emf
Summary
Emf (electromotive force) is the voltage a cell can produce when no net current flows. It is energy per unit charge, not a force. A positive E°cell = E°cathode − E°anode means a spontaneous galvanic cell, and ΔG = −nFE links emf to free energy.
Detail
Meaning: Emf (electromotive force) is the potential difference a cell can deliver, in volts, when no net current flows. It is not a force: 1 V = 1 J/C. Find it from reduction potentials: E°cell = E°cathode − E°anode, with reduction at the cathode. A positive emf means a spontaneous (galvanic) cell; a negative emf means nonspontaneous (electrolytic).
Equation: ΔG = −nFE, where ΔG is the free energy change (J/mol), n is the moles of electrons transferred (mol), F is the Faraday constant, about 96,500 C/mol, and E is the emf (V).
On the MCAT: Passages give a table of standard reduction potentials. You pick the pair with the largest emf, find the sign of E°cell, or predict spontaneity. The Nernst equation, E = E° − (RT/nF) ln Q, predicts that raising reactant concentration raises emf and raising product concentration lowers it. At equilibrium E = 0 and Q = Keq.
Common confusion: Do not multiply E° by coefficients when you scale a balanced reaction. Potential is energy per charge, so it stays the same, while ΔG scales with n. Also, emf is the ideal voltage with no current drawn. The voltage measured across a working cell is lower because of internal resistance.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
- Chemistry: The Molecular Nature of Matter and Change
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.