Aluminium trifluoride is insoluble in anhydrous HF but dissolves on addition of NaF. Aluminium trifluoride precipitates out of the resulting solution when gaseous $\mathrm{BF}_3$ is bubbled through. Give reasons.

Question

Aluminium trifluoride is insoluble in anhydrous HF but dissolves on addition of NaF. Aluminium trifluoride precipitates out of the resulting solution when gaseous $\mathrm{BF}_3$ is bubbled through. Give reasons.,

MARKS App · Accepted Answer

(i) Anhydrous $\mathrm{HF}$ is a covalent compound and is strongly $\mathrm{H}$ - bonded. Therefore, it does give $\mathrm{F}^{-}$ions hence $\mathrm{AlF}_3$ does not dissolve in $\mathrm{HF}$. In contrast, $\mathrm{NaF}$ being an ionic compound contains $\mathrm{F}^{-}$ions and hence combines with $\mathrm{AlF}_3$ to form the soluble complex. (ii) Because of smaller size and higher electronegativity, $\mathrm{B}$ has much higher tendency to form complexes than $\mathrm{Al}$, therefore, when $\mathrm{BF}_3$ is added to the above solution, $\mathrm{AlF}_3$ gets precipitated.

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