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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: jensro on August 26, 2020, 06:52:11 AM

Title: Oxidation of Fe with H2SO4
Post by: jensro on August 26, 2020, 06:52:11 AM
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
Title: Re: Oxidation of Fe with H2SO4
Post by: Borek on August 26, 2020, 08:29:57 AM
It is not sulfuric acid that is an oxidizer here.
Title: Re: Oxidation of Fe with H2SO4
Post by: jensro on August 26, 2020, 11:24:53 AM
MnO4- is an oxidizer for Fe2+ (to Fe3+), but H2SO4 is an oxidizer for Fe to Fe2+.
Title: Re: Oxidation of Fe with H2SO4
Post by: AWK on August 26, 2020, 11:30:10 AM
Write down reaction of iron with sulfuric acid!
Title: Re: Oxidation of Fe with H2SO4
Post by: jensro on August 26, 2020, 11:40:57 AM

Fe + 2H+ --> Fe2+  + H2
 
or

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

Title: Re: Oxidation of Fe with H2SO4
Post by: AWK on August 26, 2020, 11:45:30 AM
Forget the second reaction.
Title: Re: Oxidation of Fe with H2SO4
Post by: jensro on August 26, 2020, 11:49:45 AM
Why?

And still; the reaction with 2H+ +2e- --> H2 will theoretically be able to oxidize Fe to Fe3+ (from standard reduction potential).
Title: Re: Oxidation of Fe with H2SO4
Post by: Borek on August 26, 2020, 01:00:47 PM
Quote from: jensro on August 26, 2020, 11:49:45 AM
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.

Quote
Why?

Yes, concentrated sulfuric acid is an oxidizer and in some specific cases can get reduced to SO32-. Not in the presence of excess water though, and 3M solution is quite diluted.
Title: Re: Oxidation of Fe with H2SO4
Post by: jensro on August 26, 2020, 01:21:16 PM
Fe--> Fe3+  + 3e-    E0=0.04V, so theoretically it will, but fair enough.

Thanks!
Title: Re: Oxidation of Fe with H2SO4
Post by: Borek on August 26, 2020, 01:48:04 PM
Quote from: jensro on August 26, 2020, 01:21:16 PM
Fe--> Fe3+  + 3e-    E0=0.04V, so theoretically it will

None of the sources I consider reliable confirms this number.
Title: Re: Oxidation of Fe with H2SO4
Post by: jensro on August 26, 2020, 03:14:03 PM
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.
Title: Re: Oxidation of Fe with H2SO4
Post by: Enthalpy on August 27, 2020, 05:35:40 PM
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.