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) \(\mathrm{Al}\) 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

Statement (d) is not correct for the context of Hall's process.

Step 1: Analyze reaction of Al with NaOH

Aluminum reacts with sodium hydroxide to produce hydrogen gas. The balanced chemical equation is \( 2Al + 2NaOH + 6H_2O \rightarrow 2NaAl(OH)_4 + 3H_2 \). Therefore, this statement (a) is correct.

Step 2: Determine if AlCl3 is a Lewis acid

A Lewis acid is a compound that can accept a pair of electrons. \( \text{AlCl}_3 \) has an empty p-orbital and can accept an electron pair, making it a Lewis acid. Therefore, statement (b) is correct.

Step 3: Examine the use of Al in electrical cables

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

Step 4: Verify the role of NaOH in Hall's process

The Hall-Héroult process is used to extract aluminum by electrolysis after bauxite is purified to alumina through the Bayer process, where \( \text{NaOH} \) is used to dissolve the bauxite. Therefore, statement (d) is correct for Bayer process, not specifically the Hall’s process. Hence, it can be misleading to say \( \text{NaOH} \) is used in Hall's process.

Key Concepts

Understanding Lewis Acids

In the world of chemistry, a Lewis acid is defined by its ability to accept a pair of electrons. This might sound a bit abstract, so let's break it down with some simple examples. Typically, Lewis acids are substances that lack electrons to complete their valence shells. This electron deficiency means they are pretty eager to snatch electron pairs from other molecules or ions. This behavior is important in many types of chemical reactions, particularly those involving the formation of coordination complexes.
For example, \( ext{AlCl}_3\) acts as a Lewis acid because it possesses an empty p-orbital. This emptiness allows \( ext{AlCl}_3\) to accept an electron pair when it interacts with Lewis bases, which are usually electron-rich substances. Understanding this concept helps us predict and explain the reactivity of substances in various chemical contexts.

The Role of Electrolysis in Chemistry

Electrolysis is a fascinating process that plays a significant role in the extraction of metals and other chemical transformations. At its core, electrolysis involves using an electric current to drive a non-spontaneous chemical reaction. This method is critical in industry and in the lab for separating elements from naturally occurring sources.
In the case of aluminum extraction, electrolysis is used in the well-known Hall-Héroult process. This process involves passing an electric current through a molten mixture of alumina \(\( ext{Al}_2 ext{O}_3\)\) and cryolite \(\( ext{Na}_3 ext{AlF}_6\).\) This setup causes the breakdown of the alumina into aluminum metal and oxygen gas. The aluminum then sinks to the bottom of the cell due to its high density. This efficient extraction method has made aluminum widely available and affordable, revolutionizing its use in everyday products.

Aluminum Extraction: From Bauxite to Metal

Aluminum extraction is a multi-step process that begins with mining bauxite ore, which is rich in aluminum oxides. The first step in refining bauxite is the Bayer process, where bauxite is mixed with sodium hydroxide \(\( ext{NaOH}\)\) to dissolve the aluminum-containing compounds from the ore. This creates a solution containing sodium aluminate, leaving impurities like iron oxides behind as waste.
After refining through the Bayer process to obtain pure alumina, the material undergoes electrolysis in the Hall-Héroult process. This involves introducing the refined alumina into a large electrolytic cell filled with molten cryolite. An electrical current passes through the cell, reducing the alumina to liquid aluminum while releasing oxygen gas. This vital process transforms raw bauxite into aluminum, which is then cast into ingots or used to create various products, ranging from soda cans to aircraft parts.

• Mining of bauxite is the first step.
• Bayer process refines the ore to alumina.
• Hall-Héroult process extracts pure aluminum.