Heat of Combustion of Fuels


The objective of the exercise is to calculate the energy density of several hydrocarbon fuels and alcohol substitutes.


Concerns about finite petroleum reserves have led to a search for renewable energy sources to supplement or replace traditional hydrocarbon fuels. Ethanol has been proposed as an alternative fuel source for automobiles because it can be prepared from fermentation of almost any plant product. In some areas, "gasohol" consisting of 90% gasoline and 10% ethanol is already in use.

The heat of combustion DcH for a fuel is defined as enthalpy change for the following reaction when balances:

Fuel + O2 (g) ® CO2 (g) + H2O(l)

Balance the combustion reaction for each fuel below.

octane: C8H18 + 12.5 O2 ® 8 CO2 + 9 H2O
butane: C4H10 + 6.5 O2 ® 4 CO2 + 5 H2O
butanol: C4H9OH + 6 O2 ® 4 CO2 + 5 H2O
ethanol: C2H5OH + 3 O2 ® 2 CO2 + 3 H2O

Heats of combustion can be calculated from heats of formation DfH. Use PM3 to calculate DfH for octane, butane, butanol, and ethanol. Fill in the first blank column on the following table.

Fuel PM3 DfH Mass % oxygen DcH
n-C8H18 -212.9 0 -5508.9 -48.23 -33.76
n-C4H10 -121.9 0 -2881.9 -49.59 -28.26
C4H9OH -290.9 21.6 -2712.9 -36.60 -29.65
C2H5OH -238.0 34.8 -1406.8 -30.54 -24.12

Use the NIST Chemistry WebBook to look up literature DfH values for O2, CO2, and H2O. Use one of the many online MSDS archives (Cornell, University of Vermont) to obtain density (denoted as specific gravity in a MSDS) values for the four fuels. Then use this information to complete the remaining four blank columns of the above table.

Species DfH
O2 (g) 0.0
CO2 (g) -393.7
H2O (l) -285.8
Species Molar Mass
C8H18 114.22 0.70
C4H10 58.12 0.57
C4H9OH 74.12 0.81
C2H5OH 46.07 0.79

Explain the observed trend for the computed values of kJ/mL.

We can see that, in general, he heat of combustion (kJ/g) is greater for the hydrocarbons than for the alcohols. This can be attributed to the fact that the alcohols are already in a partially oxidized state, while the hydrocarbons are not at all oxidized. The densities of the species also play a significant role in determining the heats of combustion in kJ/mL. Since the alcohols are significantly more dense than the hydrocarbons, this slightly increases their "bang" per mL. Ethanol, the most popular additive, has a energy density about 3/4 that of octane.

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