Determination of the standard molar enthalpies of formation and heat capacities of Bi25FeO39 (s) and Bi2Fe4O9 (s) by calorimetry

The standard enthalpies of formation of Bi25FeO39 (s) and Bi2Fe4O9 (s), at 298.15 K were determined by acid solution calorimetry and are found to be (−7.55 ± 0.32) × 103 kJ mol−1 and (−2.17 ± 0.07) × 103 kJ mol−1, respectively. Heat capacities of Bi25FeO39 (s) and Bi2Fe4O9 (s) were measured using differential scanning calorimetry (DSC). The measured heat capacities as a function of temperature are expressed as: Cp (J K−1 mol−1) ± 49.9 = 1529.9 + 200.6 × 10−3 T − 21.6 × 106 T−2 (T/K: 308-818), Cp (J K−1 mol−1) ± 11.0 = 395.5 − 23.0 × 10−3 T − 7.6 × 106 T−2 (T/K: 313-773). Deviation of measured Cp values from the estimates by the Neumann-Kopp rule (NKR) has also been discussed.