CBSE Class 12 · 2011 · Redox Reactions and ElectrochemistryShort Answer

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The chemistry of corrosion of iron is essentially an electrochemical phenomenon. Explain the reactions occurring during the corrosion of iron in the atmosphere.

Answer: Anodic spots: Fe→Fe²⁺+2e⁻; cathodic: O₂+4H⁺+4e⁻→2H₂O; Fe²⁺ then oxidised by air to hydrated Fe₂O₃ (rust).

Solution

A water film with dissolved CO

_{2}

_{2}

on the iron surface sets up tiny electrochemical cells.
At anodic spots iron is oxidised:

Fe (s) ⟶ F e^{2 +} + 2 e^{-}

At cathodic spots oxygen is reduced in the presence of H

^{+}

O_{2} (g) + 4 H^{+} (aq) + 4 e^{-} ⟶ 2 H_{2} O (l)

The Fe

^{2 +}

ions are further oxidised by atmospheric O

_{2}

to Fe

^{3 +}

, which deposits as hydrated ferric oxide, Fe

_{2}

_{3} \cdot x

_{2}

O — rust.

CBSE Class 12 Board · 2011 · Subjective

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