Question

Aluminium is not commercially produced by carbo-thermic reduction because:

• A. It melts at a low temperature
• B. It does not vaporise at reasonable temperature
• C. Al–Al\(_2\)O\(_3\) line is too low in the Ellingham diagram and need excessively high temperature
• D. Aluminium metal will have excessive dissolved oxygen

Hint:

The Al–Al\(_2\)O\(_3\) line lies below the C–CO and C–CO\(_2\) lines in the Ellingham diagram, making carbon reduction of Al\(_2\)O\(_3\) thermodynamically unfeasible.

Answer 1

The Correct Option is C

Step 1: Understanding Carbo-Thermic Reduction
Carbo-thermic reduction involves reducing metal oxides using carbon as the reducing agent. However, not all metals can be commercially extracted this way due to thermodynamic limitations. Step 2: Role of the Ellingham Diagram
The Ellingham diagram is a graph of Gibbs free energy (\( \Delta G \)) vs. temperature for various metal oxides. A more negative line indicates a more stable oxide and harder reduction. In the case of aluminium: - The Al–Al\(_2\)O\(_3\) line lies far below the carbon–CO and carbon–CO\(_2\) lines. - This means carbon cannot reduce Al\(_2\)O\(_3\) because the reaction is not thermodynamically favourable at practical temperatures. Step 3: Alternative Method — Electrolytic Reduction
Due to this, aluminium is commercially extracted by electrolysis of alumina (Hall-Héroult process), not by carbo-thermic methods. Conclusion: Aluminium is not reduced by carbon because the Al–Al\(_2\)O\(_3\) line is too low in the Ellingham diagram and would require excessively high temperature for reduction.