The correct sequence of steps during the corrosion of iron is
(A) Ferrous ions are oxidized by atmospheric oxygen
(B) Hydrated ferric oxide \( (Fe_2O_3.xH_2O) \) formed with production of \( H^+ \) ions
(C) At a particular spot of iron object, oxidation takes place and that spot behaves as anode
(D) Electron released at anode moves through metal and reduces oxygen in the presence of \( H^+ \)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (C), (D), (A), (B)
Rusting of iron is an electrochemical process: tiny cells are set up on the surface of the metal in the presence of water and air. One spot acts as the anode, another as the cathode, the metal carries electrons between them, and water with dissolved \( \ce{CO2} \) (as \( \ce{H2CO3} \)) supplies \( \ce{H+} \) ions. The steps follow in this order.
(C) At one spot iron is oxidised, and that spot acts as the anode.
(D) The electrons travel through the metal to another spot, the cathode, where they reduce oxygen in the presence of \( \ce{H+} \).
(A) and (B) The ferrous ions are then oxidised by atmospheric oxygen to ferric ions, which separate as rust, hydrated ferric oxide, with production of \( \ce{H+} \) ions:
\( \displaystyle \ce{4Fe^2+(aq) + O2(g)} + (4 + 2x)\,\ce{H2O(l)} \rightarrow \ce{2Fe2O3.xH2O(s) + 8H+(aq)} \)
Shortcut: Rust (B) can form only after the ferrous ions have been oxidised to the ferric state (A), so (A) must come before (B). Only Option (4) has (A) before (B).
NCERT (2026-27 edition):
Corrosion – “… In corrosion, a metal is oxidised by loss of electrons to oxygen and formation of oxides. Corrosion of iron (commonly known as rusting) occurs in presence of water and air. The chemistry of corrosion is quite complex but it may be considered essentially as an electrochemical phenomenon. At a particular spot of an object made of iron, oxidation takes place and that spot behaves as anode and we can write the reaction
Anode: \( \ce{2Fe(s) -> 2Fe^2+ + 4e-} \)
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+ … This spot behaves as cathode with the reaction
Cathode: \( \ce{O2(g) + 4H+(aq) + 4e- -> 2H2O(l)} \)
… 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.xH2O) and with further production of hydrogen ions.”