Electromotive Force
Definition and meaning of Electromotive Force in chemistry.
Electromotive force, or EMF, is the maximum voltage a galvanic cell can produce, measured when no current is flowing through the circuit. It represents the full driving force pushing electrons from the anode to the cathode. EMF reflects the cell's total thermodynamic potential before any energy is lost.
In more detail
EMF arises from the difference between the reduction potentials of a cell's two half-reactions. It captures the complete driving force of the reaction before any energy is lost to internal resistance. Scientists measure EMF with a potentiometer or a high-impedance voltmeter, an instrument that draws almost no current.
This matters because any current flow causes a voltage drop across the cell's internal resistance, which would make the measured voltage lower than the true EMF. Students sometimes confuse EMF with the voltage read off a battery under load, but the two are only equal when no current flows.
Under standard conditions, the standard EMF equals the standard reduction potential of the cathode minus that of the anode. EMF also connects directly to thermodynamics through the equation delta G standard equals negative nFE standard, linking a cell's voltage to whether its reaction is spontaneous and to its equilibrium constant.
A cell with a positive EMF has a negative Gibbs free energy change, meaning the reaction proceeds spontaneously as written.
Key facts
| Symbol | E or ε |
|---|---|
| SI Unit | volt (V) |
| Key Relation | ΔG° = −nFE°cell |
| Field | Physical Chemistry |
| Measurement condition | Zero current (open circuit) |
| Example value | 1.10 V for the standard Daniell cell |
For the Daniell cell, written as Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s), the standard EMF equals the standard reduction potential of copper minus that of zinc. Using the standard reduction potential of copper(II) ion, 0.34 V, and zinc(II) ion, -0.76 V, gives a standard EMF of 1.10 V. This positive value confirms that zinc spontaneously reduces copper ions when the two half-cells are connected.
Frequently asked questions
Is EMF the same as terminal voltage?
No. EMF is the theoretical maximum, measured at zero current. The terminal voltage under load is lower, V = E minus Ir, because of the cell's internal resistance.
How is EMF measured experimentally?
With a potentiometer or a high-impedance voltmeter that draws negligible current, avoiding the voltage drop that would otherwise reduce the measured value.
What does a positive EMF tell you about a reaction?
A positive EMF means the cell reaction is spontaneous as written, which corresponds to a negative Gibbs free energy change.