Thermodynamics Research Center / ThermoML | Journal of Chemical Thermodynamics

ABSTRACT

Thermodynamic properties of monoclinic Fe2(SO4)3 were measured by acid solution calorimetry, adiabatic calorimetry, and semi-adiabatic calorimetry. The standard molar enthalpy of formation ?fHmo was determined as -(2585.2+-4.9) kJ*mol-1 by an appropriate thermochemical cycle with enthalpies of solution of monoclinic Fe2(SO4)3, a-MgSO4, g-FeOOH, H2O, and MgO in 5 N HCl at T = 298 K. Heat capacity data were collected from T = 0.5 K to T = 400 K. The standard molar entropy Smo at T = 298.15 K is (305.6+-0.6) J*K-1*mol-1. While the enthalpy of formation agrees well with a previous ?fHmo determination, the entropy differs significantly because magnetic entropy was entirely overlooked in the previous work. The heat capacity shows a sharp magnetic transition with the peak located at 30.0 K. The Cp,m data in the temperature range 273 to 395 K are represented by a Maier-Kelley polynomial Cp,m / J*K-1*mol-1 = 213.0 + 0.3121(T/K) - 2.959'106 (T/K)-2. The calculated standard molar Gibbs free energy of formation from the elements ?fGmo at T = 298.15 K is -(2264.5+-4.9) kJ*mol-1.