Why does magnesium rust instead of iron in sodium hypochlorite?

[tanya234]

Hi I was wondering what would be the equation for the rusting of magnesium?

I did an experiment where an iron nail was covered by a strip of magnesium and was put into diluted NaOCl (diluted sodium hypochlorite solution). Apparently the magnesium protects the iron from rusting and hence rusts instead.



Thanks,

Tanya

 

[IceSeeD]

reply from HONG KONG CHEM LOVER

Hi I was wondering what would be the equation for the rusting of magnesium?

I did an experiment where an iron nail was covered by a strip of magnesium and was put into diluted NaOCl (diluted sodium hypochlorite solution). Apparently the magnesium protects the iron from rusting and hence rusts instead.

1st: NaOCl+ H2O <>(reversible) NaCl + 2OH-
2nd: Mg + 2OH->MgO (rust) +H2O
is it right, i am not sure

 

[chem_tr]

Hello

I am not sure the equations in the first reply is totally correct, the products are okay though; hypochlorite has 1+ valence on chlorine, but chloride consists 1- valence. So there has to be a redox chemistry, and therefore, a reduction step. The electrons must have released from magnesium atom, so magnesium atom must undergo an oxidation step. Let me summarize these below:

Mg ---> Mg²⁺ + 2e^-
NaClO + 2e^- + H₂O ---> NaCl + 2OH^-

Mg + NaClO + H₂O---> Mg(OH)₂ + NaCl

But magnesium oxide may also be produced if the temperature of the medium is sufficiently high to cause dehydratation (water removal):

Mg(OH)₂ ---> MgO + H₂O

PS: Please note that this approach is the ideal one; you may also predict that mixed products like Mg(OH)Cl and MgCl₂ are likely to occur, but with a limited probability. Therefore, sodium hydroxide is another possible side product.

Regards
chem_tr

 

[IceSeeD]

YES~ mine one is incorrect..

sorry, i hve overlooked the Cl in OCl is +1