Ethanol biofuel is produced by fermenting glucose and then distilling the resulting solution. A researcher wants to compare the thermal energy that can be extracted by combusting ethanol with the thermal energy released directly by aerobic respiration of glucose.
Relevant sequential transformations at 298 K and 1 bar:
Step
Chemical transformation
ΔH (kJ mol⁻¹)
1
C₆H₁₂O₆(aq) → 2 C₂H₅OH(aq) + 2 CO₂(g)
–67.0
2
C₂H₅OH(aq) → C₂H₅OH(l)
–42.3
3
C₂H₅OH(l) + 3 O₂(g) → 2 CO₂(g) + 3 H₂O(l)
–1366.8
For comparison, the direct aerobic oxidation of glucose is simply
C₆H₁₂O₆(aq) + 6 O₂(g) → 6 CO₂(g) + 6 H₂O(l) ΔH = –2801.0 kJ mol⁻¹.
Assume that the aqueous phase is dilute so that solutions can be treated as ideal.
(a) Combine the sequential transformations (Steps 1–3) to obtain one overall chemical equation relating 1 mol of glucose to the final combustion products. Show that the stoichiometry of the combined pathway matches that of direct aerobic oxidation.
(b) Calculate the net enthalpy change (in kJ mol⁻¹ glucose) for the two-stage pathway (fermentation followed by ethanol combustion).
(c) Determine the percentage difference between the energy obtained from the two-stage pathway and the energy obtained from direct aerobic oxidation.
(d) Suggest one thermodynamic and one practical factor that could explain why industrial biofuel production proceeds through the two-stage pathway even though it is not the most exothermic route.