Question

How do you find E^o of a cell?

Is the formula E^o cell= cathode - anode?
Or is it E^o= Oxidation + Reduction?

i am very confused as to when I should add the cell potentials together and when I should subtract. If at all possible, please provide examples!

Answer 1

E^o_cell = E^o_reduction + E^o_oxidation

Generally,anode is the place where oxidation takes place and cathode is the place where reduction takes place.

The above stated formula can be used without any fuss for determining standard state cell potentials.

And regarding your addition/subtraction of the cell potentials,you always need to follow the formula and be careful when a specific potential has a negative sign attached to it.Okay,for example we are asked to determine the cell potential of the below reaction:

Zn(s) + Cu²⁺(aq) Zn²⁺(aq) + Cu(s)

Write down the half-reactions for each process.

Zn(s) Zn²⁺(aq) + 2 e^-

Cu²⁺(aq) + 2 e^- Cu(s)

E^o_{reduction of Cu2+} = + 0.339 V

E^o_{reduction of Zn2+} = - 0.762 V

E^o_{oxidation of Zn} = - ( - 0.762 V) = + 0.762 V(probably,this is where you're getting confused,we looked upon the reduction potential of Zn2+ but here we need the oxidation potential as zinc is getting reduced)

• Write the half-reactions for each process.
• Look up the standard potentials for the redcution half-reaction.
• Look up the standard reduction potential for the reverse of the oxidation reaction and change the sign.
• Add the cell potentials together to get the overall standard cell potential.

oxidation:	Zn(s) Zn2+(aq) + 2 e-	Eoox. = - Eored. = - (- 0.762 V) = + 0.762 V
reduction:	Cu2+(aq) + 2 e- Cu(s)	Eored. = + 0.339 V
overall:	Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)	Eocell = + 1.101 V