• # question_answer 24) 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.

Anhydrous HF is a covalent compound and is strongly hydrogen bonded. It does not ionise and therefore, ions do not react with $Al{{F}_{3}}$ to give a ${{[Al{{F}_{6}}]}^{3-}}$complex ion. However, when$NaF$, an ionic compound, is added, ${{F}^{-}}$ ions are furnished and a complex is formed which is soluble. $Al{{F}_{3}}+3NaF\to \underset{\text{Soluble}\,\text{complex}}{\mathop{N{{a}_{3}}[Al{{F}_{6}}]}}\,$ Boron, on account of small size and high electro negativity, has much higher tendency to form complexes than Al, therefore, when $B{{F}_{3}}$ is added to the aluminium complex, $Al{{F}_{3}}$ gets precipitated. $N{{a}_{3}}Al{{F}_{6}}+3B{{F}_{3}}\to 3Na[B{{F}_{4}}]+Al{{F}_{3}}$