Question

Aluminium trifluoride is insoluble in anhydrous HF but dissolves on addition of NaF. Aluminium trifluoride precipitates out of the resulting solution when gaseous $BF_{3}$ is bubbled through. Give reasons.

Answer 1

Anhydrous HF molecule will not give the F ions, due to the "covalent nature" of "HF molecule", i.e., it has Strong hydrogen bonds. Hence, "Aluminium trifluoride" is insoluble in "anhydrous HF".

NaF molecule gives the F ions, due to its ionic nature. These ions combines with Aluminium trifluoride $Al{ F }_{ 3 }$ and forms "Sodium hexa flouro aluminate (III)".

$3NaF\quad +\quad Al{ F }_{ 3 }\quad \rightarrow \quad { Na }_{ 3 }\left[ Al{ F }_{ 6 } \right]$

When the gaseous ${ BF }_{ 3 }$ is passed through the solution of Sodium hexa flouro aluminate (III), precipitation of Aluminium triflouride $Al{ F }_{ 3 }$ takes place. Since Boron has a higher tendency than Aluminium to form complexes due to its small size and higher electro negativity.

${ Na }_{ 3 }\left[ Al{ F }_{ 6 } \right] \quad +\quad 3B{ F }_{ 3 }\quad \rightarrow \quad 3Na\left[ { BF }_{ 4 } \right] \quad +\quad Al{ F }_{ 3 }\quad$

Answer 2

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