Determination of Gibbs energies of formation of CaFe2O4 and Ca2Fe2O5 from solid-state EMF measurements using CaF2 as solid electrolyte

The standard Gibbs energy change for the reactions CaO + Fe2O3 --> CaFe2O4 and 2 CaO + Fe2O3 --> Ca2Fe2O5 has been determined from solid-state electromotive force (emf) measurements, using CaF2 as solid electrolyte. The temperature dependence of the standard Gibbs energies of formation from the elements is represented by the equations Delta(f)G(o)(CaFe2O4) = - 1448.4 x 10(3) + 128.0 T + 27.95 T In (T) (+/- 1800) J mol(-1) 1030 < T/K < 1280 Delta(f)G(o)(Ca2Fe2O5) = -2049.9 x 10(3) - 12.84 T + 55.91 T In (T) (+/-2800) J mol(-1) 1140 < T/K < 1340 By applying a "third-law" analysis of the experimental data, the standard enthalpy of formation of CaFe2O4 from constituent oxides at 298 K was evaluated as DeltaH(298)(o)(CaFe2O4) = - 17.5 +/- 0.4 kJ mol(-1). A description of the developed apparatus for emf measurements is given.