Question 11.16 Aluminium trifluoride is insoluble in anhydrous HF but dissolves on addition of NaF. Aluminium trifluoride precipitates out of the resulting solution when gaseous BF3 is bubbled through. Give reasons.
Class XI The p-Block Elements Page 324
Class XI The p-Block Elements Page 324
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(i) Anhydrous HF is a covalent compound and is strongly H-bonded. Therefore , it doesn't give F– ions and hence AlF3 doesn't dissolve in HF. NaF is an ionic compound. it contains F– ions which combine with electron deficient AlF3 to form the soluble complex .3NaF + AlF3 ------> Na3[AlF6] {sod. hexafluoroaluminate(III)}
(ii) Boron due to its small size and higher electronegativity has greater tendency to form complexes than Aluminum. Hence, precipitation of AlF3 takes place when BF3 is passed through Na3[AlF6] solution.
Na3[AlF6] + 3BF3 -------> 3Na[BF4] {sod. tetrafluoroborate(III) }+ AlF3
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Explanation:
Anhydrous HF is a covalent compound and is strongly H-bonded. Therefore , it doesn't give F– ions and hence AlF3 doesn't dissolve in HF. NaF is an ionic compound. it contains F– ions which combine with electron deficient AlF3 to form the soluble complex .
3NaF + AlF3 ------> Na3[AlF6]