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

Question

Aluminium reacts with concentrated HCl and concentrated \(\mathrm{NaOH}\) to liberate the gases ...... 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

Option (a): \( \text{H}_2 \) and \( \text{H}_2 \).

Step 1: Reaction with HCl

When aluminium reacts with concentrated hydrochloric acid \( \text{HCl} \), it displaces hydrogen from the acid. The reaction can be represented as: \[\text{2Al} + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2\]. This shows that hydrogen gas \( \text{H}_2 \) is liberated when aluminium reacts with concentrated \( \text{HCl} \).

Step 2: Reaction with NaOH

Aluminium also reacts with concentrated sodium hydroxide \( \text{NaOH} \) in a similar manner, where it displaces hydrogen. The reaction is: \[\text{2Al} + 2\text{NaOH} + 6\text{H}_2\text{O} \rightarrow 2\text{NaAl(OH)}_4 + 3\text{H}_2\]. Here, hydrogen gas \( \text{H}_2 \) is again liberated during the reaction with \( \text{NaOH} \).

Step 3: Identify Liberated Gases

From the reactions in Step 1 and Step 2, it is clear that aluminium liberates \( \text{H}_2 \) gas in both reactions—with concentrated \( \text{HCl} \) and \( \text{NaOH} \).

Step 4: Select the Correct Option

Among the given options, the one which shows \( \text{H}_2 \) being liberated in both cases (reactions with \( \text{HCl} \) and \( \text{NaOH} \)) is option (a): \( \text{H}_2 \) and \( \text{H}_2 \).

Key Concepts

Hydrochloric Acid

Hydrochloric acid, commonly known as HCl, is a strong and corrosive acid. It's found naturally in the stomach as gastric acid and is used industrially in the production of many substances. When aluminium, a reactive metal, comes into contact with concentrated hydrochloric acid, a chemical reaction occurs.
This reaction involves the metal aluminium displacing the hydrogen component of HCl due to aluminium's higher reactivity. The simplified equation for this process is:

• \( 2\text{Al} + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2 \)

This results in the formation of aluminium chloride (\(\text{AlCl}_3\)) and the liberation of hydrogen gas \(\text{H}_2\). This reaction is an excellent example of a displacement reaction, where a more reactive element displaces a less reactive one from its compound.

Sodium Hydroxide

Sodium hydroxide, often called lye or caustic soda, is a strong base represented by the chemical formula \(\text{NaOH}\). It is highly soluble in water and is used in many industrial processes such as soap making and paper production. When aluminium comes into contact with concentrated sodium hydroxide, hydrogen gas is liberated through a chemical reaction.
In this interaction, aluminium reacts in the NaOH medium to displace hydrogen. The chemical equation for this reaction is:

• \( 2\text{Al} + 2\text{NaOH} + 6\text{H}_2\text{O} \rightarrow 2\text{NaAl(OH)}_4 + 3\text{H}_2 \)

Here, aluminium reacts with water in the presence of sodium hydroxide, leading to the formation of sodium aluminate \(\text{NaAl(OH)}_4\), alongside the liberation of hydrogen gas \(\text{H}_2\). This reflects the tendency of metals to react with hydroxides under certain conditions.

Hydrogen Gas

Hydrogen is the lightest and most abundant element in the universe, commonly seen in its gaseous form \(\text{H}_2\). In the chemical reactions involving aluminium and either hydrochloric acid or sodium hydroxide, hydrogen gas is a primary product.
Hydrogen gas is colorless, odorless, and highly flammable, making it useful as a fuel and in various chemical processes. The production of hydrogen in these reactions showcases its role in displacement reactions, highlighting its potential as a clean energy source. Aluminium's ability to liberate \(\text{H}_2\) when reacting with acids and bases emphasizes its reactive nature and the fundamental chemical processes of oxidation-reduction.

Displacement Reactions

Displacement reactions are a type of chemical reaction where an atom or a group in a molecule is replaced by another atom or group. In the context of aluminium reactions, it is clear how aluminium plays a crucial role in displacing hydrogen atoms.
For instance, when reacting with hydrochloric acid or sodium hydroxide, the more reactive aluminium displaces hydrogen from its compounds, resulting in hydrogen gas liberation. Such reactions are not only pivotal in understanding basic chemistry principles but also have numerous practical applications in industry.

• In metals, reactivity is seen through their ability to displace less reactive elements.
• This principle helps to predict and explain the outcomes of metal-acid and metal-base reactions.

Thus, understanding displacement reactions helps in identifying the behavior of various elements when subjected to different conditions.