Faraday law

FARADAY LAW

• Amount of substance produce of each electrode is directly proportional to quantity of charge flowing through the cell
  
• Also called Faraday's First Law of electrolysis.
  

CALCULATING USING FARADAY'S LAW

• Main steps
  • Balance half reaction to find number of moles of electrons.
  • Needed per mole product.
  • Use Faraday's constant (96500C / mol ) to find corresponding charge.
  • Use molar mass / mole to find charge needed for a given mass / mole of product

ELECTRIC CHARGE (Q)

Charge ( Q )         =    Current ( I )    ×    time ( t )

Unit Coulomb , C        Ampere , A        Second , s

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.

Standard hydrogen potential

Standard Reduction Potentials

Standard reduction potential (E0) is the voltage associated with a reduction reaction at an electrode when all solutes are 1 M and all gases are at 1 atm.

Standard hydrogen electrode (SHE)

 

Reduction Reaction

                                       

             E0 = 0 V

 

Galvanic cell

                           Oxidation
is defined as

• the addition of oxygen to a substance
• the loss of hydrogen from a substance
• the process of electron loss from a substance
• an increase in the oxidation number of a substance

                         Reduction

is define as

• the loss of oxygen from a substance
• the addition of hydrogen to a substance
• the process of electron gain in a substance
• a decrease in oxidation number of a substance

                     Redox reaction


are reaction involving oxidation and reduction

Electrochemical

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Electrochemical

Electrochemical processes are oxidation-reduction reactions in which:

•the energy released by a spontaneous reaction is converted to electricity or

•electrical energy is used to cause a nonspontaneous reaction to occur

  

Oxidation half-reaction (lose e-)

 

          Reduction half-reaction (gain e-)