Problem 84 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is not correct? (a) Al reacts with \(\mathrm{NaOH}\) and liberates \(\mathrm{H}_{2}\) (b) \(\mathrm{AlCl}_{3}\) is a Lewis acid (c) Al is used in the manufacture of electrical cables (d) \(\mathrm{NaOH}\) is used during Halls process of purification of bauxite

Step-by-Step Solution

Answer

(d) NaOH is not used in Hall's process of bauxite purification.

Step 1: Understanding the Reaction of Al with NaOH

Aluminum ( Al ) reacts with sodium hydroxide ( NaOH ) to produce sodium aluminate ( NaAlO_2 ) and liberate hydrogen gas ( H_2 ). Therefore, statement (a) is correct.

Step 2: Reviewing AlCl3 as a Lewis Acid

Aluminum chloride ( AlCl_3 ) can accept electron pairs due to the vacant orbitals on the aluminum atom, thus it acts as a Lewis acid. Hence, statement (b) is correct.

Step 3: Evaluating the Use of Al in Electrical Cables

Aluminum is a good conductor of electricity and is lightweight, so it is commonly used in the manufacture of electrical cables. Therefore, statement (c) is correct.

Step 4: Verifying the Use of NaOH in Hall's Process

In the Hall’s process, used for the extraction of aluminum from bauxite, cryolite is used but sodium hydroxide ( NaOH ) is not involved during the electrolytic reduction step. Thus, statement (d) is incorrect.

Key Concepts

Lewis acid

A Lewis acid is a substance that can accept an electron pair. In chemical reactions, Lewis acids play an important role by interacting with Lewis bases, which are electron pair donors.
Aluminum chloride (\(\text{AlCl}_3\)) is a classic example of a Lewis acid. The aluminum atom in \(\text{AlCl}_3\) has vacant orbitals, meaning it is electron-deficient and can accept electrons from a donor.
- When \(\text{AlCl}_3\) reacts with a Lewis base, it forms a coordinate bond.- This reaction is quite useful in various industrial and chemical processes, such as the Friedel-Crafts reaction, which introduces alkyl or acyl groups into aromatic compounds.
Understanding Lewis acids is crucial when studying reactions where electron transfer occurs, highlighting the importance of electron-rich and electron-deficient species interactions.

Hall's process

The Hall's process is a significant industrial method for extracting aluminum from bauxite ore. This process is essential for obtaining aluminum in large quantities efficiently and economically.
In the Hall's process:

• Bauxite is first purified through the Bayer process to obtain alumina (\(\text{Al}_2\text{O}_3\)).
• The purified alumina is then dissolved in molten cryolite to lower its melting point.
• In the electrolytic cell, aluminum is extracted as it is deposited at the cathode.

Cryolite plays a critical role, serving as a solvent to dissolve aluminum oxide, making the extraction energy-efficient. Although \(\text{NaOH}\) is used in the Bayer process for purifying bauxite by removing impurities, it is not directly used in the electrolytic stage of the Hall's process.
This separation between the Bayer and Hall's processes emphasizes how different techniques complement one another in the complete extraction procedure.

Sodium aluminate

Sodium aluminate is formed when aluminum reacts with sodium hydroxide (\(\text{NaOH}\)). This compound showcases the versatility of aluminum in various chemical reactions.
- The reaction occurs as follows: \[ \text{2Al} + 2\text{NaOH} + 6\text{H}_2\text{O} \rightarrow 2\text{NaAlO}_2 + 3\text{H}_2 \]- In this reaction, hydrogen gas (\(\text{H}_2\)) is liberated as a byproduct.
Sodium aluminate (\(\text{NaAlO}_2\)) has multiple industrial applications:

• It is used in water treatment as a coagulant, helping to remove impurities.
• It serves as an intermediate in the production of zeolites and other aluminum-based products.

This reaction and the roles of sodium aluminate highlight aluminum's reactivity and its significance in both industrial processes and applied chemistry.