Ncert Exercises & Solutions

Aluminium trifluoride is insoluble in anhydrous HF but dissolves on addition of NaF. Aluminium trifluoride precipitates out of the resulting solution when gaseous $B F_{3}$ 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:

$A I F_{3} + 3 N a F \to N a_{3} [A I F_{6}]$
$S o d i u m h e x a f l u o r o a l u m i n a t e (I I I)$

When boron trifluoride ( $B F_{3}$ ) is added to the solution, aluminium fluoride precipitates out of the solution. This happens because the tendency of boron to form complexes is much more than that of aluminium. Therefore, when BF3 is added to the solution, B replaces Al from the complexes according to the following reaction:

$N a_{3} [A I F_{6}] + 3 B F_{3} \to 3 N a [B F_{4}] + A I F_{3}$

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