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how rusting of iron is envisaged as setting up of


How rusting of iron is envisaged as setting up of an electrochemical cell?

Solution)

Rusting of Iron is due to corossion.

Rust is an Fe2O3.xH2O

Theory of Rusting :

1. Water present on the metal surface dissolves CO2 and O2 from the air.

       H2O + CO2 --------->     H2CO3

       O2 + 4H+ + 4e- -------> 2H2O

2. Fe in contact with dissolved CO2 and O2 undergoes oxidation.

    Fe ------------> Fe2+ + 2e-  Anode (EFe+2/Fe = -0.44V)

 

3. Electrons lost by Fe are taken by H+

      H+ + e-  ------------>  H

      4H+O2 --------> 2H2O

The dissolved O2 can be take electrons directly also

  O2 + 2H2O + 4e-  ----------->  3OH-   cathode

  Overall 2Fe(s) + O2(g)  +4H+ (aq)   ------------->    2Fe+2  + 2H2O

 

4. Fe+2 react with dissolved O2 and water

Fe+2 + 2OH-  ----------->  Fe(OH)2

4Fe(OH)2 + O2 +2H2O ----------> 4Fe(OH)3

2Fe(OH)3  --------->  Fe2O3 + 3H2O

Fe2O3 + XH2O  ------------>  Fe2O3.XH2O

Rust (Hydrated ferric oxide)

 

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