The electrode potential in volts of 2H$^+$(aq) / H$_2$(q) system is

Analyze the question: It asks for the standard electrode potential of the 2H⁺(aq)/H₂(g) system, which is the hydrogen electrode.

Concept: Electrode potentials cannot be measured in isolation. They are measured relative to a reference, and by international convention the standard hydrogen electrode (SHE) is the reference. It is assigned a potential of exactly 0.00 V under standard conditions:

• Temperature: 298 K (25°C)
• Pressure of H₂ gas: 1 atm
• Concentration of H⁺: 1 mol/dm³
• Electrode: platinum coated with platinum black

Half-equation: 2H⁺(aq) + 2e⁻ ⇌ H₂(g) E° = 0.00 V

Checking the other options:

• A (-2.87 V) is close to the value for F₂/F⁻ in magnitude (+2.87 V), but with the wrong sign, so it is not hydrogen.
• B (-2.71 V) is the value for Na⁺/Na.
• C (0.8 V) is close to Ag⁺/Ag (+0.80 V).
• D (1.68 V) is close to PbO₂/Pb²⁺ (about +1.69 V).

None of these is the hydrogen system. The answer is E, 0 V.

Exam tip: Whenever you see "hydrogen electrode" or "2H⁺/H₂", think zero. It is the reference point for the whole electrochemical series. Metals above hydrogen in the series have negative E° values, and those below it have positive values.