10.2 Nernst equation
Electrochemistry

10.2 Nernst equation


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.




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