Question

This figure shows the corrosion of iron.

1. What happens to a metal during corrosion – oxidation or reduction?

2. Explain the electrochemical theory of rusting of iron.

3. Write two examples for metals which undergo corrosion.

Answer 1

1. Oxidation.

2. The impure surface of iron behaves like as electrochemical cell. Pure iron functions as the anode while the impure surface acts as cathode. The electrolyte is moisture containing dissolved O₂ or CO₂ .

At anode : 2Fe(s) → 2Fe²⁺(aq) + 4e^-

At cathode : 4H⁺(eq) + O₂(g) + 4e → 2H₂O(l)

The overall reaction is

2Fe(s) + O₂(g) + 4H⁺(aq) → 2Fe²⁺(aq) + 2H₂O(l)

Ferrous ions so formed are oxidised by atmospheric oxygen to ferric ions. The rust formed is chemically hydrated ferric oxide (Fe₂O₃ .xH₂O)

2Fe²⁺(aq) + 2H₂O + 1/2 O₂ → Fe₂O₃ + 4H⁺

Fe₂O₃ + xH₂O → Fe₂O₃ .xH₂O (rust)

3. Iron and Copper.