JEE Main 2024 · 27 January · evening · Single correct · +4 / −1
Which of the following statements is not correct about rusting of iron?
- (a) Rusting of iron is envisaged as setting up of electrochemical cell on the surface of iron object.
- (b) Dissolved acidic oxides SO₂, NO₂ in water act as catalyst in the process of rusting.
- (c) Coating of iron surface by tin prevents rusting, even if the tin coating is peeling off.
- (d) When pH lies above 9 or 10, rusting of iron does not take place.
Answer and solution
Answer (c) Coating of iron surface by tin prevents rusting, even if the tin coating is…
Solution
Chemistry ma’am · 4 steps
Tin is nobler than iron, so once the tin cracks, iron becomes the anode and rusts even faster.
- Rusting is a cell. At one spot on the surface, iron is oxidised: Fe → Fe²⁺ + 2e⁻. At another spot, oxygen is reduced: O₂ + 4H⁺ + 4e⁻ → 2H₂O. So statement (a) is correct.
- Role of pH. The cathode reaction uses up H⁺. Dissolved acidic oxides like SO₂ and NO₂ supply more H⁺ and speed up rusting, so (b) is correct. Above pH 9 to 10, there is too little H⁺ and rusting practically stops, so (d) is also correct.
E°_(Sn²⁺/Sn) = -0.14 V > E°_(Fe²⁺/Fe) = -0.44 V
- Tin coating. Tin has the higher reduction potential, so it is harder to oxidise than iron. An intact tin layer protects iron only as a barrier. Once the layer peels off, iron and tin form a cell in which iron is the anode, so the exposed iron rusts faster.
- So. Statement (c) is the incorrect one. Tin does not prevent rusting once its coating is broken.
The idea to keep. A coating metal that is more reactive than iron, like zinc, protects it even when scratched. A less reactive one, like tin, protects only while it is intact. Compare their E° values.
Answer from the NTA final answer key. Solution by PrepWiser.