Exploring the Iron Rusting Reaction with KMnO4

[tanya234]

Hi everyone,

I'm not sure what would happen in the following reaction (it has something to do with the rusting of iron):

Fe (s) + KMnO4 (aq) ----> ?

KMnO4 is potassium permanganate and I know it is a very strong oxidizer. So I am assuming one of the products may be Fe2O3, which is rust?



- Tanya

 

[chem_tr]

Hello,

I am afraid permanganate has nothing to do with rusting; it is indeed an oxidizer, but is not involved in this very process. The oxidant in rusting of iron is elemental oxygen, and the product is iron(III)hydroxide, Fe(OH)₃.

4/Fe ---> Fe³⁺ + 3e^-
3/O₂ + 4e^----> 2O^2-
--------------------------
4Fe+3O₂ ---> 4Fe³⁺ + 6O^2- (which means that two moles of Fe₂O₃ is produced)

This reaction takes place in a very fine aqueous layer on iron; so water molecules are also involved:

Fe₂O₃ + 3H₂O ---> 2Fe(OH)₃

So, we may conclude that rust contains both iron(III)hydroxide and iron(III)oxide; the product depends of the relative humidity and water content present on iron.

Regards
chem_tr

 

[chem_tr]

What about Fe and permanganate?

By the way, you might wonder what would be the result of reaction between solid iron and potassium permanganate. The product totally depends on which acid (or base) is used as an auxilary to drive this redox process faster.

Let me give some examples about this:

a) Acidic media (Mn²⁺; five-electron reduction)

5Fe ---> 5Fe³⁺ + 15e^-
3MnO₄^- + 15e^- + 24H⁺ ---> 3Mn²⁺ + 12H₂O

5Fe + 3MnO₄^- + 24H⁺ ---> 5Fe³⁺ + 3Mn²⁺ + 12H₂O

When you carry out this reaction in the presence of hydrochloric acid, you will obtain this:

5Fe + 3KMnO₄ + 24HCl ---> 5FeCl₃ + 3MnCl₂ + 3KCl + 12H₂O

b) Excessive alkaline media (oxidative alkaline fusion; MnO₄^2-; one electron reduction)

I have found that this reaction is responsible for iron's oxidation and production of manganate:

Fe + 3MnO₄^- + 3OH^- ---> Fe(OH)₃ + 3MnO₄^2-

However, writing a full-balanced equation is excessively difficult in this case. It is possible that, a mixture of potassium and sodium manganates (if sodium hydroxide is used) and even iron(III)manganate is expected to occur.

c) Nearly neutral media (mild acidic, MnO₂; mild basic, MnO(OH)₂): 3 electron-reduction

There are two alternative reactions here:

Fe + MnO₄^- + 2H₂O ---> Fe³⁺ + MnO₂ + 4OH^-

Fe + MnO₄^- + 2H₂O ---> Fe³⁺ + MnO(OH)₂ + O^2-

Regards
chem_tr