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Reduction of MnO4- to MnO2

elloL

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Hi,

I just ran across a practice problem during my content review that mentioned that when MnO4- reacts to become MnO2 this is a reduction. I am confused about this because it has fewer electrons since there are fewer double bonds? The practice problem was about a whole reaction, so if more information is needed to clear up my confusion, I can post the whole reaction.
 

JimKimSlim

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Have you tried writing down the whole balanced redox equation? In my case, I know permanganate is a strong oxidizing agent (should know this from orgo). Also, this tip doesn’t ALWAYS work, but the opposite of reduction is oxidation, and less oxygen usually means reduction. Again, if you want to approach this more systematically, just look up permanganate reduction and balance the equation yourself.
 
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Kumorebi

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Hi,

I just ran across a practice problem during my content review that mentioned that when MnO4- reacts to become MnO2 this is a reduction. I am confused about this because it has fewer electrons since there are fewer double bonds? The practice problem was about a whole reaction, so if more information is needed to clear up my confusion, I can post the whole reaction.
To clarify: oxygen is pretty electronegative. They pull electron density AWAY from Mn. LEO the lion goes GER - loss of electrons is oxidation, gain of electrons is reduction. By removing oxygens, more electrons are available for Mn reducing it. In general, adding oxygens will oxidize, adding hydrogens in place of oxygen will reduce. I hope this helps!

edit: also review calculating oxidation states and what your answer means.
 
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elloL

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To clarify: oxygen is pretty electronegative. They pull electron density AWAY from Mn. LEO, GER - loss of electrons is oxidation, gain of electrons is reduction. By removing oxygens, more electrons are available for Mn reducing it. In general, adding oxygens will oxidize, adding hydrogens in place of oxygen will reduce. I hope this helps!

edit: also review calculating oxidation states and what your answer means.

I was being silly and not considering how electronegative oxygen was and relying solely on the number of bonds that an atom has, so that makes a lot of sense, thank you @Kumorebi. Theoretically, if more bonds are added to an atom that are not associated with an electronegative atom or hydrogen, would this be considered a reduction? And @JimKimSlim - it's been years since I have taken organic chemistry, that specific compound isn't one that I remember explicitly.
 
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Kumorebi

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I was being silly and not considering how electronegative oxygen was and relying solely on the number of bonds that an atom has, so that makes a lot of sense, thank you @Kumorebi. Theoretically, if more bonds are added to an atom that are not associated with an electronegative atom or hydrogen, would this be considered a reduction? And @JimKimSlim - it's been years since I have taken organic chemistry, that specific compound isn't one that I remember explicitly.
It would depend on the relative difference of electronegativity between the atoms you are combining. Let's do an example - below is a periodic table with electronegativity values.

Electronegative.jpg


You can see that hydrogen is less electronegative than carbon. If you take an alkene and add hydrogens to it, the alkene is reduced (gain of electrons) because carbon is more electronegative than hydrogen. Hydrogen adds more electrons to the system, and the carbon will pull the electron density away from hydrogen towards itself. This by definition means the carbon is reduced. A common reaction is reducing alkenes with a pt catalyst. You might remember this one since it is pretty common (platinum is excessively used in industry for petroleum, used to create higher octane level for gasoline).

Ethene.jpg


On the flip side, what happens if we add a halogen (let's use bromine) to this? Bromine is more electronegative than carbon and therefore the addition of bromine to an alkene is technically an oxidation reaction! You can confirm this by calculating your oxidation numbers. Bromine will pull electron density away from the system towards itself, oxidating (loss of electrons) carbon. When we think of this in organic chemistry this is how leaving groups work! So to answer your question - adding bonds does not always mean reduction. It depends on the relative electronegativity of the atoms you combine. Sorry for the lecture lol. I was in PhD chemistry before switching. ;)
1920px-Alkene-bromine-addition-2D-skeletal.png

Edit: spelling
 
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Kumorebi

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The way I thought of this question was: MnO4-(Mn has +7 oxidation state here) -> MnO2 (Mn has +4 oxidation state here)
Mn gained 3 electrons and is reduced.
But if you know the foundation behind oxidation/reduction you don’t even have to calculate it! I do however recommend knowing how to calculate oxidation numbers for the MCAT.
 
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DrPatrickStar

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But if you know the foundation behind oxidation/reduction you don’t even have to calculate it! I do however recommend knowing how to calculate oxidation numbers for the MCAT.
True, but the concept works better in organic molecules while this is an inorganic molecule and oxidation numbers will be much easier to solve this problem.
 
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elloL

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But if you know the foundation behind oxidation/reduction you don’t even have to calculate it! I do however recommend knowing how to calculate oxidation numbers for the MCAT.
That was an incredible explanation @Kumorebi , thank you so much! I'm pretty weak in chemistry at the moment because I am a Psych/neuroscience undergrad newly graduated, and took my pre-reqs really early, but that helped refresh many fundamentals about chemistry.
 
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Kumorebi

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That was an incredible explanation @Kumorebi , thank you so much! I'm pretty weak in chemistry at the moment because I am a Psych/neuroscience undergrad newly graduated, and took my pre-reqs really early, but that helped refresh many fundamentals about chemistry.
Shoot me PMs if you have any other questions on chemisty. Best of luck!
 
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