Electrolysis: Oxidation at the anode (positive electrode)

Steps:

• In the Hall-Héroult process, the positive electrode is the anode where oxidation reactions occur.
• Reaction 1 shows Al³⁺ gaining electrons (reduction), so it occurs at the cathode (negative electrode).
• Reaction 2 shows O²⁻ losing electrons to form O₂ (oxidation), so it occurs at the anode.
• Reaction 3 shows carbon (anode material) reacting with O₂ from reaction 2 to form CO₂, consuming the anode.

Why D is correct:

• At the anode, oxide ions oxidize to oxygen (reaction 2), which immediately reacts with the carbon anode (reaction 3) per the overall anodic process in aluminium electrolysis.

Why the others are wrong:

• A: Reaction 1 is reduction, occurring at the cathode, not anode.
• B: Reaction 3 also occurs at the anode as the carbon electrode reacts with produced oxygen.
• C: Reaction 1 is at the cathode; only 3 is partially correct but misses 2.

Final answer: D