ixxon2001
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MnO + Br2 = ???
This question is regarding Manganese Oxide(II) and Bromine. The equation at 298K is:
MnO(s) + Br2(l) = ???
Using gibbs free energy values (rounded to nearest whole) I get:
MnO(s) + Br2(l) -> MnBr2(s) + 1/2O2(g)
dG = products-reactants = -373 + 0 - (-363) - 0 = -10 kJ/mol
This indicates the reaction should go forward. However, I can't find clear documentation of this reaction as most discussions talk about MnO2 and the
need for H2SO4 and not MnO. Again using gibbs free energy values:
MnO2(s) + Br2(l) -> MnBr2 + O2(g)
dG = -373 + 0 -(-465) - 0 = +92 kJ/mol
so this shouldn't go forward. Why is there no literature on the reaction with MnO? What am I missing or what did I do wrong?
Thanks for any insight.
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spirocycle
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maybe with MnO2, the reaction is heated.
remember that thermodynamics will tell you the direction of the reaction, but sometimes it progresses painfully slow.
also are you using dG(o) or the formula dG=dH-tdS?
because you need to account for temperature if you haven't already
[Edited on 20-10-2010 by spirocycle]
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ixxon2001
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Thanks for the reply!
I am using dG corrected for temperature (dG=dH-tdS) and have pulled normalized values from published tables (NIST has a 2-volume set of these for pure
compounds).
I haven't actually tested the reaction yet, I am simply frustrated and concerned at the lack of literature on the reaction. Granted, MnO2 is common
and MnO is not as MnO is produced by decarbonation of MnCO3 under vacuum - but I am still worried I have missed something.
Any more insights? Anyone? Bueller? Bueller? (teacher from Ferris Bueller's Day Off)
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woelen
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I would expect hardly any reaction, maybe a little of the Mn is oxidized to oxidation state +4, but that will not be a major reaction.
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ixxon2001
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| Quote: |
I would expect hardly any reaction, maybe a little of the Mn is oxidized to oxidation state +4, but that will not be a major reaction.
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I'm confused by this response as my source material is MnO (s) and Br2 (l), so the MnO should be in oxidation state +2 (Mn (+2) and O (-2)) and would
remain in state +2 if Br2 was to replace O. Is this not correct?
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woelen
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If Br replaces O, then the manganese remains in oxidation state +2, but it might also be that the Br2 oxidizes some of the Mn to the +4 oxidation
state. In that case there will be no replacement of oxygen by bromine, but there will be addition of bromine to the MnO. I'm quite sure that bromine
will not replace oxygen, if it reacts, then the only thing I can imagine is oxidation or disproportionatio: O(2-) can give OBr(-) and Br(-) with Br2.
Whether this reaction really occurs or not will be hard to tell. I do not expect much of this. I do not have any MnO to test this myself, I only have
MnO2.
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not_important
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As MnO is a sold, same for MnB2, and I would not expect either to show much if any solubility in liquid Br2, any reaction would likely be slow and
slow further as a layer of MnBr2 formed over the MnO.
The fact that you find no journal discussions of that reaction suggests that it simply may not occur.
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