Aluminium trifluoride is insoluble in anhydrous HF but dissolves on the addition of NaF. Aluminium trifluoride precipitates out of the resulting solution when gaseous BF3 is bubbled through. Give reasons.
Hydrogen fluoride (HF) is a covalent compound and has a very strong intermolecular hydrogen-bonding. Thus, it does not provide ions and aluminium fluoride (AlF) does not dissolve in it. Sodium fluoride (NaF) is an ionic compound and when it is added to the mixture, AlF dissolves. This is because of the availability of free F–. The reaction involved in the process is:
`AlF_3 + 3 NaF -> Na_3[AlF_6]`
Sodium hexafluoroaluminate (III)
Since, anhydrous HF is covalent compound and weak acid due to high bond dissociation energy. AlF3does not dissolve in HF.
Whereas NaF is ionic compound.
3NaF + AlF3 ———> Na3[AlF6]
Na3[AlF6] + 3BF3(g) ——-> AlF3 + 3Na+ [BF]–