A thermogravimetric method for accurate determination of thermodynamic quantities at high temperatures

Abstract

A method for the determination of the change in enthalpy, entropy, and specific heat capacity for monovariant heterogenous equilibria is presented. These quantities are obtained indirectly by measuring the temperature dependence of equilibrium pressures. At a given pressure of the relevant gas the corresponding equilibrium temperature is selected from the continuously recorded temperature cycling around this equilibrium temperature, with the aid of the mass of the condensed phase recorded at the same time. For the equilibrium: CaCO3(s) = CaO(s) + CO2(g), from 900 to 1175 K the results are as follows. For ΔrCp = 0: log10(p/atm) = −8.828 × 103(K/T) + 7.572, with ΔrHo = 40.39 kcalth mol−1, and ΔrSo = 34.64 calth K−1 mol−1. For ΔrCp = 13.1 calth K−1 mol−1: log10(p/atm) = −15.227 − 5.819 × 103(K/T)+ 6.596log10(T/K), with ΔrHo(T) = “26.62 + 13.1 × 10−3 (T/K)’ kcalth mol−1, and ΔrSo(T) = “−56.53 + 30.16 log10(T/K)’ calth K−1 mol−1.