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

a. Balanced equations

2C8H18 + 25O2 ---> 16CO2 + 18H2O

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

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

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

C2H5OH + 3O2 ---> 2CO2 + 3H2O

2. Heat released by combustion of,

(a) C8H18

dH = (16 x -393.5 + 18 x -241.8) - (2 x -208.4) = -10231.6 kJ/mol

(b) C12H26

dH = (24 x -393.5 + 26 x -241.8) - (2 x -291.0) = -15148.8 kJ/mol

(c) CH3OH

dH = (2 x -393.5 + 4 x -241.8) - (2 x -238.4) = -1277.4 kJ/mol

(d) CH3NO2

dH = (4 x -393.5 + 6x -241.8) - (4 x -401.6) = -1418.4 kJ/mol

(e) C2H5OH

dH = (2 x -393.5 + 3 x -241.8) - (-277) = -1235.4 kJ/mol

3. For 15 gallons = 56.782 L of fuel

(a) C8H18 moles = 56.782 x 0.7/114.23 = 0.348 mols

energy released = -3560.18 kJ/mol

(b) C12H26 moles = 56.782 x 0.8/170.34 = 0.267 mols

energy released = -4039.82 kJ/mol

(c) CH3OH moles = 56.782 x 1.14/61.04 = 1.06 mols

energy released = -1354 kJ/mol

(d) CH3NO2 moles = 56.782 x 1.14/61.04 = 1.06 mols

energy released = -1504.18 kJ/mol

(e) C2H5OH moles = 56.782 x 0.79/40.07 = 1.12 mols

energy released = -1588 kJ/mol

4. C12H26 gives highest amount of heat for combustion

5. Addition of ethanol would increase the gas mileage