1. The Nernst Equation

2. Concentration and Ecell
The voltage of a cell depends on the conditions of the cell.
In non-standard conditions the voltage of a cell will be different:
Ecell = E°cell - (RT/nF)ln(Q)
Ecell = E°cell - (0.0591/n)log(Q)
The Nernst Equation

3. Q not liquids or solids Q is the reaction Quotient: aA + bB  cC + dD
Q = [C]c [D]d [A]a [B]b
not liquids or solids

4. Concentration and Ecell
With the Nernst Eq., we can determine the effect of concentration on cell potentials.
Ecell = E°cell - (0.0591/n)log(Q)
Example. Calculate the cell potential for the following:
Fe(s) + Cu2+(aq) Fe2+(aq) + Cu(s)
Where [Cu2+] = 0.3 M and [Fe2+] = 0.1 M

5. Concentration and Ecell (cont.)
• First, need to identify the two half reacions
Fe(s) + Cu2+(aq) Fe2+(aq) + Cu(s)
Cu2+(aq) + 2e- Cu(s)
E°1/2 = 0.34 V
Fe(s) Fe 2+(aq) + 2e-
E°1/2 = V
Fe(s) + Cu2+(aq) Fe2+(aq) + Cu(s)
E°cell = V

6. Concentration and Ecell (cont.)
• Now, calculate Ecell
Fe(s) + Cu2+(aq) Fe2+(aq) + Cu(s)
E°cell = V
Ecell = E°cell - (0.0591/n)log(Q)
Ecell = 0.78 V - ( /2)log(0.33)
Ecell = 0.78 V - ( V) = V

7. Determining Concentration
• If [Cu2+] = 0.3 M, what [Fe2+] is needed so that
Ecell = 0.76 V?
Fe(s) + Cu2+(aq) Fe2+(aq) + Cu(s)
E°cell = V
Ecell = E°cell - (0.0591/n)log(Q)
0.76 V = 0.78 V - (0.0591/2)log(Q)
0.02 V = (0.0591/2)log(Q)
0.676 = log(Q)
4.7 = Q

8. Concentration and Ecell (cont.)
Fe(s) + Cu2+(aq) Fe2+(aq) + Cu(s)
4.7 = Q
[Fe2+] = 1.4 M