Cell Potential Or Emf Of The Cell
Category : JEE Main & Advanced
(1) “The difference in potentials of the two half – cells of a cell known as electromotive force (emf) of the cell or cell potential.”
The difference in potentials of the two half – cells of a cell arises due to the flow of electrons from anode to cathode and flow of current from cathode to anode.
\[Anode\underset{Flow\,\,of\,\,current}{\overset{Flow\,\,of\,\,electrons}{\longleftrightarrow}}Cathode\]
(2) The emf of the cell or cell potential can be calculated from the values of electrode potentials of two the half – cells constituting the cell. The following three methods are in use :
(i) When oxidation potential of anode and reduction potential of cathode are taken into account
\[E_{\text{cell}}^{0}=\] Oxidation potential of anode + Reduction potential of cathode \[=E_{\text{ox}}^{0}\,(\text{anode})+E_{\text{red}}^{\text{0}}(\text{cathode})\]
(ii) When reduction potentials of both electrodes are taken into account
\[=E_{\text{Cathode}}^{\text{0}}-E_{\text{Anode}}^{\text{0}}\]\[=E_{\text{right}}^{\text{o}}-E_{\text{left}}^{o}\]
(iii) When oxidation potentials of both electrodes are taken into account
\[E_{\text{cell}}^{o}=\] Oxidation potential of anode – Oxidation potential of cathode \[=E_{\text{ox}}^{0}(\text{anode})-E_{\text{ox}}^{0}(\text{cathode})\]
(3) Difference between emf and potential difference
Emf | Potential difference |
It is the potential difference between two electrodes when no current is flowing in the circuit. | It is the difference of the electrode potentials of the two electrodes when the cell is under operation. |
It is the maximum voltage that the cell can deliver. | It is always less then the maximum value of voltage which the cell can deliver. |
It is responsible for the steady flow of current in the cell. | It is not responsible for the steady flow of current in the cell. |
(4) Cell EMF and the spontaneity of the reaction : We know, \[\Delta G=-nF{{E}_{cell}}\]
Nature of reaction | \[\Delta \mathbf{G(or}\,\Delta {{\mathbf{G}}^{\mathbf{o}}}\mathbf{)}\] | \[{{\mathbf{E}}_{\mathbf{cell}}}\mathbf{(or}\,\mathbf{E}_{\mathbf{cell}}^{\mathbf{o}}\mathbf{)}\] |
Spontaneous | - | + |
Equilibrium | 0 | 0 |
Non - spontaneous | + | - |
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