Problem 31 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Electrolytic reduction of alumina to aluminium by Hall-Heroult process is carried out [2000S] (a) in the presence of \(\mathrm{NaCl}\) (b) in the presence of fluorite (c) in the presence of cryolite which forms a melt with lower melting temperature (d) in the presence of cryolite which forms a melt with higher melting temperature

Step-by-Step Solution

Answer

(c) Cryolite forms a melt with lower melting temperature.

Step 1: Understanding the Hall-Heroult Process

The Hall-Heroult process is an industrial method for the reduction of alumina (\(\mathrm{Al_2O_3}\)) to aluminium. The process requires alumina to be dissolved in a molten electrolyte for the electrolysis to occur efficiently.

Step 2: Identify the Role of Cryolite

In the Hall-Heroult process, cryolite (\(\mathrm{Na_3AlF_6}\)) is used as a solvent for alumina. Its main purpose is to lower the melting point of the mixture, making the process more energy-efficient.

Step 3: Comparing Melting Point Effects

Cryolite forms a melt with a lower melting temperature than pure alumina. This characteristic is crucial because it allows the electrolysis to be conducted at a lower temperature, thereby saving energy and reducing costs in the industrial process.

Step 4: Analyzing the Options

Given the options, identify the one that describes the correct usage of cryolite in the process: (a) \(\mathrm{NaCl}\) is not used in this context, (b) fluorite is not used, (c) cryolite which forms a melt with a lower melting temperature, and (d) cryolite which forms a melt with a higher melting temperature.

Step 5: Final Selection

The accurate statement about the Hall-Heroult process is option (c): cryolite forms a melt with a lower melting temperature, making it the correct choice. It enables efficient electrolysis by lowering operational temperatures.

Key Concepts

Hall-Heroult Process

The Hall-Heroult process is a pivotal industrial method used to convert alumina (\(\mathrm{Al_2O_3}\)) into aluminum metal. This process is crucial because aluminum is a highly sought after material known for its lightweight and strong properties. The key principle behind this method is electrolysis, which involves passing an electric current through the alumina to break its bonds and isolate pure aluminum. This is not just about using electricity; it's about having the right conditions to make the reaction feasible. We need alumina to be in a particular form, specifically dissolved in a special liquid, for the process to effectively occur. This leads us to the need for "melting point reduction techniques" to ensure the alumina is readily dissolved in the liquid solution used for electrolysis.

Alumina

Alumina, with the chemical formula \(\mathrm{Al_2O_3}\), is the primary raw material for aluminum production. Extracted from bauxite ore, alumina is a white powder that must be processed into liquid form for electrolysis to be viable. In the context of the Hall-Heroult process, it's important that alumina is dissolved in a molten electrolyte, which requires high temperatures typically beyond what is feasible without assistance. The choice of the right solvents, like cryolite, is pivotal in ensuring that alumina melts effectively at these lower temperatures, thus enabling efficient conversion to aluminum.

Cryolite

Cryolite, denoted as \(\mathrm{Na_3AlF_6}\), is an essential solvent in the Hall-Heroult process. Its primary role is to dissolve alumina lower than its natural melting point, thereby facilitating effective electrolysis. This substance, when melted, forms a conductive liquid mixture that helps lower the overall temperature required for the alumina to enter a molten state. The use of cryolite means that less energy is required to maintain these conditions, making the production process more environmentally and financially sensible. Furthermore, cryolite amplifies the electrical conductivity of the solution, ensuring that the electric current flows efficiently during the electrolysis.

Melting Point Reduction

Melting point reduction is a strategy used to make processes like the Hall-Heroult method more efficient. By lowering the temperature at which alumina can dissolve, cryolite makes it possible to conduct electrolysis at lower, more manageable temperatures. In essence, this involves reducing the energy needed to reach and maintain the necessary high temperatures for the reaction. This not only ensures that the equipment used can withstand the process but also reduces operational costs. The controlled reduction of the melting point is crucial because it makes the daunting task of extracting aluminum from alumina much more practical and less energy-intensive, fostering sustainability in aluminum production.