III.Electrochemical Cells (continued): Calculating Cell Voltages  The standard voltage for any electrochemical cell can be calculated from the theoretical standard voltage of each of the “half-cells”. eg: Calculate the standard cell voltage for a zinc // copper cell. E° cell = E° oxid + E° red Standard cell potential Standard oxidation potential Standard reduction potential = + Note: values on SRP table are for reduction half-reactions. To get standard oxidation potential, change sign of SRP. E° cell = 0.76V V = 1.10V

The Hydrogen Reference Electrode:  All half cell potentials are measured relative to H 2. eg: H 2 + Cu 2+  Cu + 2H +  If measured voltage under standard conditions is 0.34V, how do we know how much voltage comes from each half-reaction? H 2  2H + + 2e - Cu e -  Cu or  In reality neither half-reaction alone provides any voltage.  We arbitrarily define the H 2 half-reaction to be 0.0V.

E° cell = E° oxid + E° red 0.34V = 0.0V + E° red The voltage of the reduction of Cu 2+ is 0.34V.  A substance with a positive SRP is more easily reduced than H + and a substance with a negative SRP is not as easily reduced H +.

Notes:  Voltage is independent of the number of e - ’s transferred.  A negative value for E° cell means the forward reaction is not spontaneous (the reverse reaction is.)  Increasing reactant concentration or decreasing product concentration will increase the voltage of the cell.  If a cell reaches equilibrium E° cell = 0V (dead battery)