Question

Aluminum fuoride is insoluble in pure liquid HF, but dissolves readily when Naf is added. When boron trifluoride is bubbled into the solution, aluminum fluoride precipitates. Write two equations of reaction that explain these observations. Use acid-base reaction concepts. Include state designations. Indicate solvation by HF as (HF) in the equation, analogous to (aq). Aluminum fluoride plus sodium fluoride in HF liquid reaction 1: A1F3(s) + 3 NaF(aq) + Nag [A1F6](aq) Aluminum product from above plus boron trifluoride gas in HF liquid reaction 2: Na A1F6(aq) + 3 BF,(g) + A1F,(s) + 3 NaBF (aq)

Answer 1

1.

Aluminium fluoride in HF when NaF is added, forms soluble compound of aluminium, Cryolite.

AlF3 (s) + 3 NaF (HF) ---------> Na3[AlF6] (HF)

Cryolite

2.

The soluble compound formed above, react with boron trifluoride to form a precipitate of aluminium fluoride again.

Na3[AlF6] (HF) + 3 BF3 (g) --------------> AlF3 (s) + 3 NaBF4 (HF)

Note: (HF) indicates the medium liquid HF