Question

Consider four possible liquid fuels used in vehicles.

• C8H18 Octane - gasoline

• C12H26 Dodecane - diesel

• CH3OH ethanol - gasoline additive

• CH3NO2 nitromethane - racing fuel

Determine the heat released upon combustion with O2 for 15 gallons of each fuel.

Hint: Perform the following.

1. Balance the combustion reaction for each fuel. The products are CO2(g), H2O(g); for nitromethane, N produces N2(g) as one of the products.

2. Determine the enthalpy of each combustion reaction.

3. Determine the enthalpy of combustion for 15 gallons of fuel reactant.

4. Which fuel gives the most heat upon combustion?

5. Does adding ethanol to gasoline increase or decrease gas mileage?

Answer 1

1. Balanced equations,

C8H18 : 2C8H18 + 25O2 ---> 16CO2 + 18H20

C12H26 : 2C12H26 + 37O2 ---> 24CO2 + 26H2O

CH3OH : 2CH3OH + 3O2 ---> 2CO2 + 4H2O

CH3NO2 : 4CH3NO2 + 3O2 ---> 4CO2 + 6H2O + 2N2

2. Enthalpy of each combustion reaction,

C8H18

dHrxn = (16 x -393.5 + 18 x -241.82) - (2 x -208.4) = -10231.96 kJ/mol

C12H26

dHrxn = (24 x -393.5 + 26 x -241.82) - (2 x -353) = -15024.8 kJ/mol

CH3OH

dHrxn = (2 x -393.5 + 4 x -241.82) - (2 x -201) = -1352.28 kJ/mol

CH3NO2

dHrxn = (4 x -393.5 + 6 x -241.82) - (4 x -81) = -2700.92 kJ/mol

3. Enthalpy of combustion for 15 gallon = 56.7812 L fuel

moles of C8H18 = 56.78 x 0.7495 x 1000/114.23 = 372.55 mols

dHcomb = -10231.96 kJ/mol x 372.55 = -3.81 x 10^6 kJ

moles of C12H26 = 56.78 x 0.7495 x 1000/170.33= 249.85 moles

dHcomb = -15024.8 kJ/mol x 249.85 = -3.75 x 10^6 kJ

moles of CH3OH = 56.78 x 0.792 x 1000/32.04 = 1403.55 mols

dHcomb = -1352.28 kJ/mol x 1403.55 = -1.90 x 10^6 kJ

moles of CH3NO2 = 56.78 x 1.14 x 1000/61.04 = 1060.44 mols

dHcomb = -2700.92 kJ/mol x 1060.44 = -2.86 x 10^6 kJ

4. C8H18 gives the highest amount of heat upon combustion

5. Adding ethanol would increase the fuel value and gas mileage.