When a reaction is given, and it is able to be broken up into half reactions, you are able to look up the standard reduction potentials of the reactions. Adding up the standard reduction potentials of the half reactions will give you the cell potential of the overall reaction.
Things to remember:
1.Flipping a half reaction causes the sign of the standard reduction potential to reverse (negative becomes a positive sign, and vice versa).
2.Multiplying a half reaction by a constant does not mean multiplying a standard reduction potential by the same constant. Leave the standard reduction potential alone unless you are flipping the half reaction around.
When a cell potential is negative, the overall reaction is nonspontaneous (does not happen by itself). A positive cell potential indicates that the process is spontaneous.