Explain how rusting of iron is envisaged as setting up of an electroCHMemical cell.

Explanation

Water layer present on the surface of iron (particularly during rainy season ) dissolves acidic oxides present in air such as `CO_2 , SO_2` , etc to form acids which dissociate to give `H^+` ions :
`H_(2) O + CO_(2) to H_(2) CO_(3) hArr 2 H^(+) + CO_(3)^(2-)`
In the presence of `H^(+)` ions , iron starts losing electrons at a particular spot of iron object to form ferrous ions . Therefore , oxidation takes place and that spot behaves as anode .
`Fe (s) to Fe^(2+) (aq) + 2 e^(-)`
The electrons released at anodic spot move through the metal to reach another spot where `H^(+)` ions and the dissolved oxygen take up these electrons and reduction takes place . Hence this spot behaves as cathode
`O_2 (g) + 4 H^+ (aq) + 4 e^(-) to 2 H_2O (l)`
The overall reaction is :
`2 Fe (s) + O_(2) (g) + 4 H^(+) (aq) to 2 Fe^(2+) (aq) + 2 H_(2) O (l)`
Thus , an electrochemical cell sets up on the surface .
Ferrous ions are further oxidised by atmospheric oxygen to ferric ions which combine with water molecules to form hydrated ferric oxide , `Fe_2 O_3 , x H_2 O `, which is rust .