The transported entropy of Na⁺ in solid state cryolite

Abstract

The transported entropy of Na⁺ in mixtures of NaF (s) and Na₃AlF₆ (s) is determined from thermocell experiments. The experiments were favorably described by the electric work method. The variation observed in the thermocell electromotive force (emf) with composition can be explained from the probable path of charge transfer in the electrolyte. The transported entropies are S*^cry _Na+ = 140 ± 7 J K⁻¹ mol⁻¹ for cryolite and S*^NaFNa+ = 81 ± 8 J K⁻¹ mol⁻¹ for sodium fluoride between 380 °C and 500 °C. The value obtained for sodium in the solid cryolite makes us predict that the transported entropy for Na⁺ in themolten electrolyte mixture for aluminum production is substantial and that the reversible heat effects in the aluminum electrolysis cell are the same.