Answer: Al 2 O 3 has a very large and negative Delta_G of formation; the C/CO Ellingham line never crosses below Al 2 O 3 at any practical temperature.
- A Al<sub>2</sub>O<sub>3</sub> has a very large and negative Delta_G of formation; the C/CO Ellingham line never crosses below Al<sub>2</sub>O<sub>3</sub> at any practical temperature
- B Carbon is generally far too expensive a reducing agent to ever be used for large-scale aluminium extraction in typical laboratory settings
- C Aluminium metal instead reacts with the carbon furnace lining itself to contaminate the final metal product under usual circumstances
- D Electrolysis is preferred mainly as a matter of historical industrial convention rather than for any thermodynamic reason according to most researchers
Correct answer: A. Al<sub>2</sub>O<sub>3</sub> has a very large and negative Delta_G of formation; the C/CO Ellingham line never crosses below Al<sub>2</sub>O<sub>3</sub> at any practical temperature
Explanation: Even at 3000°C, the C→CO line is above the Al→Al<sub>2</sub>O<sub>3</sub> line on the Ellingham diagram; thermodynamics forbids C from reducing Al<sub>2</sub>O<sub>3</sub>. At extremely high T, aluminium carbide forms instead.
An Ellingham diagram plots ΔG° of oxide formation against temperature for different metals; whichever line is LOWER (more negative ΔG°) at a given temperature reduces the oxide of any metal whose line sits above it - the basis of carbon reduction (Fe, Zn) vs electrolytic reduction (Al, Mg, Na) decisions.
Concept context
The science of extracting metals from ores and refining them for use. Covers concentration methods, reduction techniques, refining processes, and the thermodynamic principles that govern metal extraction.