Thermodynamics of monoclinic Fe(SO)

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 5N HCl at T=298K. Heat capacity data were collected from T=(0.5 to 400)K. The standard molar entropy Sm∘ at T=298.15K 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 30.0K. 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=298.15K is −(2264.5±4.9)kJ·mol−1.