G for a reaction at any moment in time tells us two things. The sign of 
G tells us in what direction the reaction has to shift to reach equilibrium. The magnitude of 
G tells us how far the reaction is from equilibrium at that moment.Energy drives all changes including chemical reactions. In a redox reaction, the energy released in a reaction due to movement of charged particles give rise to a potential difference. The maximum potential difference is called the electromotive force, (EMF), E and the maximum electric work W is the product of charge q in Coulomb (C), and the potential DE in Volt (= J / C) or EMF.
W = q DE (units)
Note that the EMF DE is determined by the nature of the reactants and electrolytes, not by the size of the cell or amounts of material in it. The amount of reactants is proportional to the charge and available energy of the galvanic cell.
The Gibb's free energy DG is the negative value of maximum electric work,
DG = - W = - q DE
A redox reaction equation represents definite amounts of reactants in the formation of also definite amounts of products. The number (n) of electrons in such a reaction equation, is related to the amount of charge transferred when the reaction is completed. Since each mole of electron has a charge of 96485 C (known as the Faraday's constant, F),
q = n F and DG = - n F DE At standard conditions, DG° = - n F DE°
The value of G for a reaction at any moment in time tells us two things. The sign of G tells us in what direction the reaction has to shift to reach equilibrium. The magnitude of G tells us how far the reaction is from equilibrium at that moment.
The potential of an electrochemical cell is a measure of how far an oxidation-reduction reaction is from equilibrium. The Nernst equation describes the relationship between the cell potential at any moment in time and the standard-state cell potential.
F= Faraday Constant = 9.6485338x104 C/mol
The amount of electric charge carried by one mole of electrons.
Let's rearrange this equation as follows.
nFE = nFEo - RT ln Q
We can now compare it with the equation used to describe the relationship between the free energy of reaction at any moment in time and the standard-state free energy of reaction.
G = 
Go + RT ln Q
These equations are similar because the Nernst equation is a special case of the more general free energy relationship.
Problems using Nernst :
Calculate the EMF of the cell
Zn(s) | Zn2+ (0.024 M) || Zn2+ (2.4 M) | Zn(s)
Solution
Zn2+ (2.4 M) + 2 e = Zn Reduction
Zn = Zn2+ (0.024 M) + 2 e Oxidation
--------------------------------------------
Zn2+ (2.4 M) = Zn2+ (0.024 M), DE° = 0.00 - - Net reaction
Using the Nernst equation:
DE = 0.00 - 0.0592 / 2 log ((0.024) / (2.4)
= (-0.296)(-2.0) = 0.0592 V
1) Using the balanced redox reaction and the concentrations given, determine the potential.
2Al(s) + ClO3-(aq) + 6H+(aq) => 2Al3+(aq) + Cl-(aq) + 3H2O
Eo = 3.12;[ClO3-] = 0.93M;[H+] = 0.91M;[Al3+] = 0.66M;[Cl-] = 0.88M;
2) Using the balanced redox reaction and the concentrations given, determine the potential.
3Pb(s) + 2NO3-(aq) + 8H+(aq) => 3Pb2+(aq) + 2NO(g) + 4H2O
Eo = 1.0812;[NO3-] = 0.53M;[H+] = 0.32M;[Pb2+] = 0.95M;[NO] = 0.71M;