Electromotive Force
The electrochemical cell consists of two half cells. The electrodes in these half cells have different electrode potentials. When the circuit is completed the loss of electrons occurs at the electrode having lower reduction potential whereas the gain of electrons occurs at the electrode with higher reduction potential.
The
difference in the electrode potentials of the two electrodes of the cell is
termed as electromotive force (abbreviated as EMF) or cell voltage (Ecell).
The
EMF of the cell at the standard state conditions is called standard EMF. The
Emf of the cell or cell potential can be calculated from the values of
electrode potentials of the two half-cells constituting the cell
Ø When oxidation potential of
anode and reduction potential of cathode are taken into account:
ECello =
Oxidation potential of anode + Reduction potential of cathode
ECello
= Eoxo (anode) + Eredo (cathode)
Ø When reduction potentials of
both electrodes are taken into account:
ECello =
Reduction potential of cathode - Reduction potential of anode
ECello =
ECathodeo - EAnodeo
Consider
the following example: If standard potential for the following reactions is
given:
A2+ + 2e– → A
E0 =
+2.5 V
B2+ + 2e– → B
E0 =
– 1.0V
C2+ + 2e– → C
E0 =
+1.5 V
We
can deduce from the above values that A2+ has the maximum tendency
to get reduced as it has the most positive reduction potential and the lowest
tendency to get reduced is shown by B2+ in the above three cases.
In
an electrochemical cell the electrode with higher electrode potential (Eº) acts
as cathode while that with lower electrode potential will act as anode.