Aluminum 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 test 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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Solution

$A l$ reacts with both acid and base and hence, it is an amphoteric metal.

On reacting with $H C l$ and $N a O H$ , $A l$ liberates $H_{2}$ gas which burns in air with a pop sound.

$\begin{matrix} 2 A l (s) + 2 N a O H (a q) + 6 H_{2} O_{(l)} ↓ 2 N a^{+} [A l (O H)_{4})]^{-} + 3 H_{2} (g) \end{matrix}$

$\begin{matrix} 2 A l (s) + 6 H C l (a q) ↓ 2 A l^{3 +} (a q) + 6 C l^{-} + 3 H_{2} (g) i . e 2 A l C l_{3} + 3 H_{2} \end{matrix}$

While $H N O_{3}$ is a very strong oxidizing agent and it oxidises $A l$ to $A l_{2} O_{3}$ (Aluminium oxide) forming a thin layer of $A l_{2} O_{3}$ on the surface of $A l$ .

$\begin{matrix} 2 A l (s) + 6 H N O_{3 (a q .)} ↓ A l_{2} O_{3} (s) + 6 N O_{2} (g) + 3 H_{2} O (l) \end{matrix}$

This layer protects the further reaction of $A l$ with $H N O_{3}$ and thus no $H_{2}$ gas is liberated.

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Similar questions

Name the gas produced when aluminium foil reacts with :

(a) dilute hydrochloric acid.

(b) sodium hydroxide solution.

Give reason for the following :

(a) Though lead is above hydrogen in the activity series, it does not react with dilute hydrochloric acid or dilute sulphuric acid.

(b) Potassium and sodium are not used fro reaction with dilute hydrochloric acid or dilute sulphuric acid in laboratory preparation of hydrogen.

(i)

(i i)

(i i)

X, Y

Z

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