Thermodynamics of monoclinic Fe2(SO4)3

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 ΔfHm∘ was determined as −(2585.2 ± 4.9) kJ · mol−1 by an appropriate thermochemical cycle with enthalpies of solution of monoclinic Fe2(SO4)3, α-MgSO4, γ-FeOOH, H2O, and MgO in 5 N HCl at T = 298 K. Heat capacity data were collected from T = (0.5 to 400) K. The standard molar entropy Sm∘ 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 ΔfHm∘ 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 ΔfGm∘ at T = 298.15 K is −(2264.5 ± 4.9) kJ · mol−1.