Question 7 on Midterm 2013

[Alexandra Martinez 3D]

can someone explain to me why we reverse the oxidation half reaction with the E0=+1.49V, resulting in a negative voltage, instead of the reduction half reaction with the E0=+1.07V?

[MelanieAu1G]

You somewhat answered your question in your question. We switch the reaction involving Mn because it is the oxidation half reaction, meaning that the charge of the products is being reduced and therefore that the electrons must be on the right side of the equation. Switching this equation also allows its cell potential/ voltage to become the most negative of the two, which means that it will be a better reducing agent than the Br2 half reaction and thus, an oxidation.

[Alexandra Martinez 3D]

So we reverse it simply because we want the electrons in that equation on the right side? I was under the impression that we always wanted a positive voltage for our E0cell, and we flipped which ever equation allowed us to obtain such an answer.