Calculate the standard cell potential given the following standard reduction potentials: Al3++3e-→Al; E∘=-1.66 V Cu2++2e-→Cu; E∘=0.340 V

To calculate the cell potential we use the relation:

Eº cell = Eº oxidation + Eº reduction

Now in order to determine which of the species is going to be oxidized, we have to remember that the more the value of the reduction potential is negative, the greater its tendency to be oxidized is. In electrochemistry we use the values of the reductions potential in the tables for simplicity because the only thing we need to do is change the sign of the reduction potential for the oxized species.

So the species that is going to be oxidized is the Aluminium, and therefore:

Eº cell = - (-1.66 V) + 0.340 V = 5.06 V

Equally valid is to write the equation as:

Eº cell = Eº reduction for the reduced species - Eº reduction for the oxidized species

These two expressions are equivalent, choose the one you fell more comfortable but be careful with the signs.