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The p-Block Elements — Chemistry STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 ScienceChemistryThe p-Block Elements5 Marks
Question
When metal X is treated with sodium hydroxide, a white precipitate (A) is obtained, which is soluble in excess of NaOH to give soluble complex (B). Compound (A) is soluble in dilute HCl to form compound (C). The compound (A) when heated strongly gives (D), which is used to extract metal. Identify (X), (A), (B), (C) and (D). Write suitable equations to support their identities.
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Answer

The given metal X gives a white precipitate with sodium hydroxide and the precipitate dissolves in excess of sodium hydroxide. Hence, X must be aluminium.
The white precipitate (compound A) obtained is aluminium hydroxide. The compound B formed when an excess of the base is added is sodium tetrahydroxoaluminate(III).
$ \ \ \ \ \ \ \ \ \ \ 2\text{Al} \ \ \ \ \ \ \ \ + \ \ \ \ \ \ \ \ 3\text{NaOH} \ \ \ \ \ \ \ \ \rightarrow \ \ \ \ \ \ \ \ \text{Al(OH)}_3\downarrow+3\text{Na}^+ \\ \text{Aliminium (X)} \ \ \ \ \text{Sodium hydroxide} \ \ \ \ \ \ \ \ \ \ \ \ \text{White ppt. (A)}$
$\text{Al(OH)}_3 \ \ \ \ \ \ \ + \ \ \ \ \ \ \ \text{NaOH} \ \ \ \ \ \ \ \ \rightarrow \ \ \ \ \ \ \ \ \text{Na}^+\big[\text{Al(OH)}_4\big]^- \\ \ \ \ \ \text{(A)} \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{Sodium tetrahydroxoaliminate (iii)} \\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{(Soluble complex B)}$
Now, when dilute hydrochloric acid is added to aluminium hydroxide, aluminium chloride (compound C) is obtained.
$\text{Al(OH)}_3+3\text{HCl}\rightarrow\text{AlCl}_3+\text{H}_2\text{O} \\ \ \ \ (\text{A}) \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{(C)}$
Also, when compound A is heated strongly, it gives compound D. This compound is used to extract metal X. Aluminium metal is extracted from alumina. Hence, compound D must be alumina.
$2\text{Al(OH)}_3\xrightarrow{\Delta}\text{Al}_2\text{O}_3+3\text{H}_2\text{O} \\ \ \ \ \ \text{(A)} \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{(D)}$

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