What is the standard reduction potential of the hydrogen couple H+/H2 at standard conditions?

• A. +0.50 V
• B. 0.00 V
• C. -0.50 V
• D. +1.00 V

Answer: (B)

The main idea is that this value is defined as the reference point for all electrode potentials. The hydrogen couple involves the reduction of protons to hydrogen gas: 2H+ + 2e- → H2. Under standard conditions—1 M H+, 1 atm H2, and 25°C—the standard reduction potential is set to zero by convention. This makes the hydrogen electrode the zero-energy benchmark, so other half-reactions are measured relative to this point. The Nernst equation shows that changing conditions away from the standard state will shift the observed potential, but at standard conditions the hydrogen couple has E° = 0.00 V. That’s why the correct value is zero; the other numbers would imply a non-standard state or a different reference.