Module 3  

Lesson 4.2  Considerations when Comparing Fuels



Key Concepts


When comparing fuels, it is important to consider the issue from various perspectives. For example, a given fuel may be the most energy efficient (i.e. have the highest energy content per gram) however that same fuel may not necessarily be the best choice for the environment (Refer back to the section entitled "Combustion Reactions" in Lesson 2 of Module 2 to review the environmental impact of fossil fuels).

Other questions must also be asked when assessing fuels, such as:

  • Is the fuel toxic?
  • Is the fuel unstable?
  • Is the fuel readily available?
  • Is the fuel economically viable?

Check Your Understanding


Methane (molar mass = 16.05 g/mol)

\( \mathrm { CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g) } \)
\( \mathrm { \Delta_cH = -802.5~kJ } \)

Propane (molar mass = 44.11 g/mol)

\( \mathrm { C_3H_8(g) + 5O_2(g) \rightarrow 3CO_2(g) + 4H_2O(g) } \)
\( \mathrm { \Delta_cH = -2043.9~kJ } \)

Octane (molar mass = 114.26 g/mol)

\( \mathrm { C_8H_{18}(g) + \frac{25}{2}O_2(g) \rightarrow 8CO_2(g) + 9H_2O(g) } \)
\( \mathrm { \Delta_cH = -5074.1~kJ } \)

Self-Check Question 1. Rank the above fuels from lowest to highest molar enthalpy of combustion.

Self-Check Question 2. Is there a trend relating molar enthalpy of combustion and molar mass of the fuels?

Self-Check Question 3. Rank the three fuels from lowest to highest heat of combustion as expressed in kJ/g.

Self-Check Question 4. Is there an advantage of communicating the energy associated with combustion of a hydrocarbon using kJ/g rather than kJ/mol?

Self-Check Question 5. Another factor that is of high importance when comparing fuels is cleanliness of combustion. For each fuel, calculate the enthalpy change per mole of carbon dioxide produced. (Recall that the production of carbon dioxide has an influence on global warming and global climate change)

Click on the link below to check your answers.

Self-Check Question 1.

Methane, propane, octane


Self-Check Question 2.
Hydrocarbon Molar enthalpy of combustion
(kJ/mol) 		Molar mass
(g/mol)
Methane, CH4 -802.5 16.05
Propane, C3H8 -2043.9 44.11
Octane, C8H18 -5074.1 114.26

Trend: As molar mass of a hydrocarbon increases, so does its molar enthalpy of combustion


Self-Check Question 3.

To convert from kJ/mol to kJ/g, divide by the molar mass.

Methane -802.5 kJ/mol x 1 mol/16.05 g = - 50.00 kJ/g

Propane -2043.9 kJ/mol x 1 mol/44.11 g = - 46.34 kJ/g

Octane -5074.1 kJ/mol x 1 mol/114.26 g = - 44.408 kJ/g

Hydrocarbon Enthalpy of combustion (kJ/g)
Octane -44.41 
Propane -46.34 
Methane -50.00 

Note that the order is reversed.


Self-Check Question 4.

Fuels are usually not sold or distributed by the mole. Volume or mass of a fuel are directly measurable. An energy value per unit volume or mass can easily be used to predict net energy output from a measured volume or mass.


Self-Check Question 5.

Methane

-802.5 kJ/1 mol of CO2 = -802.5 kJ per mole of CO2(g) produced

Propane

-2043.9 kJ/3 mol of CO2 = - 681.3 kJ per mole of CO2(g) produced

Octane

-5074.1 kJ/8 mol of CO2 = - 634.3 kJ per mole of CO2(g) produced

In other words, methane releases the most energy per mole of carbon dioxide produced whereas octane releases the least. So from an environmental aspect, burning methane is preferable to burning octane.

Several other factors can also be examined to determine the "best fuel" such as price and availability. Can you think of other factors?