Dehydrogenating ethane in the gas phase,
C2H6(g) → C2H4(g) + H2(g), ΔH = +136 kJ mol−1,
is highly endothermic. An engineer considers coupling this step with one of the following hydrogen reactions carried out simultaneously (all species in the gas phase):
Option A H2 + ½ O2 → H2O
Option B H2 + Cl2 → 2 HCl
Option C H2 + I2 → 2 HI
Using the average bond energies listed, calculate the overall ΔH for each combined pathway (dehydrogenation plus the chosen option) and indicate which option brings the net enthalpy change closest to zero.
Average bond energies (kJ mol−1): H–H 436, O=O 498, Cl–Cl 243, I–I 151, O–H 463, H–Cl 431, H–I 297