m65mx
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K2MnO4 disproportionation
Hello,
I'm looking for a solution of that:
Equation of heating KMnO4 is:
2 KMnO4 ––––> K2MnO4 + MnO2 + O2
And I have to find out, which of the products dissolves in water to a green solution. I think it's K2MnO4, but I'm not sure...
Then I have to write equations (reactions) for the disproportionation of that chemical compound (that one that I think it's K2MnO4) in neutral and
acidic medium (specifically, a reaction with H2SO4).
Thank you very much for your help...
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12AX7
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Well, you can find out easily enough, UTFSE, www.google.com. Hint: MnO2 is insoluble, so either everything drops out of solution, which it doesn't since you say it remains green, or else,
the manganate remains dissolved, with a green color.
As for the equations, I would guess stoichiometry with the MnO2, so you basically need to add two sulfate ions to your equation, putting Mn(4+)
sulfate in solution. Simple enough, huh?
Tim
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woelen
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MnO2 is insoluble. The MnO4(2-) ion disproportionates to MnO2 and MnO4(-) in acidic media (and also slowly in neutral media).
Mn(4+) will NOT go into solution, not even with strong sulphuric acid. Only if very concentrated hydrochloric acid is used for dissolving the K2MnO4,
the Mn(4+) will go in solution as an unstable chloro-complex. This complex, however, quickly decomposes, giving chloride ion, chlorine gas and Mn(2+).
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m65mx
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thanks
Thank you very much for your help, that's what I needed.
______
m65mx
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12AX7
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| Quote: | Originally posted by woelen
Mn(4+) will NOT go into solution, not even with strong sulphuric acid. Only if very concentrated hydrochloric acid is used for dissolving the K2MnO4,
the Mn(4+) will go in solution as an unstable chloro-complex. This complex, however, quickly decomposes, giving chloride ion, chlorine gas and Mn(2+).
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Ah. Then what was the light purple solution I got with (not concentrated) sulfuric acid and (ceramic grade) MnO2? I'm pretty sure it's Mn(higher
than 2+) since it bubbled Cl2 when salt was added (I'm pretty sure it wasn't HCl coming off from the strong acidity), and the color disappeared.
Tim
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woelen
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That most likely is Mn(3+). The commercial and cheap MnO2 in fact is MnOx, with x < 2. It contains Mn(III) and Mn(IV). At very low pH, the Mn(3+)
ion can exist in aqueous solution. With HCl this is converted to Mn(2+), and Cl2 is formed.
A picture of the Mn(3+) ion is on my website:
http://woelen.scheikunde.net/science/chem/solutions/mn.html
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12AX7
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Hmm, could be, don't remember any oxygen production although it dissolves quite slowly to begin with. Seems to me it's pinker, too. Suppose I should
prepare some and take a picture...
Tim
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woelen
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Oxygen production? How do you mean that? I nowhere mention the production of oxygen in making Mn(3+).
Oxygen is produced when making Mn(VI) as the manganate (also shown on the same page), but that is a totally different story.
I, btw, made my Mn(3+) solution by very careful oxidation of Mn(2+) with permanganate in strongly acidic solution. Too much permanganate or too high a
pH leads to formation of MnO2 instead of the delicate Mn(3+).
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12AX7
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Well yeah, but if I have a product that is mostly Mn(IV), but it dissolves as Mn(III), where'd my oxidation state go? Persulfate formation??!
Tim
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woelen
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The cheap MnO2 from the pottery suppliers has quite some Mn(III), it is oxygen-deficient. Did all of the solid dissolve or just a small part of it? If
only a small part of it dissolved, then only the Mn(III) part dissolved and then you need not get any oxygen. You certainly will not get persulfate
from MnO2 and dilute H2SO4.
If all dissolved, then I seriously doubt that you have normal MnO2. I also have MnO2 from a pottery supplier and that stuff is dark grey fine
crystalline material and it does not dissolve in dilute H2SO4.
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12AX7
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Hmm, ok then...
It certainly doesn't dissolve quickly.
Tim
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