Question

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

Answer 1

Aluminium trifluoride is insoluble in anhydrous HF, this because of the covalent nature of HF molecule. In HF molecule Strong hydrogen bonds are present. Therefore, no free fluoride ions are produced.
On the other hand the NaF molecule is ionic in nature and produce free fluoride ions.
These fluoride ions combines with AlF3 to form soluble complex sodium hexa flouro aluminate(III)
3NaF+ AlF3 → Na3 [AlF6]
Aluminium triflouride precipitates out of the resulting solution when gaseous BF3 is bubbled through
Reason: BF3 is a stronger lewis acid than AlF3. This is due to the small size and higher electro negativity of boron than aluminum . Hence, Boron has more tendency to form complexes when compared to aluminium.
So, BF3 forms a complex compound precipitating AlF3.
Na3 [AlF6] + 3BF3→ 3Na[BF4] + AlF3 .

Answer 2

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