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Aluminium dissolves in mineral acids and aqueous alkalies and thus shows amphoteric character. A piece of aluminium foil is treated with dilute hydrochloric acid or dilute sodium hydroxide solution in a test tube and on bringing a burning matchstick near the mouth of the rest tube, a pop sound indicates the evolution of hydrogen gas. The same activity when performed with concentrated nitric acid, reaction doesn't proceed. Explain the reason.
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Verified Answer
Al being amphoteric dissolves both in acids and alkalies evolving $\mathrm{H}_2$ gas which burns with a pop sound.
$$
\begin{aligned}
&2 \mathrm{Al}+6 \mathrm{HCl} \rightarrow 2 \mathrm{AlCl}_3+3 \mathrm{H}_2 \\
&2 \mathrm{Al}+2 \mathrm{NaOH}+2 \mathrm{H}_2 \mathrm{O} \rightarrow 2 \mathrm{NaAlO}_2+3 \mathrm{H}_2
\end{aligned}
$$
Sod. meta aluminate
Conc $\mathrm{HNO}_3$ renders aluminium passive by forming a protective oxide layer on the surface.
$$
2 \mathrm{Al}+6 \mathrm{HNO}_3 \rightarrow \mathrm{Al}_2 \mathrm{O}_3+6 \mathrm{NO}_2+3 \mathrm{H}_2 \mathrm{O}
$$
$$
\begin{aligned}
&2 \mathrm{Al}+6 \mathrm{HCl} \rightarrow 2 \mathrm{AlCl}_3+3 \mathrm{H}_2 \\
&2 \mathrm{Al}+2 \mathrm{NaOH}+2 \mathrm{H}_2 \mathrm{O} \rightarrow 2 \mathrm{NaAlO}_2+3 \mathrm{H}_2
\end{aligned}
$$
Sod. meta aluminate
Conc $\mathrm{HNO}_3$ renders aluminium passive by forming a protective oxide layer on the surface.
$$
2 \mathrm{Al}+6 \mathrm{HNO}_3 \rightarrow \mathrm{Al}_2 \mathrm{O}_3+6 \mathrm{NO}_2+3 \mathrm{H}_2 \mathrm{O}
$$
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