of the curve of the heat capacity with temperature. The temperature of the phase transition was observed at 273.04 K, and the enthalpy and entropy of the phase transition were calculated to be ΔHm = 4.650 kJ·mol-1 and ΔSm = 17.03 J·K-1·mol-1, respectively. According to the polynomial equations and thermodynamic relationship, the values of the thermodynamic function of the aqueous Li2B4O7 solution relative to 298.15 K were calculated in the temperature range from 80 K to 355 K with an interval of 5 K. The relative apparent molar heat capacities of the aqueous Li2B4O7 solution, Cp,φ, were calculated every 5 K in the temperature range from 80 K to 355 K from the experimental heat capacities of the aqueous Li2B4O7 solution and the heat capacities of pure water.