Problem 48 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The major role of fluorspar \(\left(\mathrm{CaF}_{2}\right.\) ), which is added in small quantities in the electrolytic reduction of alumina dissolved in fused cryolite \(\left(\mathrm{Na}_{3} \mathrm{AlF}_{6}\right)\), is [1993-1 Mark] (a) as a catalyst (b) to make the fused mixture very conducting (c) to lower the temperature of the melt (d) to decrease the rate of oxidation of carbon at the anode.

Step-by-Step Solution

Answer

Option (c), to lower the temperature of the melt, is the correct answer.

Step 1: Understand the Role of Fluorspar

Fluorspar, also known as calcium fluoride (\(\mathrm{CaF}_2\)), is used in the electrolytic reduction of alumina. It is important to understand that its function is related to its properties when dissolved in the cryolite-alumina mixture.

Step 2: Analyze Properties of the Mixture

The mixture of alumina in cryolite is used in the Hall-Héroult process for aluminum production. It is a high-temperature electrolytic process requiring efficient molten phases.

Step 3: Assess Fluorspar’s Functions

Fluorspar is known to lower the melting point of the cryolite-alumina mixture. This makes the fusion process more energy-efficient by reducing the energy requirement to maintain the electrolyte in molten form.

Step 4: Compare with Answer Options

Let us go through the provided options: - (a) catalyst: No direct catalytic role is played by fluorspar. - (b) very conducting: Fluorspar doesn't significantly affect conductivity. - (c) lower the temperature of the melt: This matches the property of fluorspar. - (d) decrease oxidation rate at the anode: Fluorspar has no role here. The correct answer is (c).

Key Concepts

Fluorspar

Fluorspar, chemically known as calcium fluoride (\(\text{CaF}_2\)), plays a crucial role in the electrolytic reduction of alumina when added to the cryolite mixture. Its primary purpose is to improve the efficiency of the process. This is due to its ability to lower the melting point of the cryolite-alumina mixture. By reducing the melting point, fluorspar helps minimize the energy required to keep the mixture liquid during the electrolysis.

• Fluorspar is not a catalyst and does not directly enhance the reaction speed.
• It also does not significantly impact the conductivity of the mixture.
• Nor does it play a role in reducing the oxidation rate at the anode.

By lowering the temperature needed for the molten state, fluorspar is essential in making the aluminum production process more energy-efficient, contributing to cost-effectiveness and sustainability.

Hall-Héroult process

The Hall-Héroult process is a widely used technique for extracting aluminum from alumina, the main aluminum oxide, using electrolysis. It is distinctive because it operates at high temperatures to maintain the alumina dissolved in molten cryolite, which serves as a solvent and lowers the melting point of alumina, facilitating the extraction of aluminum.
Some key aspects of the Hall-Héroult process include:

• Using large carbon-lined steel pots as electrolytic cells.
• Applying direct electrical currents to separate aluminum and oxygen ions.
• The liquid aluminum deposits at the cathode (bottom of the cell), while oxygen forms carbon dioxide at the anode.

Lowering the required operational temperature through additives like fluorspar is crucial. This not only makes energy consumption more efficient, but also helps protect the carbon anodes from excessive consumption and oxidation.

Melting Point Reduction

Melting point reduction is a critical factor in the efficiency of the electrolytic reduction process used in aluminum production. By using substances like fluorspar, the Cryolite (\(\text{Na}_3\text{AlF}_6\))-Alumina (\(\text{Al}_2\text{O}_3\)) mixture's melting point can be significantly lowered. This ensures that the mixture remains molten at a more manageable temperature, leading to substantial energy savings.

• Melting point lowering allows for the operational temperature to stay within the material tolerance of the equipment used.
• A reduced melting temperature results in less thermal stress on the installation, improving its lifespan.
• Energy efficiency is enhanced, leading to a reduction in operational costs and environmental impact.

Overall, the concept of melting point reduction is not only key to energy and cost savings but also essential for optimal production and durability of the involved equipment.