10.2 Nernst equation

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 work.