Question

Explain the chemical reactions in rusting of iron.

Answer 1

The rust on iron does not form by a simple reaction of oxygen with the iron surface. The rust is formed by an electrochemical reaction. Different regions on the surface of iron become anode and cathode.

1. Fe is oxidised to Fe²⁺ in the anode region.
   \[\ce{Fe (s) -> Fe^{2+} (aq) + 2e-}\]
2. O₂ is reduced to form water in the cathode region.
   \[\ce{O2(g) + 4H^+ (aq) + 4e- -> 2H2O(l)}\]
   When Fe²⁺ ions migrate from the anode region, they react with water and further get oxidised to form Fe³⁺ ions. A reddish-coloured hydrated oxide is formed from Fe³⁺ ions. It is called rust. It collects on the surface. \[\ce{2Fe^{3+}(aq) + 4H2O(l) -> Fe2O3.H2O (s) + 6H^+ (aq)}\]