Question

Aluminium trifluoride is insoluble in anhydrous HF but dissolves on addition of NaF.  Aluminium trifluoride precipitates out of the resulting solution when gaseous BF₃ is buddled through. Give reasons.

Answer 1

Anhydrous HF is a covalent compound and is strongly H-bonded. Therefore, it does not give free F^- ions and hence AlF₃ does not dissolve in HF. In contrast, NaF is an ionic compound and hence F^- ions are easily available. As a result, it combines with AlF₃ to form the soluble complex. 

3NaF + AlF₃ → Na₃ [AlF₃] 

On bubbling gaseous BF₃ , AlF₃ is precipitated. It is because BF₃ is a stronger Lewis acid than AlF₃ . This is attributed to smaller size and higher electronegativity of Boron. As a result of this B has much higher tending to form complexes than Al. Therefore, when BF₃ is added to the above solution, AlF₃ gets precipitated. 

Na₃ [AlF₆] + 3BF₃ → 3Na[BF₄] + AlF₃ (s)