Electrochemistry

Which of the following statements is not correct about rusting of iron?

1. Rusting of iron is envisaged as setting up of electrochemical cell on the surface of iron object.
2. Dissolved acidic oxides SO2, NO2 in water act as catalyst in the process of rusting.
3. Coating of iron surface by tin prevents rusting, even if the tin coating is peeling off.
4. When pH lies above 9 or 10, rusting of iron does not take place.

Answer

(3) Coating of iron surface by tin prevents rusting, even if the tin coating is peeling off.

Solution

(A): anode Fe Fe2+ + 2e-; cathode O2 + 4H+ + 4e- 2H2O ⇒ (A) is correct

(B): SO2, NO2 dissolve to give H+ (electrolyte) ⇒ rusting is faster ⇒ (B) is correct

(C): E (Sn2+/Sn) = −0.14 V is higher than E (Fe2+/Fe) = −0.44 V, so once tin peels off, exposed Fe becomes the anode and rusts ⇒ (C) is not correct

(D): at pH above 9–10, H+ needed at the cathode is absent ⇒ (D) is correct

Answer: (3)