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2021 O Level Chem P1 Q28 — Metals / Prevention of Rusting Solution

Metals / Prevention of Rusting 2021 O Level Pure Chemistry · Paper 1 · Q28

Question

Iron can be protected from rusting by galvanising and by sacrificial protection. Which reactions take place in either or both of these methods of protection? 1 Zn(s) → Zn²⁺(aq) + 2e⁻. 2 Mg(s) → Mg²⁺(aq) + 2e⁻. 3 Cu(s) → Cu²⁺(aq) + 2e⁻.

A1 and 2 only
B1 and 3 only
C2 and 3 only
D1, 2 and 3

Correct answer: A

Video Solution

Explanation

Answer: A (1 and 2 only). Both galvanising and sacrificial protection work by attaching a metal more reactive than iron, which is then oxidised in preference to the iron, protecting it.

Galvanising coats iron with zinc; if the coating is scratched, the zinc (above iron in the reactivity series) is oxidised first: Zn → Zn²⁺ + 2e⁻ (equation 1). Sacrificial protection bolts a block of a more reactive metal, usually magnesium or zinc, onto the iron so the block corrodes instead: Mg → Mg²⁺ + 2e⁻ (equation 2).

Equation 3, Cu → Cu²⁺ + 2e⁻, is not used: copper is below iron in the reactivity series, so it cannot protect iron. So only reactions 1 and 2 take place.

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