[jensro]

Hi!

My task is to find the mass percentage of Fe in a razer blade. I do understand the calculations and the use of the  chemical equation; 5Fe2+ + 8H+ + MnO4- --- Mn2+ + 4H2O + 5Fe3+.

What I don't understand is that we use 3M H2SO4 for the oxidation of Fe to Fe2+, before starting the titration.

The reduction potential of SO42- + 4H+ +2e-  or 2H+ +2e- is high enough to oxidize Fe ---> Fe3+ + 3e-.

SO42- + 4H+ +2e-   are also able to oxidize Fe2+ ---> Fe3+ + e-. Why or how do I know that the sulfuric acid only oxidizes Fe to Fe2+?

Thanks!
Jens

[Borek]

It is not sulfuric acid that is an oxidizer here.

[jensro]

MnO4- is an oxidizer for Fe2+ (to Fe3+), but H2SO4 is an oxidizer for Fe to Fe2+.

[AWK]

Write down reaction of iron with sulfuric acid!

[jensro]

Fe + 2H+ --> Fe2+  + H2
 
or

Fe+ 4H+ + SO42-  --> Fe2+ + H2SO3 + H2O.

[AWK]

Forget the second reaction.

[jensro]

Why?

And still; the reaction with 2H+ +2e- --> H2 will theoretically be able to oxidize Fe to Fe3+ (from standard reduction potential).

[Borek]

And still; the reaction with 2H+ +2e- --> H2 will theoretically be able to oxidize Fe to Fe3+ (from standard reduction potential).

Nope. As far as I am aware H⁺ (not sulfuric acid, just H⁺) is capable of oxidizing solid Fe to Fe²⁺ but not further.

Why?

Yes, concentrated sulfuric acid is an oxidizer and in some specific cases can get reduced to SO₃^2-. Not in the presence of excess water though, and 3M solution is quite diluted.

[jensro]

Fe--> Fe3+  + 3e-    E0=0.04V, so theoretically it will, but fair enough.

Thanks!

[Borek]

Fe--> Fe3+  + 3e-    E0=0.04V, so theoretically it will

None of the sources I consider reliable confirms this number.

[jensro]

Well, that is another discussion.

The values of the standard reduction potentials I've been given in school will set the start of my argumentation. In school context the principles are the most important.

Thanks again.

[Enthalpy]

A razor blade is not made of iron. It uses to be stainless steel, with some 18% Cr that make a stable oxide layer to change the chemical behaviour completely. I feel unreasonable to apply iron redox potentials and make any deduction from them.