Wednesday, December 22, 2010

Cell Potential

CELL POTENTIAL (Ecell)

  1. Difference in electrical potential of electrodes.
  2.  Also called voltage or electromotive force (emf).
  • Cell emf depends on the concentration of ions in the cell, the temperature of solutions and pressure of gasses involved.
·         Unit : Volt (V).

Ecell > 0
Ecell = 0
Ecell < 0
Spontaneous cell reaction.
Reaction has reach equilibrium.
Non spontaneous cell reaction.

STANDARD CELL POTENTIAL (EOcell)
  1. Difference in electrical potential of electrodes measured at specified temperature (usually 298 K) with all components in their standard state which are :
  •  1 atm (for gases)
  •  1M for solutions
  • Pure solid for electrodes 
 2. The Eocell can be calculated using the equation:

              Eocell = Eocathode - Eoanode

    ·       STANDARD ELECTRODE (HALF-CELL) POTENTIAL (EOhalf-cell)
    1. Potential associated with a given half reaction (electrode compartment) when all components are in their standard states.
    2. Also called standard reduction potential.
    3. Changing the balancing coefficients of a half-reaction does not change Eo value because electrode potentials are intensive properties.
    4. Eg. :

                        Oxidation (anode)     :      Zn(s) → Zn2+(aq) + 2e-          Eoanode = -0.76 V
                        Reduction (cathode)  :     Cu2+(aq) + 2e- → Cu(s)         Eocathode = +0.34 V

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
                       Overall                      :     Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)

    Eocell = Eocathode - Eoanode
            = Eocopper - Eozinc
            = +0.34 V – (-0.76 V)
            = +1.10 V

    STANDARD HYDROGEN ELECTRODE (SHE)

    1. SHE is the standard reference electrode to measure the electrode potentials of other half-cell.
    2. SHE is assign as zero potential (0 V)SHE consists of H2 gas at 1 atm,25°C bubbling around a platinum electrode which is immersed in 1 M of solution H+.
    3.  The electric potential of the half reaction is due to the reduction of H+ to H2 (g).

                   2H+ (aq) +2e-→H2 (g)      E° =0.00V

    STANDARD REDUCTION POTENTIAL TABLE

    From the table,
    1. The element with more +ve E° becomes cathode.
    2. The element with more -ve E° becomes anode.
    3. The more +ve Eo favours reaction to the right.
    4. The more -ve Eo favours reaction to the left.
    5. The more +ve E° , the stronger the oxidising agent.
    6. The more -ve E° , the stronger the reducing agent.

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