Q. Standard free energies of formation (in kJ/mol) at 298 K are −237.2, −394.4 and −8.2 for H₂O(l), CO₂(g) and pentane(g) respectively. The value of E°cell for the pentane-oxygen fuel cell is
A) 0.0968 V
B) 1.968 V
C) 2.0968 V
D) 1.0968 V
Explanation
[Solution from source] C₅H₁₂ + 8O₂ → 5CO₂ + 6H₂O. ∆G° = 5(−394.4) + 6(−237.2) − (−8.2) = −3387.0 kJ. Eight O₂ molecules take 4 electrons each, so n = 32, and E° = −∆G°/nF = 3387000/(32 × 96500) ≈ 1.0968 V. Using a smaller n (for example 16) would double the voltage to about 2.19 V, which is not offered. Answer (4).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.