Question

Give reason. $Al{{F}_{3}}$ in insoluble anhydrous $\left( HF \right)$ but dissolve on addition of $NaF$, $Al{{F}_{3}}$ precipitate out when gaseous $B{{F}_{3}}$ is bubbled through it.

Answer 1

Precipitation is the process in which the considered compound comes out of solution.
-When a solute is added to a solution, the interaction between the molecules of the solute and the molecules which were formerly present in the solution, decides whether the solute will dissolve in the solution or would form a precipitate.

Complete step by step answer:
$Al{{F}_{3}}$ is produced by treating alumina with hydrogen fluoride. This adopts three dimensional polymeric structure and also has a high melting point.
We know that anhydrous $HF$ is a covalent compound and held together by strong intramolecular hydrogen bonding.
In $HF$, fluorine is strongly held together to hydrogen because of hydrogen bonding through which fluoride ions is not easy to dissociate. IT does not dissolve in $Al{{F}_{3}}$ and does not have fluoride ions to furnish the reaction.
In Spite of which $NaF$ is an organic compound and can easily dissociate on hydrolysis because they are held together by weak forces and have fluoride ions also. So we can say that it can combine with $Al{{F}_{3}}$ to form a soluble complex. And the reaction may be given as –
$Al{{F}_{3}}+3NaF\to N{{a}_{3}}\left[ Al{{F}_{6}} \right]$
Whereas, when $B{{F}_{3}}$ is passed through the solution, $Al{{F}_{3}}$ precipitated out because boron having small size and high electronegativity as compare to $Al$. Boron has a high tendency to form complex after formation of soluble complex of aluminium. Due to this reason $Al{{F}_{3}}$ is precipitated out when boron trifluoride is added to the solution. The reaction may be given as,
$N{{a}_{3}}\left[ Al{{F}_{6}} \right]+3B{{F}_{3}}\to 3NaB{{F}_{3}}+Al{{F}_{3}}$
So we can say that due to the high tendency of boron to form complex aluminium trifluoride is precipitated out.

Note: The Aluminium Trifluoride readily dissolves in sodium fluoride because of the presence of weak forces between the bonds, and the presence of more fluoride ions.
-Boron trifluoride when passed through a solution of aluminium trifluoride, it forms a precipitate because of the small size of boron, it has a higher tendency to form complexes than aluminium.