Problem 123 - A Complete Resource Book in Chemistry for JEE Main

Question

small quantities in the electrolytic reduction of alumina dissolved in fused cryolite \(\left(\mathrm{Na}_{3} \mathrm{AlF}_{6}\right)\) is 1\. as a catalyst 2\. to make the fused mixture very conducting 3\. to lower the temperature of melt 4\. to decrease the rate of oxidation of carbon at the anode (a) 2,3 (b) 1,2 (c) \(2,3,4\) (d) 3,4

Step-by-Step Solution

Answer

The correct answer is (a) 2, 3.

Step 1: Understand the Problem

The problem is asking about the role of small quantities of certain substances in the electrolytic reduction of alumina (Aluminum oxide) dissolved in molten cryolite (Na₃AlF₆). We need to select which statements correctly describe their purpose.

Step 2: Analyze the Statements

Review each statement to determine whether it accurately describes the role of substances in the electrolysis process: 1. As a catalyst - Catalysts are not typically needed for this process. 2. To make the fused mixture very conducting - True, cryolite enhances electrical conductivity. 3. To lower the temperature of melt - True, cryolite lowers the melting point, making the process more efficient. 4. To decrease the rate of oxidation of carbon at the anode - Not exactly applicable or primary role.

Step 3: Evaluate the Options

Evaluate the given options by associating them with the statements: - Option (a) refers to statements 2 and 3, both of which are true. - Option (b) refers to statements 1 and 2; statement 1 is false. - Option (c) refers to statements 2, 3, and 4; statement 4 is not accurate. - Option (d) refers to statements 3 and 4; statement 4 is not accurate.

Step 4: Select the Correct Option

Based on the analysis, option (a): '2, 3' is the correct answer because it accurately describes the roles of increasing conductivity and lowering the melting point.

Key Concepts

Alumina

Alumina, also known as aluminum oxide, is a critical component in the production of aluminum metal. It is represented by the chemical formula \(\text{Al}_2\text{O}_3\). Alumina is a white powdery substance derived from bauxite through a refining process. Bauxite is the primary ore of aluminum and contains a mixture of mineral phases, from which alumina is extracted.
Alumina serves as the feedstock for the electrolytic process where it is dissolved in molten cryolite to produce pure aluminum. When it melts, alumina can conduct electricity properly when combined with cryolite. This process requires high energy inputs to break the strong ionic bonds inside alumina, which then releases aluminum ions for reduction at the cathode in an electrolytic cell.

Cryolite

Cryolite is an uncommon, naturally occurring mineral with the chemical formula \(\text{Na}_3\text{AlF}_6\). It plays a crucial role in the electrolytic reduction of alumina during aluminum production. Cryolite serves multiple purposes during the electrolysis process. It significantly lowers the melting point of alumina, which is originally very high, making the molten mixture more manageable.
Additionally, cryolite improves the electrical conductivity of the solution, enabling the electrolysis process to run more efficiently. Without cryolite, achieving the necessary temperatures and electrical conditions for the extraction of aluminum would be energy-intensive and costly. Though natural cryolite sources are scarce, synthetic cryolite is often used to sustain aluminum production.

Electrolysis

Electrolysis is a chemical process that uses electricity to drive a non-spontaneous chemical reaction, such as the separation of elements from compounds. For alumina dissolved in molten cryolite, electrolysis is the method employed to extract aluminum. An electrolytic cell is set up in an industrial setting with electrodes immersed in the electrolyte solution, where alumina serves as the primary source of aluminum.
In the electrolysis of alumina, aluminum ions migrate towards a negatively charged cathode where they gain electrons and form solid aluminum. At the same time, oxygen ions move toward the positively charged anode, where they lose electrons, typically forming oxygen gas, which can also lead to the oxidation of carbon-based electrodes in the process. This step is energy-intensive, highlighting the importance of cryolite in reducing energy demands.

Melting Point

The melting point is the temperature at which a solid becomes a liquid. Alumina has a high melting point of over 2000°C, making its handling during aluminum production challenging and energy-intensive without assistance. The introduction of cryolite into the electrolytic process helps by lowering the melting point of the mixture significantly, to about 1000°C.
This reduction in the melting point allows the alumina to dissolve in a molten state in the cryolite at more manageable temperatures. This not only saves on energy costs but also simplifies the operations in aluminum smelting facilities. It minimizes wear and tear on equipment subjected to extreme heat, hence enhancing the durability and efficiency of the electrolytic cells.

Electrical Conductivity

Electrical conductivity refers to the ability of a substance to conduct electricity. Cryolite plays an essential role in the electrolysis of alumina due to its ability to enhance the electrical conductivity of the solution. By allowing electric current to pass through the molten mixture effectively, cryolite ensures that the electrolytic process proceeds smoothly without excessive energy loss.
Higher electrical conductivity means that electrons can move more freely through the liquid, facilitating the reduction of aluminum ions and oxidation at the electrodes. This characteristic not only reduces the production costs of aluminum by lowering electricity consumption but also improves the overall efficiency of the electrolysis process. Cryolite's ability to maintain such conductivity is integral within industrial electrochemical reactions.