Oxidation state change in oxalate half-reaction

Steps:

• Assign oxidation state to C in C₂O₄²⁻: 2x + 4(-2) = -2, so 2x = 6, x = +3 per C.
• Assign oxidation state to C in CO₂: x + 2(-2) = 0, so x = +4.
• Oxidation state rises from +3 to +4, indicating oxidation (loss of electrons).
• Half-equation shows 2e⁻ released, confirming oxidation process.

Why A is correct:

• Carbon starts at +3 and increases to +4, defining oxidation from the +3 state per IUPAC oxidation number rules.

Why the others are wrong:

• B: Reduction decreases oxidation state, but +3 to +4 increases it; +5 irrelevant.
• C: Oxidation occurs, but initial state is +3, not +5.
• D: Process is oxidation (electron loss), not reduction.

Final answer: A