Electrochemistry : 10.2 Nernst Equation
NERNST EQUATION
- Cell potential (E°cell) under any condition
Ecell = E°cell – (RT/nF) ln Q
R: universal gas constant. Q : reaction quontient.
N: no. of e- transferred in T : absolute temperature (X)
balanced redox reaction.
F : charge of 1 mol of e"96.500 C/ mol e-
Example:
Cd (s) + 2Ag+ (aq) ------ Cd2+(aq) + 2Ag (s)
Q=[Cd2+]/[Ag+]2
Ecell At 25°C (298K)
Ecell = E°cell - (RT/nF) ln Q (convert to logarithm)
Ecell = E°cell - (0.0592/N) log Q
EFFECT OF CONC . ON CELL POTENTIAL.
Ecell = E°cell - (RT/nF) ln Q
EXAMPLE 1:
Zn (S) +Cu2+ (aq) ------ Zn2+ (aq) + Cu (s)
Q=[Cd2+]/[Ag+]2
WHEN Q < 1 = [reactant] > [product]
= in Q < 0 , so Ecell > E°cell
WHEN Q > 1 = [reactant] = [product]
= in Q = 0 , so Ecell = E°cell
WHEN Q > 1 = [reactant] < [product]
= in Q > 0 , so Ecell < E°cell
WHEN Q = Kc
Ecell = E°cell - (0.0592/N) log Q
- Ecell = O
- The system reach equilibrium.
- No more energy release.
- Cell can do no more work.
