Question

IIydrogen is not liberated when aluminium reacts with (1) boiling water (2) hot conc. \(\mathrm{NaOII}\) (3) conc. IICl (4) conc. \(\mathrm{II}_{2} \mathrm{SO}_{4}\)

Short answer

Concentrated \(\mathrm{H}_2SO_4\).

Step-by-step solution

Identify the reactions

List out the reactions of aluminium with each of the given substances:\(\mathrm{(1)}\ Boiling\ water\ (2)\ Hot\ concentrated\ \mathrm{NaOH}\ (3)\ Concentrated\ \mathrm{HCl}\ (4)\ Concentrated\ \mathrm{H}_2\mathrm{SO}_4\).

Reaction with boiling water

Aluminium reacts with boiling water to form aluminium hydroxide and hydrogen gas. The reaction is: \(2Al + 6H_2O \rightarrow 2Al(OH)_3 + 3H_2\).

Reaction with hot concentrated \(\mathrm{NaOH}\)

Aluminium reacts with hot concentrated \(\mathrm{NaOH}\) to form sodium aluminate and hydrogen gas: \(2Al + 2NaOH + 6H_2O \rightarrow 2NaAl(OH)_4 + 3H_2\).

Reaction with concentrated \(\mathrm{HCl}\)

Aluminium reacts with concentrated \(\mathrm{HCl}\) to produce aluminium chloride and hydrogen gas: \(2Al + 6HCl \rightarrow 2AlCl_3 + 3H_2\).

Reaction with concentrated \(\mathrm{H}_2SO_4\)

Concentrated \(\mathrm{H}_2SO_4\) causes passivation of aluminium due to the formation of a protective oxide layer on the metal's surface, which prevents further reaction. Hence, hydrogen is not liberated in this reaction.

Conclusion

Based on the reactions, hydrogen is not liberated when aluminium reacts with concentrated \(\mathrm{H}_2SO_4\).

Key concepts

Reaction with Boiling Water

Aluminium reacts noticeably with boiling water. When it does, it forms aluminium hydroxide, \(\text{Al(OH)}_3\), and hydrogen gas. This tells us that hydrogen gas is liberated during this reaction. The equation for this reaction is: \[2Al + 6H_2O \rightarrow 2Al(OH)_3 + 3H_2 \].
It's important to understand that this reaction is quite aggressive, often requiring a little time to start due to the presence of an oxide layer on aluminium's surface.
Once the reaction begins, it continuously produces hydrogen gas and aluminium hydroxide as long as contact between aluminium and boiling water is maintained.

Reaction with Hot Concentrated NaOH

Aluminium's reaction with hot concentrated sodium hydroxide \(NaOH\) is fascinating. In this scenario, aluminium reacts to form sodium aluminate \(NaAl(OH)_4\) and releases hydrogen gas. The chemical equation for this reaction is: \[ 2Al + 2NaOH + 6H_2O \rightarrow 2NaAl(OH)_4 + 3H_2 \].
This reaction can be quite vigorous and exothermic (releases heat).
Sodium hydroxide corrodes the aluminium, breaking down the oxide layer, which normally protects the aluminium from reacting. This exposure allows the reaction to progress fully, producing hydrogen gas rather quickly.

Passivation by Concentrated H2SO4

Concentrated \( \text{H}_2\text{SO}_4 \) causes a unique phenomenon when it comes into contact with aluminium. Instead of a typical vigorous reaction, the concentrated sulfuric acid induces a process called passivation.
During passivation, a protective oxide layer forms on the surface of the aluminium, making it inert to reactive substances. Consequently, no hydrogen gas is liberated in this reaction.
The protective oxide layer formed is dense and adherent, effectively shielding the underlying aluminium from further reactions with the acid. This property is extensively utilized in industries to safeguard aluminium structures from corrosion by acidic environments.