Standard Reduction Potentials

The reaction in a galvanic cell is always a redox reaction that can be broken down into two half-reactions.  We can determine the cell potential by assigning a potential to each half-reaction and summing them.
Standard reduction potentials are the E° values corresponding to reduction half-reactions with all solutes at 1 M and all gases at 1 atm.
Combining the half-reactions to get a balanced redox reaction often requires one of the reactions to be reversed (in order to cancel out the electron transfers and since redox reactions must involve a substance being oxidized and a substance being reduced).  Also, the half-reaction with the largest possible potential (the reduction) will not be the reaction that is reversed.  The other half-reaction will be forced to run in reverse (the oxidation).  During this manipulation, the net potential of the cell will be the difference between the two.  (cathode - anode)  Also, because subtraction means "change the sign and add", we will change the sign of the oxidation (anode) reaction when we reverse it and add it to the reduction (cathode) reaction.
Another possible manipulation is multiplying the half-reaction by integers to make the number of electrons lost equal the number gained.  However, when this is done, the value of  E° is not changed.  Since standard reduction potential is an intensive property, the potential is not multiplied by the integer.
Another question is which reaction must run in reverse.  This can be answered by considering the sign of the potential of a working cell.  A cell will always run spontaneously in the direction that produces a positive cell potential.

A complete description of a galvanic cell usually contains these four items:
-the cell potential
-the direction of electron flow (obtained by inspecting the half-reactions and using the direction that gives a positive Ecell)
-designation of the anode and cathode
-the nature of each electrode and the ions present in each compartment