Determination of thermodynamic properties of Na 2S using solid-state EMF measurements

Abstract

To obtain reliable thermodynamic data for Na 2S(s), solid-state EMF measurements of the cell Pd(s)|O 2(g)|Na 2S(s), Na 2SO 4(s)|YSZ| Fe(s), FeO(s)|O 2(g) ref| Pd(s) were carried out in the temperature range 870 < T/K < 1000 with yttria stabilized zirconia as the solid electrolyte. The measured EMF values were fitted according to the equation E fit/V (±0.00047) = 0.63650 − 0.00584732( T/K) + 0.00073190( T/K) ln ( T/K). From the experimental results and the available literature data on Na 2SO 4(s), the equilibrium constant of formation for Na 2S(s) was determined to be lg K f(Na 2S(s)) (±0.05) = 216.28 − 4750( T/K) −1 − 28.28878 ln ( T/K). Gibbs energy of formation for Na 2S(s) was obtained as Δ f G ∘(Na 2S(s))/(kJ · mol −1) (±1.0) = 90.9 − 4.1407( T/K) + 0.5415849( T/K) ln ( T/K). By applying third law analysis of the experimental data, the standard enthalpy of formation of Na 2S(s) was evaluated to be Δ f H ∘(Na 2S(s), 298.15 K)/(kJ · mol −1) (±1.0) = −369.0. Using the literature data for C p and the calculated Δ f H ∘, the standard entropy was evaluated to S ∘(Na 2S(s), 298.15 K)/(J · mol −1 · K −1) (±2.0) = 97.0.