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.  

Answer:

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. \[N{{a}_{3}}Al{{F}_{6}}+3B{{F}_{3}}\to 3Na[B{{F}_{4}}]+Al{{F}_{3}}\]