In the rusting of iron in moist air, the electrons released at an ano…

The correct answer is Option (3) → \( \ce{O2(g) + 4H+(aq) + 4e^- -> 2H2O(l)} \)

Rusting is an electrochemical process. At the anodic spot iron is oxidised; the electrons it gives up travel through the metal to another spot, where they reduce oxygen in the presence of H⁺ ions. The H⁺ ions come from carbonic acid, formed when carbon dioxide of the air dissolves in the water.

    \( \displaystyle \text{Anode: } \ce{2Fe(s) -> 2Fe^2+ + 4e^-} \qquad E^\circ_{\ce{Fe^2+/Fe}} = -0.44\ \mathrm{V} \)

    \( \displaystyle \text{Cathode: } \ce{O2(g) + 4H+(aq) + 4e^- -> 2H2O(l)} \qquad E^\circ = 1.23\ \mathrm{V} \)

    \( \displaystyle \text{Cell reaction: } \ce{2Fe(s) + O2(g) + 4H+(aq) -> 2Fe^2+(aq) + 2H2O(l)} \qquad E^\circ_{\text{cell}} = 1.67\ \mathrm{V} \)

Why the other options are incorrect

Option (1) \( \ce{Fe^2+(aq) + 2e^- -> Fe(s)} \) — Incorrect. This is the reverse of the anode reaction. In rusting iron goes into solution as Fe²⁺; it is not deposited back.

Option (2) \( \ce{2H+(aq) + 2e^- -> H2(g)} \) — Incorrect. At a cathode the reduction with the higher E° is preferred. Oxygen (1.23 V) is far above H⁺/H₂ (0.00 V), so the electrons reduce oxygen and no hydrogen is given off.

Option (4) \( \ce{2Fe^2+(aq) + 2H2O(l) + 1/2O2(g) -> Fe2O3(s) + 4H+(aq)} \) — Incorrect. This is the later oxidation of the ferrous ions by atmospheric oxygen, which gives rust (Fe₂O₃·xH₂O). It is not an electrode reaction and takes no electrons from the metal.

NCERT (2026-27 edition):
2.8 Corrosion – “…Electrons released at anodic spot move through the metal and go to another spot on the metal and reduce oxygen in the presence of H⁺ (which is believed to be available from H₂CO₃ formed due to dissolution of carbon dioxide from air into water. …). This spot behaves as cathode with the reaction Cathode: O₂(g) + 4 H⁺(aq) + 4 e^– → 2 H₂O (l) … The ferrous ions are further oxidised by atmospheric oxygen to ferric ions which come out as rust in the form of hydrated ferric oxide (Fe₂O₃. x H₂O) and with further production of hydrogen ions.”