An energy company is evaluating three candidate liquid fuels—ethanol, 1-propanol, and 1-butanol—for use in a new line of portable camping stoves. The complete combustion of each fuel in excess oxygen at standard conditions is represented below.
1. C2H5OH(l) + 3 O2(g) → 2 CO2(g) + 3 H2O(l)
2. C3H7OH(l) + 9⁄2 O2(g) → 3 CO2(g) + 4 H2O(l)
3. C4H9OH(l) + 6 O2(g) → 4 CO2(g) + 5 H2O(l)
A selection of standard enthalpies of formation, ΔHf∘, is provided.
Species ΔHf∘ (kJ·mol⁻¹)
C2H5OH(l) −277.0
C3H7OH(l) −302.0
C4H9OH(l) −328.0
CO2(g) −393.5
H2O(l) −285.8
O2(g) 0.0
(a) Calculate the standard enthalpy change, ΔHrxn∘, for each combustion reaction listed above. Show all work, including proper use of the data table and units.
(b) For each fuel, determine the amount of heat released per gram of fuel burned. Express each result in kJ·g⁻¹ with three significant figures.
(c) Rank the three fuels from greatest to least in terms of heat released per gram. Justify your ranking using your results from part (b).
(d) Ethanol has a lower molar enthalpy of combustion than 1-butanol yet is often preferred as a biofuel. Propose TWO chemical or practical factors (other than enthalpy of combustion) that might explain this choice, and briefly explain how each factor is relevant.