Problem 178 - A Complete Resource Book in Chemistry for JEE Main

Question

In the context of the Hall-Heroult process for the extraction of \(\mathrm{A} l\), which of the following statement is false? (a) \(\mathrm{CO}\) and \(\mathrm{CO}_{2}\) are produced in this process (b) \(\mathrm{Al}_{2} O_{3}\) is mixed with \(\mathrm{CaF}_{2}\) which lowers the melting point of the mixture and brings conductivity (c) \(\mathrm{A} P^{\beta+}\) is reduced at the cathode to form \(\mathrm{Al}\) (d) \(\mathrm{Na}_{3} \mathrm{~A} / \mathrm{F}_{6}\) serves as the electrolyte

Step-by-Step Solution

Answer

Statement (c) is false.

Step 1: Understanding Statement (a)

The Hall-Heroult process is used for the extraction of aluminum. In this process, carbon anodes are used, and one of the outcomes of the anode reaction is the formation of carbon dioxide \( (\mathrm{CO}_2) \). Therefore, the statement \'\( \mathrm{CO}\) and \( \mathrm{CO}_2 \) are produced\' is true since \( \mathrm{CO}_2 \) is indeed a byproduct.

Step 2: Evaluating Statement (b)

During the Hall-Heroult process, alumina (\( \mathrm{Al}_2\mathrm{O}_3 \)) is dissolved in molten cryolite (\( \mathrm{Na}_3\mathrm{AlF}_6 \)), which lowers the melting point of the mixture and improves electrical conductivity. \( \mathrm{CaF}_2 \) is often added to further reduce the melting point, thus making it easier to maintain the molten state. This statement is true.

Step 3: Analyzing Statement (c)

The reduction at the cathode involves aluminum ions. \( \mathrm{Al}^{3+} \) ions from the alumina are reduced to form aluminum metal. The mention of \( \mathrm{A} P^{\beta+} \) does not logically fit into this context, as \( \mathrm{Al}_2\mathrm{O}_3 \) provides \( \mathrm{Al}^{3+} \) ions, not \( \mathrm{A} P^{\beta+} \). Therefore, this statement is false.

Step 4: Verifying Statement (d)

Cryolite, chemically known as \( \mathrm{Na}_3\mathrm{AlF}_6 \), is used as the major component in the electrolyte mixture during the Hall-Heroult process. Hence, this statement accurately describes the role of cryolite in the process, making it true.

Key Concepts

Aluminum Extraction

The Hall-Héroult process is the most widely used method for extracting aluminum from its ore, bauxite. This electrolytic process involves separating aluminum metal from the compounds found in the ore. The ore is first purified to produce alumina (aluminum oxide, \( \mathrm{Al}_2\mathrm{O}_3 \)).In the Hall-Héroult process, purified alumina is dissolved in a molten mixture of cryolite. Under high temperatures, the mixture becomes electrically conductive, allowing the flow of electricity to trigger the separation of aluminum from oxygen. This process occurs in a large electrolytic cell, where the cathode and anode play crucial roles.The reaction:

• At the cathode: \( \mathrm{Al}^{3+} + 3e^- \rightarrow \mathrm{Al} \), aluminum ions are reduced to form pure aluminum metal.
• At the anode: Carbon reacts with oxygen in the melted alumina, forming carbon dioxide, hence contributing to the carbon emissions.

This method is essential because of aluminum's extensive use in various industries, from automotive to construction and packaging.

Cryolite

Cryolite, which has the chemical formula \( \mathrm{Na}_3\mathrm{AlF}_6 \), plays an essential role in the Hall-Héroult process. This mineral acts as a solvent for alumina, lowering its melting point significantly. Pure alumina possesses a very high melting point of around 2072°C (3762°F), which is impractical for industrial processing.However, when alumina is dissolved in cryolite:

• The melting point of the mixture drops to approximately 950-1000°C (1742-1832°F), making the process more energy-efficient.
• Cryolite enhances the conductivity of the molten alumina, facilitating the transfer of electric current needed for the electrolytic reaction.

Additionally, synthetic cryolite is often used in the process due to the rarity of natural cryolite. This innovation allows continuous operation of the Hall-Héroult process on a global industrial scale.

Electrical Conductivity

Electrical conductivity is a vital element in the Hall-Héroult process. For aluminum to be extracted efficiently, the molten mixture of alumina and cryolite must be electrically conductive, allowing the passage of electric currents. This conductivity is crucial because:

• It enables the electrolytic reaction that separates aluminum ions from oxygen ions in the alumina.
• Ensures sufficient energy is available for the reduction of aluminum ions at the cathode, forming pure aluminum metal.
• The proper current flow supports the continued operation of the cell, ensuring a consistent output of aluminum metal.

With efficient conductivity, the Hall-Héroult process becomes less energy-consuming and more cost-effective, which is essential due to the large energy demands of aluminum production.

Carbon Emissions

Carbon emissions are an environmental concern associated with the Hall-Héroult process. During this process, carbon anodes and oxygen from alumina react at high temperatures.The byproducts include:

• Carbon dioxide (\( \mathrm{CO}_2 \)), which is a significant greenhouse gas contributing to climate change.
• Small amounts of carbon monoxide (\( \mathrm{CO} \)), which may occasionally form under specific conditions.

Efforts are continually made to reduce these emissions through improved technologies and using renewable energy sources in the process.Furthermore, replacing the consumable carbon anodes with more sustainable materials is being researched. Lowering the carbon footprint of aluminum extraction is crucial as the demand for aluminum continues to rise globally.