Thermodynamics of monoclinic Fe2(SO4)3

Thermodynamic properties of monoclinic Fe 2 (SO 4 ) 3 were measured by acid solution calorimetry, adiabatic calorimetry, and semi-adiabatic calorimetry. The standard molar enthalpy of formation Δ f H m ∘ was determined as −(2585.2 ± 4.9) kJ · mol −1 by an appropriate thermochemical cycle with enthalpies of solution of monoclinic Fe 2 (SO 4 ) 3 , α-MgSO 4 , γ-FeOOH, H 2 O, 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 S m ∘ 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 Δ f H m ∘ 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 C p,m data in the temperature range (273 to 395) K are represented by a Maier–Kelley polynomial C p,m /J · K −1  · mol −1  = 213.0 + 0.3121( T /K) − 2.959 × 10 6 ( T /K) −2 . The calculated standard molar Gibbs free energy of formation from the elements Δ f G m ∘ at T  = 298.15 K is −(2264.5 ± 4.9) kJ · mol −1 .