What is the standard reduction potential for the hydrogen half-reaction 2H+ + 2e- ⟶ H2 under standard conditions?

• A. -0.23 V
• B. 0.00 V
• C. +0.34 V
• D. +1.23 V

Answer: (B)

Standard reduction potentials are defined relative to a fixed reference, and that reference is the standard hydrogen electrode. This electrode is assigned a value of 0 volts under standard conditions (1 M H+, 1 atm H2, 25°C). For the hydrogen half-reaction 2H+ + 2e- → H2, this convention sets its standard reduction potential to 0 V. This zero point lets you compare other redox couples directly: any other couple’s E° is given as a voltage relative to this hydrogen reference. The other numbers correspond to different reactions (for example, +1.23 V is the O2/H2O couple under acidic conditions), not the hydrogen couple itself.