Winter 2016 final Q3A [ENDORSED]

[katlanoue2I]

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]

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]

Thank you! That makes sense

[Stephanie Demo 2N]

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

[Benjamin_Oh_3O]

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

[Chem_Mod]

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