Explain how rusting of iron is envisaged as setting up of an electrochemical cell.
In the corrosion reaction, due to the presence of air and moisture, oxidation takes place at a particular point of an object made of iron. That spot behaves as the anode. The reaction at the anode is given by,
Fe(s) ⇒ Fe2+ (aq) + 2e-
Electrons released at the anodic spot move through the metal and go to another spot of the object, wherein presence of H+ ions, the electrons reduce oxygen. This spot behaves as the cathode. These H+ ions come either from H2CO3, which are formed due to the dissolution of carbon dioxide from the air into water. The cathodic reaction is given by
O2(air) + 4H(aq)++4e-⇒ 2H2O
The overall reaction is given by,
2Fe(s) + O2(air) + 4H(aq)+⇒ 2Fe2++2H2O