For the reaction 2H+ + 2e− → H2, what is E° at 25°C?

• A. 0.00 V
• B. +0.50 V
• C. -0.41 V
• D. +0.00 V

Answer: (A)

The key idea is that the standard hydrogen electrode is the reference for electrode potentials, and its potential is defined to be zero at 25°C when the activities are H+ = 1 and H2 = 1 atm. The half-reaction shown, 2H+ + 2e− → H2, is exactly the reaction used to define that reference. Therefore its standard potential E° is 0.00 V. This is a convention, so all other electrode potentials are measured relative to this value. If conditions change, the measured potential would shift according to the Nernst equation, but under standard conditions it sits at zero. The numerical values other than zero do not apply to this couple under standard conditions because they would correspond to different references or nonstandard states.