Electrochemistry : 10.2 Nernst Equation

NERNST EQUATION 

• Cell potential (E°_cell) under any condition

                  E_cell = 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)


 
    E_cell = E°_cell - (RT/nF) ln Q         (convert to logarithm)


    E_cell = E°_cell - (0.0592/N) log Q  

 
            

EFFECT OF CONC . ON CELL POTENTIAL.

  

 E_cell = 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 E_cell > E°_cell



WHEN Q > 1 = [reactant] = [product]

                       = in Q = 0 , so E_cell = E°_cell



WHEN Q > 1 = [reactant] < [product]

                       = in Q > 0 , so E_cell < E°_cell


 
WHEN Q = K_c


    E_cell = E°_cell - (0.0592/N) log Q
        

• E_cell = O                           
  
• The system reach equilibrium.
  
• No more energy release.
  
• Cell can do no more wore.