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Combustion of Fossil Fuels Teacher Sheet

Combustion of Fossil Fuels Teacher Sheet

Part 1. The Combustion of Fossil Fuels
http://chemistry.elmhurst.edu/vchembook/511natgascombust.html

What do we mean that a hydrocarbon can go through combustion? What are we really doing to the hydrocarbon?
We are burning it.

Why do you think hydrocarbons make good fuels?
They produce a lot of energy when combusted.

Write the chemical reaction for the combustion of methane below.
CH4 + 2O2 à CO2 +2H2O

How does the combustion of methane compare to the combustion of other hydrocarbon fossil fuels, like coal and oil?
The combustion of methane is cleaner, producing only water and carbon dioxide. The combustion of oil and coal is more complicated, and harmful air pollutants are produced.

Par II. Energy of Combustion
http://chemistry.elmhurst.edu/vchembook/512energycombust.html

What is the general chemical formula for the combustion of fossil fuels, or any hydrocarbons?
Fossil fuel plus oxygen produces carbon dioxide, water, and energy.

Approximately what percentage of the world’s energy needs is fulfilled by the combustion of fossil fuels?
Approximately 75%.

Combustion produces energy. Where does all this energy come from?
Energy is stored in the chemical bonds that hold the carbon and hydrogen atoms together. When the atoms are rearranged, the bonds are broken, releasing energy.

Based on the diagram for the energy used and produced in the combustion of methane, fill in the chart. Make sure to indicate whether the energy is positive or negative. Also indicate the units.

Step

Description of Combustion Step

Energy Released or Produced

1

How much energy is needed to break all the Carbon – Hydrogen bonds in one methane molecule?

+1644 kJ

2

How much energy is needed to break the Oxygen – Oxygen bonds in two oxygen molecules?

+988 kJ

3

How much energy is produced to make the Carbon – Oxygen bonds in two carbon dioxide molecules?

-1598 kJ

4

How much energy is produced to make the Hydrogen – Oxygen bonds in two water molecules?

-1836 kJ

What is the total net energy?

-802 kJ

 

Based on your calculations, is energy needed or produced to combust methane? How do you know this based on your calculation?
Energy is produced in the combustion of methane. We know this because the total net energy is negative, indicating that energy is produced, not needed.What does the term “endothermic” mean?
It means energy is required.

In the steps above, which ones are endothermic?
Steps 1 and 2.

What does the term “exothermic” mean?
It means energy or heat is produced.

In the steps above, which ones are exothermic?
Steps 3 and 4.

Overall, is the combustion of methane endothermic or exothermic?
It is exothermic.

Part III. Combustion Energy of Ethane
Different hydrocarbons produce different amounts of energy when combusted, or burned. As we know from above, the combustion of methane produces 802 kJ of energy. How much energy is produced from the combustion of ethane? Use the bond energy chart found on the top of the online resource, Energy from Fossil Fuels (http://www.wou.edu/las/physci/GS361/Energy_From_Fossil_Fuels.htm).

What is the correct chemical reaction for the combustion of ethane?
2C2H6 + 7O2 à 4CO2 +6H2O

Determine the bond energies in each step of the combustion of ethane. Two steps are provided for you as examples.

Step 1:

Breaking of C-H bonds in ethane

Bond energy of one C-H bond

Number of bonds in one molecule

Total energy needed for one molecule

Number of molecules

Total energy needed for all molecules

410 kJ/mole

6

2460 kJ/mole

2

+4920 kJ/mole

 

Step 2:

Breaking of C-C bonds in ethane

Bond energy of one C-C bond

Number of bonds in one molecule

Total energy needed for one molecule

Number of molecules

Total energy needed for all molecules

347 kJ/mole

1

347 kJ/mole

1

+347 kJ/mole

 

Step 3:

Breaking of O=O bonds in oxygen

Bond energy of one O=O bond

Number of bonds in one molecule

Total energy needed for one molecule

Number of molecules

Total energy needed for all molecules

494 kJ/mole

1

494 kJ/mole

7

+3458 kJ/mole

 

Step 4:

Making of C=O bonds in carbon dioxide

Bond energy of one C=O bond

Number of bonds in one molecule

Total energy needed for one molecule

Number of molecules

Total energy produced for all molecules

799 kJ/mole

2

1598 kJ/mole

4

- 6392 kJ/mole

 

Step 5:

Making of O-H bonds in water

Bond energy of one O-H bond

Number of bonds in one molecule

Total energy needed for one molecule

Number of molecules

Total energy produced for all molecules

460 kJ/mole

2

920 kJ/mole

6

- 5520 kJ/mole

 What is the total combustion energy for ethane?
It is - 3187 kJ/mole.         

Is energy needed for this reaction or is it produced?
It is produced.

This teacher sheet is a part of the Chemistry of Petroleum 2: What Happens to Hydrocarbons When They Burn? lesson.

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