Thermodynamics of the system NaF-Alf₃. part ii: The free energies of formation of cryolite (Na₃AlF₆) and chiolite (Na₅Al₃F₁₄)

Abstract

The theory of the solid-electrolyte cells$\begin{gathered} Al\left| \begin{gathered} Na_3 AlF_6 \hfill \\ NaF \hfill \\ \end{gathered} \right|Na^ + \left| \begin{gathered} Na_3 AlF_6 \hfill \\ Na_5 Al_3 F_{14} \hfill \\ \end{gathered} \right|Al \hfill \\ and \hfill \\ Al\left| \begin{gathered} Na_3 AlF_6 \hfill \\ Na_5 Al_3 F_{14} \hfill \\ \end{gathered} \right|Na^ + \left| \begin{gathered} Na_5 Al_3 F_{14} \hfill \\ AlF_3 \hfill \\ \end{gathered} \right|Al \hfill \\ \end{gathered}$ is given, and it is shown that cryolite itself with Ca²⁺ in solid solution is a suitable Na⁺-ion conductor. Experimental electromotive forces for the ranges 570° to 725°C and 570° to 670°C, r − 18,960 cal with a standard deviation of ±36 cal (based on a third-law calculation). For 5NaF(s) + 3AlF₃(s) = Na₅Al₃F₁₄(s), ΔG° = −38,560 − 7.081T with a standard deviation of ±130 cal. Combination of these results with recent values for Al + 3/2 F₂ = A1F₃ and for 6NaF + Al = Na₃AlF₆ + 3Na gives ΔH°_f298(Na₃AlF₆) = −792,400 cal and ΔH°_f298(NaF) = −137,530 cal. The latter is in excellent agreement with the most recent critical assessment.