Problem 36 - A Complete Resource Book in Chemistry for JEE Main

Question

Aluminium reacts with concentrated \(\mathrm{HCl}\) and concentrated \(\mathrm{NaOH}\) to liberate the gases \(\ldots \ldots\) respectively. (a) \(\mathrm{H}_{2}\) and \(\mathrm{H}_{2}\) (b) \(\mathrm{O}_{2}\) and \(\mathrm{O}_{2}\) (c) \(\mathrm{O}_{2}\) and \(\mathrm{H}_{2}\) (d) \(\mathrm{H}_{2}\) and \(\mathrm{O}_{2}\)

Step-by-Step Solution

Answer

(a) \(\mathrm{H}_{2}\) and \(\mathrm{H}_{2}\)

Step 1: Identify the reactions

Understand that aluminum reacts with both hydrochloric acid (HCl) and sodium hydroxide (NaOH) to release gases. We need to determine these gases for each reaction.

Step 2: Reaction with Hydrochloric Acid

When aluminum (\( \mathrm{Al} \)) reacts with hydrochloric acid (\( \mathrm{HCl} \)), hydrogen gas (\( \mathrm{H_2} \)) is released. The balanced chemical equation for this reaction is: \[ 2\mathrm{Al} + 6\mathrm{HCl} \rightarrow 2\mathrm{AlCl_3} + 3\mathrm{H_2} \]. The gas released from this reaction is \( \mathrm{H_2} \).

Step 3: Reaction with Sodium Hydroxide

When aluminum reacts with sodium hydroxide (\( \mathrm{NaOH} \)), hydrogen gas (\( \mathrm{H_2} \)) is also released. The balanced chemical equation is: \[ 2\mathrm{Al} + 2\mathrm{NaOH} + 6\mathrm{H_2O} \rightarrow 2\mathrm{NaAl(OH)_4} + 3\mathrm{H_2} \]. The gas released from this reaction is \( \mathrm{H_2} \).

Step 4: Conclusion

The gases released from both reactions are hydrogen (\( \mathrm{H_2} \)) and hydrogen (\( \mathrm{H_2} \)). Therefore, the correct choice is option (a).

Key Concepts

Hydrochloric acid reaction

When aluminum (2Al1) interacts with hydrochloric acid (2HCl1), an interesting chemical reaction occurs. This reaction results in the liberation of hydrogen gas (2H_21). The process begins as the aluminum displaces hydrogen from the hydrochloric acid. This happens because aluminum is a more reactive metal than hydrogen. The activity series of metals plays a crucial role in predicting this displacement reaction.
To break it down further, in the reaction between aluminum and hydrochloric acid:

• Aluminum goes from a metal to forming aluminum chloride (2AlCl_31).
• Hydrogen ions from the acid are reduced to form hydrogen gas.

The balanced chemical equation for this reaction is:\[ 2\mathrm{Al} + 6\mathrm{HCl} \rightarrow 2\mathrm{AlCl_3} + 3\mathrm{H_2} \]This equation clearly shows that for every 2 moles of aluminum reacting with 6 moles of hydrochloric acid, 3 moles of hydrogen gas are produced.

Sodium hydroxide reaction

Aluminum also reacts with sodium hydroxide (2NaOH1) to produce hydrogen gas. This might be surprising to some, as 2NaOH1 is a strong base rather than an acid like 2HCl1. However, aluminum can react with 2NaOH1 in the presence of water to liberate hydrogen gas due to its amphoteric nature.
Aluminum's amphoteric property means it can react with both acids and bases. During the reaction with sodium hydroxide:

• Aluminum forms sodium aluminate (2NaAl(OH)_41).
• In this process, water molecules also play an essential role to release hydrogen gas.

The balanced equation illustrating this reaction is:\[ 2\mathrm{Al} + 2\mathrm{NaOH} + 6\mathrm{H_2O} \rightarrow 2\mathrm{NaAl(OH)_4} + 3\mathrm{H_2} \]Here, for every 2 moles of aluminum and sodium hydroxide, in the presence of 6 moles of water, 3 moles of hydrogen gas are released.

Hydrogen gas production

Hydrogen gas production is a key result of aluminum's interaction with both hydrochloric acid and sodium hydroxide. In each of these reactions, hydrogen gas is consistently formed, albeit through slightly different pathways.
This production of hydrogen is of interest for a variety of scientific and industrial reasons:

• Hydrogen gas is a clean fuel, making its production important for energy applications.
• Knowing how to generate hydrogen via chemical reactions can aid in understanding and developing alternative energy resources.
• Additionally, learning about these reactions helps students understand core principles of chemistry, such as reactivity and the concept of amphoterism.

Reflecting on the balanced equations for both reactions, it becomes clear that aluminum serves as a versatile element capable of producing hydrogen gas in different chemical environments.