Which of the following statements best explains the corrosion of iron in moist air?

hard MCQ 3 marks Class 12 · Chemistry · Electrochemistry

A) Fe → $F e^{2 +}$ +2e⁻ at the metal surface, O₂+2H₂O+4e⁻ → 4OH⁻ at the surface; $F e^{2 +}$ combines with OH⁻ to form Fe(OH)₂, which later converts to hydrated Fe₂O₃.
B) Fe is oxidised directly to $F e^{3 +}$ by water, producing H₂ gas.
C) Corrosion occurs because carbon deposits on iron surfaces, preventing electron flow.
D) Iron corrodes only in the complete absence of oxygen.

Answer

A) Fe →

F e^{2 +}

+2e⁻ at the metal surface, O₂+2H₂O+4e⁻ → 4OH⁻ at the surface;

F e^{2 +}

combines with OH⁻ to form Fe(OH)₂, which later converts to hydrated Fe₂O₃.

Step-by-step solution

Identify the oxidation half‑reaction: Fe(s) → $F e^{2 +}$ +2e⁻.
Identify the reduction half‑reaction of dissolved oxygen: O₂+2H₂O+4e⁻ → 4OH⁻.
Combine the two half‑reactions to obtain the overall corrosion process.
Explain that $F e^{2 +}$ reacts with OH⁻ to give Fe(OH)₂, which further oxidises to Fe₂O₃·nH₂O (rust).
Recognise that the presence of both water and oxygen is essential for this sequence.

Common mistake: Neglecting the role of oxygen or assuming direct formation of

F e^{3 +}

without water

Concept tested: Mechanism of iron corrosion

Published 7 September 2026 · Last updated 16 September 2026 · How this solution was prepared and checked