Determination of Keq Values Using the Nernst Equation
The Eo values and the equilibrium constant are both measures of the tendency for a reaction to take place.  It would be reasonable for us to suspect that they are related.
 
At equilibrium the value of Eo is 0, since no net flow of elecrtons will be occurring.
 
Therefore the existing [ ]'s at equilibrium and the logrithm term from the Nernst equation become Keq.
ie.  0 = Eo - 0.059/n log [C]c[D]d / [A]a[B]b          but     Keq = [C]c[D]d / [A]a[B]b
     Therefore   0 = Eo - 0.059/n log Keq

      log Keq = n Eo / 0.059

      Keq = 10 (nEo / 0.059)
 

So the equilibrium constant for any redox reaction can be calculated using it's standard half-cell reduction potential.  If the Eo value is +ve then Keq > 1 and a reaction with an Eo value that is -ve will have a Keq < 1.
 
ex.   Calculate the Keq value for the reaction between silver nitrate and metallic zinc:  Is the reaction essentially complete or incomplete.

1.  Write the overall reaction:    2 AgNO3  +  Zno  ---->   Zn(NO3)2  +  2 Ago

2.    Write the half cell reactions:     2 Ag+1  +  2e-1  ---> 2 Ago   Eo = +0.80 V
                                                          Zno  ----->  Zn+2   + 2e-1       Eo = +0.76 V
                                                                                                         Eo = +1.56 V

3.   Solve for Keq           Keq = 10 (nEo / 0.059)
                                              =  10 (2)(1.56)/0.059
                                              =  1052.88
 

The extremely large value of Keq indicates a reaction that is essentially complete.
 
Ex #2:  Calculate the Keq value for the reaction between Zn and Copper(II) sulphate.  Is the reaction complete?

        Zno   +  CuSO4 --->  ZnSO4  + Cuo

        Zno  ---->  Zn+2   + 2e-1       Eo  = +0.76 V
        Cu+2  +  2e-1  ---->  Cuo      Eo = +0.34 V
                                                     Eo = 1.10 V
 

            Keq = 10 (nEo / 0.059)
                   =  10 (2)(1.10)/0.059
                   =  1037.29
 
Therefore it is essentially complete.