Rusting of iron, Chemistry

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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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