Heat of Combustion of Fuels

Objective

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

Procedure

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 + O2 ® CO2 + H2O
butane: C4H10 + O2 ® CO2 + H2O
butanol: C4H9OH + O2 ® CO2 + H2O
ethanol: C2H5OH + O2 ® CO2 + 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
(kJ/mol)
DcH
(kJ/gram)
DcH
(kJ/mL)
n-C8H18          
n-C4H10          
C4H9OH          
C2H5OH          

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
(kJ/mol)
O2 (g)  
CO2 (g)  
H2O (l)  
Species Molar Mass
(g/mol)
Density
(g/mL)
C8H18    
C4H10    
C4H9OH    
C2H5OH    

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

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