The combustion of liquid ethanol is represented (in the gas phase for all species) by
C2H5OH(g) + 3 O2(g) → 2 CO2(g) + 3 H2O(g)
Average bond energies (298 K):
• C–C 348 kJ mol⁻¹
• C–H 413 kJ mol⁻¹
• C–O 358 kJ mol⁻¹
• O–H 467 kJ mol⁻¹
• O=O 498 kJ mol⁻¹
• C=O (in CO2) 799 kJ mol⁻¹
• O–H (in H2O) 463 kJ mol⁻¹
An experimental value for the molar enthalpy of combustion of ethanol under these conditions is –1367 kJ mol⁻¹.
Answer the following questions, showing all work.
(a) Identify and tally the total number of each specific bond broken in the reactants and each bond formed in the products for one mole of reaction as written.
(b) Using the bond energies provided, calculate ΔH for the combustion reaction.
(c) Calculate the percent difference between your calculated ΔH and the experimental value given.
(d) Provide one well–reasoned explanation for the discrepancy between the value obtained from average bond energies and the experimental enthalpy of combustion.