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Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: filiana on November 10, 2005, 03:22:56 AM

Title: Aqueous solution of KMnO4 + conc. H2SO4
Post by: filiana on November 10, 2005, 03:22:56 AM
Hi,

I just tried following reaction and can't explain what happens chemically.
Could somebody help me on this?

Aqueous solution of KMnO4 added with:

a. a portion of conc. H2SO4 --> reddish solution, which turned yellow after some time.

b. double portion of conc. H2SO4 --> dark red solution, which stayed over time.

Actually in the system I only have: KMnO4, H2O, H2SO4.

In the forum, I found: KMnO4 + H2SO4 + H20 --> KHSO4 + MnO4- + H30+

Is KHSO4 dark red?

Due to color change, I assume that redox reaction took place. Thus, MnO4- --> Mn2+ (in acidic solution), which should be colorless. Then there should be a reducing agent. Is it (SO4)2-??

What could the reddish color be, which turn to yellow?
And what is the dark red color?
 ???

Thanks before....
Title: Re:Aqueous solution of KMnO4 + conc. H2SO4
Post by: Borek on November 10, 2005, 08:34:24 AM
Red color (into violet) is MnO4-.

MnO4- is not very stable and it may decompose producing brown MnO2.
Title: Re:Aqueous solution of KMnO4 + conc. H2SO4
Post by: Alberto_Kravina on November 10, 2005, 10:50:03 AM
Quote
Is KHSO4 dark red?

No! It is a white salt that forms colorless solutions.

Mabye the dark red color is MnO2 or manganeous acid (H2MnO3), like Borek said

Quote
KMnO4 + H2SO4 + H20 --> KHSO4 + MnO4- + H30+

I don't think that HMnO4 is stable

HMnO4 ----> MnO(OH) + O2(g)
Title: Re:Aqueous solution of KMnO4 + conc. H2SO4
Post by: filiana on November 10, 2005, 09:47:37 PM
Thank you for the hints....

I've searched further, and get that Mn2O7, which is red, is formed.
This will be further reduced to Mn2(SO4)3, which is formed quantitatively by the reaction Mn2+  +  permanganate in diluted H2SO4). What I haven't found, is whether the yellow color caused by Mn2(SO4)3 or something else.

Mabye the dark red color is MnO2 or manganeous acid (H2MnO3), like Borek saidI don't think that HMnO4 is stable

The red color should come from Mn2O7, i think.

HMnO4 ----> MnO(OH) + O2(g)
Quote
What color dies MnO(OH) have? I remember that MnO(OH) is not stable.
Title: Re:Aqueous solution of KMnO4 + conc. H2SO4
Post by: AgG on November 11, 2005, 11:28:44 PM
I am inclined to agree with Borek on this one.  Conc. MnO4- is a dark purple, dillute it is a red to pale pink. On exposure to air MnO4- is oxidized and decomposes to MnO2
Title: Re:Aqueous solution of KMnO4 + conc. H2SO4
Post by: Borek on November 12, 2005, 05:58:24 AM
Quote from: AgG on November 11, 2005, 11:28:44 PM
On exposure to air MnO4- is oxidized and decomposes to MnO2

It decomposes, but it is not oxidized, for example:

4MnO4- + 4H+ -> 4MnO2 + 3O2 + 2H2O

I doubt in existence of Mn2O7 in diluted solution - IIRC it can be made adding dry potassium permanganate to concentrated sulfuric acid. It won't exist in the presence of excess water.
Title: Re:Aqueous solution of KMnO4 + conc. H2SO4
Post by: AgG on November 12, 2005, 09:15:22 PM
oops! Borek is definately correct there, my apologies.  What I meant to say is that MnO4- decomposes in light.