Winter 2016 final Q3A  [ENDORSED]

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katlanoue2I
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Joined: Fri Jul 22, 2016 3:00 am

Winter 2016 final Q3A

Postby katlanoue2I » Thu Mar 16, 2017 12:17 am

For 3A on the 2016 final how do we know which half reaction will be oxidation and which will be reduction? Both of them seem to have increasing oxidation numbers so usually I'd think that'd mean they're oxidation?

Stephanie Demo 2N
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Joined: Thu Sep 29, 2016 3:00 am

Re: Winter 2016 final Q3A

Postby Stephanie Demo 2N » Thu Mar 16, 2017 1:38 pm

Fe initially has a charge 2+ and loses electrons to have a final charge of 3+ (lose electrons = oxidation)
The molecule MnO4 has a net charge of 1-. If you break it up into Mn and O you will see that Mn has a 6+ charge because O is 8- (oxygen is almost always 2-). The given information says that Mn has a final charge of 2+, meaning it had to have gained 4 electrons in the reaction (gain electrons = reduction)

katlanoue2I
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Joined: Fri Jul 22, 2016 3:00 am

Re: Winter 2016 final Q3A

Postby katlanoue2I » Thu Mar 16, 2017 5:05 pm

Thank you! That makes sense

Stephanie Demo 2N
Posts: 11
Joined: Thu Sep 29, 2016 3:00 am

Re: Winter 2016 final Q3A

Postby Stephanie Demo 2N » Thu Mar 16, 2017 10:22 pm

My bad, Mn would have a charge of 7+ and gain 5 electrons, apparently I can't add lol

Benjamin_Oh_3O
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Joined: Wed Sep 21, 2016 2:55 pm

Re: Winter 2016 final Q3A

Postby Benjamin_Oh_3O » Sat Mar 18, 2017 2:55 pm

For this question, where does the H+ come from? Can anyone help me??

Chem_Mod
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Re: Winter 2016 final Q3A  [ENDORSED]

Postby Chem_Mod » Sat Mar 18, 2017 3:18 pm

The H+ is present due to acidic conditions as described in the question.
I discussed this in detail in my Friday review session. See video of my review at:
https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=160&t=20492&sid=8d8d12a0f3d82b6959674cef8ce36595&sid=8ed6d164952da9378019889c6ccc1eba#p59637


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