CHEMISTRY COMMUNITY

How do you calculate the emf based only on the species?

This is from Q3 on 2014 Final, the one with the Pikachu drawing. It asks to pick the chemicals that you can use to produce the highest voltage. How do you know which ones to choose?

It later asks for emf of the cell in part D but no voltages were given at all.

I'm assuming that the reduction potentials were given on a sheet that was not included in the course reader. If you look up the reduction potentials involved in the question it works out! Here is the work:

For this question, we are trying to build a galvanic cell that has the highest overall voltage. Here are the missing reduction potentials:

Zn (2+) + 2e -> Zn (s) -.763V

Fe (2+) + 2e -> Fe (s) -.44V

Fe (3+) + e -> Fe (2+) (aq) .771

Cu (+) + e -> Cu (s) .521V


So in creating the galvanic cell we want to choose the two that combined give the greatest overall voltage which, using the E*cell= E*cathode- E*anode, would be the reduction of Zinc to Zn(s) and Fe (3+) to Fe (2+) (in the equation: .771- (-.763) which would give an overall cell potential of 1.534). As the reduction of iron is to another aqueous iron ion, we use Fe(NO3)2 instead of Fe(s) which implies that we also need to use Pt as a conductor.

Therefore, we use Zn(s), Zn(NO3)2, Fe(NO3)2, Fe(NO3)3, and Pt in the reaction.

For the final then if asked a question without the necessary reduction potentials being stated in the problem, would we be given the reduction potentials on the formula sheet to find ourselves? Like in the pikachu problem. I was confused on this as well.