Which statement best defines the standard electrode potential E° and its relation to Gibbs free energy change for a redox couple?

• A. E° is the potential of a half-reaction written as a reduction under standard conditions (1 M, 1 atm, 25°C). It relates to Gibbs free energy by ΔG° = -nF E°.
• B. E° is the potential of a full cell reaction under standard conditions.
• C. E° equals ΔG°/(nF).
• D. E° is the sum of oxidation numbers in the reaction.

Answer: (A)

Standard electrode potential E° measures the tendency of a redox couple to gain electrons when the half-reaction is written as a reduction under standard conditions (activities 1 for solutes, 1 atm for gases, 25°C). It serves as the thermodynamic driving force for that redox step. The link to Gibbs free energy is given by ΔG° = -n F E°, where n is the number of electrons transferred and F is Faraday’s constant. This means a positive E° corresponds to a negative ΔG°, indicating the reduction is favorable under standard conditions. The potential described here is for a half-reaction, not a full cell reaction, so the standard cell potential comes from the difference between two half-reactions. The relationship is not ΔG°/(nF); it is ΔG° = -n F E°. And E° has no relation to the sum of oxidation numbers.