Cell Potential

CELL POTENTIAL (E_cell)

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 (E^O_cell)

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 E^o_cell can be calculated using the equation:
              E^o_cell = E^o_cathode - E^o_anode

·       STANDARD ELECTRODE (HALF-CELL) POTENTIAL (E^O_half-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 E^o value because electrode potentials are intensive properties.
4. Eg. :

                    Oxidation (anode)     :      Zn(s) → Zn²⁺(aq) + 2e^-          E^o_anode = -0.76 V

                    Reduction (cathode)  :     Cu²⁺(aq) + 2e^- → Cu(s)         E^o_cathode = +0.34 V

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           

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

E^o_cell = E^o_cathode - E^o_anode

        = E^o_copper - E^o_zinc

        = +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 E^o favours reaction to the right.
4. The more -ve E^o 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.