Answer: CO 2 and CO form as anode gases from oxidation of C by O 2- ions: C + 2O 2- → CO 2 + 4e-.
- A CO<sub>2</sub> and CO form as anode gases from oxidation of C by O<sup>2-</sup> ions: C + 2O<sup>2-</sup> → CO<sub>2</sub> + 4e-
- B The carbon anode gradually dissolves directly into the molten aluminium product
- C The carbon anode itself chemically reduces the surrounding Al<sub>2</sub>O<sub>3</sub> to aluminium
- D The carbon anode functions as a flux that lowers the electrolyte's melting point
Correct answer: A. CO<sub>2</sub> and CO form as anode gases from oxidation of C by O<sup>2-</sup> ions: C + 2O<sup>2-</sup> → CO<sub>2</sub> + 4e-
Explanation: At the carbon anode, O<sup>2-</sup> from Al<sub>2</sub>O<sub>3</sub> oxidises the carbon: 2O<sup>2-</sup> → O<sub>2</sub> + 4e-; but the O<sub>2</sub> immediately reacts with the hot carbon anode to give CO/CO<sub>2</sub>. Anodes are regularly replaced.
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.