The standard Gibbs energy change of the reaction forming liquid water from hydrogen and oxygen, H₂ + 1/2 O₂ → H₂O(l), is −237 kJ/mol at 25 °C. Which value is closest to the theoretical emf of a fuel cell using this reaction? Take the Faraday constant as 96485 C/mol.

Answer: B. About 1.23 V

The theoretical emf is E = −ΔG/(nF), and when 1 mol of H₂ reacts, 2 mol of electrons are transferred, so n = 2. E = 237×10³ / (2 × 96485) ≈ 1.23 V. About 2.46 V takes n = 1, and about 0.61 V combines the number of electrons per mole of O₂, n = 4, with ΔG per mole of H₂. The ratio of the Gibbs energy change to the enthalpy change of the reaction (−286 kJ/mol), 237/286 ≈ 83%, is a guide to the theoretical efficiency at 25 °C. In real cells, electrode overpotentials and electrolyte resistance lower the operating voltage to about 0.6–0.8 V.