reduction potential
Summary
Reduction potential (E°red) is a measure, in volts, of how strongly a species tends to gain electrons, compared with the standard hydrogen electrode (0 V). The more positive it is, the stronger the oxidizing agent. In a galvanic cell, the half-cell with the higher value is the cathode.
Detail
Meaning: Reduction potential is the tendency of a species to gain electrons, written as a reduction half-reaction. Standard values (1 M solutions, 1 atm gases, 25 °C) are measured relative to the standard hydrogen electrode, defined as 0 V. The more positive the value, the more readily the species is reduced, so it is a stronger oxidizing agent and its reduced form is a weaker reducing agent.
Equation: E°cell = E°cathode − E°anode, and ΔG° = −nFE°cell, where E is in volts (V), n is moles of electrons transferred (mol), F is the Faraday constant 96,485 C/mol, and ΔG° is in J/mol.
On the MCAT: A passage gives a table of standard reduction potentials. You pick which species is reduced, rank oxidizing or reducing agents, calculate E°cell, and judge spontaneity. E°cell > 0 means ΔG° < 0, which describes a galvanic cell. E°cell < 0 describes an electrolytic cell, which needs an external energy source.
Common confusion: Students multiply E° by the stoichiometric coefficient when balancing electrons. E° is an intensive property, so it does not change when you scale a half-reaction. Reversing a half-reaction to show oxidation flips the sign of its potential. Also, "more positive" means a greater tendency to be reduced, not a larger magnitude: −0.76 V is a weaker oxidizing agent than −0.14 V.
Sources
- Chemistry: The Central Science
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.