Answer: 950-1000°C in cryolite (melting point lowered from 2050°C of pure Al 2 O 3 ).
- A 950-1000°C in cryolite (melting point lowered from 2050°C of pure Al<sub>2</sub>O<sub>3</sub>)
- B Ordinary room temperature, since cryolite fully dissolves Al<sub>2</sub>O<sub>3</sub> cold
- C Around 100°C, the boiling point of water used to cool the cell
- D Around 3000°C, close to the melting point of pure Al<sub>2</sub>O<sub>3</sub> alone
Correct answer: A. 950-1000°C in cryolite (melting point lowered from 2050°C of pure Al<sub>2</sub>O<sub>3</sub>)
Explanation: Pure Al<sub>2</sub>O<sub>3</sub> melts at 2050°C; dissolving it in cryolite (Na3AlF6) reduces the operating temperature to ~950-1000°C, making the process practical.
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.