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

Explanation

The moisture on the surface of iron dissolves acidic oxides in the air like`CO_(2),SO_(2)` etc., to form acids which dissociate to give `H^(+)` ions
`H_(2)O + CO_(2) to H_(2)CO_(3) In the presence of H+ ions, iron having lower reduction potential starts losing electrons at some spot to form ferrous ions, i.e., its oxidation takes place. Hence, this spot acts as the anode :
`Fe(s) to Fe^(2+) (aq) + 2e^(-)`(oxidation)
The electrons thus released move through the metal to reach another spot where H+ ions and the dissolved oxygen take up these electrons and reduction reaction takes place. Hence, this spot acts as the cathode
`O_(2)(g) + 4H^(+) (aq) + 4e^(-) to2H_(2)O (l)` reduction)
The overall reaction may be written as
`2Fe(s) + O_(2)(g) + 4H^(+)(aq) to 2Fe^(2+)(aq) + 2H_(2)O (l)`
Thus, an electrochemical cell is set up on the surface.