Electrode Potentials

We will continue our discussion of electrochemical cells, using the same example used previously, the zinc-copper cell, also known as the Daniell cell shown below.  An electric current flows from the anode to the cathode because there is a difference in electrical potential energy between the electrodes.  This flow of electric current can be likened to the flow of water down a waterfall, which occurs because there is a difference in gravitational potential energy.  Experimentally the difference in electrical potential between the anode and cathode can be measured with a voltmeter as shown in our diagram of the Daniell cell.  The reading on the voltmeter will be in volts.  However, two other terms, electromotive force or emf and cell potential are also used to denote cell voltage.  The voltage of a cell depends on the nature of the electrodes and the concentration

of ions and the temperature. 

When the concentrations of the Cu²⁺ and Zn²⁺ ions are both 1.0 M, we find that the cell voltage of the Daniell cell is 1.10 V at 25^oC.  The voltmeter can only measure the difference in potential between two electrodes, it cannot measure the potential of either electrode alone.  In order to get around this impasse we have arbitrarily chosen the hydrogen electrode as a reference and defined its potential to be zero.  Having done this, we can use the hydrogen electrode to determine the relative potentials of other electrodes.  Tables are available listing standard electrode potentials. 

We are now in a position to answer our question in the previous  as to how we determine which electrode is the cathode and which is the anode.  Lets stick with our present example of a cell made from copper metal in contact with Cu²⁺ and zinc metal in contact with Zn²⁺ ions.  If we look in a table of standard electrode potentials we will find these two entries for the copper and zinc electrodes.

Cu²⁺(aq)  + 2 e^-  = Cu(s)               E^o_Cu = 0.34 V

Zn²⁺(aq)  + 2 e^-  = Zn(s)               E^o_Zn = -0.76 V  

First notice that all of the half-reactions in the table are written as reductions, this allows easy comparison of the electrode potentials.  The electrode potential is a quantitative measure of the driving force or tendency of the half-reaction to occur as written.  Comparing the copper and zinc electrode potentials, we see that copper has the more positive potential and consequently has a greater tendency to undergo reduction than zinc.  This tells us that the copper electrode is going to be the cathode and zinc is going to be the anode.  We can  now calculate the cell potential from the following relationship:

E^o_cell = E^o_cathode - E^o_anode = 0.34 V -(-0.76 V) = 1.10 V

Strengths of Oxidizing and Reducing Agents