2016 WINTER MIDTERM Q8A

[Claire_Zhou_1A]

The question is: A standard electrochemical cell is made by placing a silver electrode into a 1M Ag+ solution and a cadmium electrode into a 1A Cd2+ solution.
What is the redox reaction and what is the maximum potential produced by this cell?
So the E° for Cd(s) → Cd2+ + 2e- is 0.4V (the reverse of the reduction potential) and for Ag+ + e- → Ag is 0.8V. So I thought the answer should be 0.8V - 0.4V which equals to 0.4V. Why is the answer 1.2V? Could someone explain it? Thanks!

[Anna_Kim_2E]

You use subtraction Ecathode-Eanode when they are both in their reduction state. Hence, since the two half reactions are already in oxidation and reduction states, you can add their E° of the both equations. 0.8V+0.4V=1.2V