Chemistry of Petroleum 2: What Happens to Hydrocarbons When They Burn?

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Chemistry of Petroleum 2: What Happens to Hydrocarbons When They Burn?


To introduce students to the energy transformations involved in the burning of hydrocarbons.


This lesson is part of the Energy in a High-Tech World Project, which examines the science behind energy. Energy in a High-Tech World is developed by AAAS and funded by the American Petroleum Institute. For more lessons, activities, and interactives that take a closer look at the science behind energy, be sure to check out the Energy in a High-Tech World Project page.

This lesson is the second in a series of four lessons about the chemistry of petroleum that are intended for upper-level chemistry students in the 11th and 12th grades. The goal of these lessons is to introduce high-school students to the use of oil as an energy source in today’s high-tech world. In the Chemistry of Petroleum 1: What are Hydrocarbons?, students explore hydrocarbons, the molecular basis of petroleum, and learn to distinguish between organic and inorganic compounds.

In the Chemistry of Petroleum 2: What Happens to Hydrocarbons When They Burn? students examine the varying amounts of energy produced by the combustion of different hydrocarbons.

In the Chemistry of Petroleum 3: Distillation of Hydrocarbons, students will be introduced to the distillation and treatment processes by which petroleum is refined to produce useful fuel oils.

The Chemistry of Petroleum 4: Treatment of Hydrocarbons will help students explore the chemical treatment processes by which distilled petroleum fractions are converted to produce useful fuel oils.

The energy produced by hydrocarbon combustion is directly relevant to our lives. Today, about 75% or more of the world’s energy needs are fulfilled by the combustion of fossil fuels, such as methane, coal, and petroleum. Today’s oil and gas industry utilizes the stored energy within fossil fuels’ hydrocarbons to drive our high-tech world. This is due to the laws of thermodynamics—although energy changes form, it is never lost or destroyed. Energy can take various forms, but each form can be measured in a way that makes it possible to keep track of how much of one form is converted into another. Whenever the amount of energy in one place or form diminishes, the amount in another place or form increases by an equivalent amount. Thus, if no energy leaks in or out across the boundaries of a system, the total energy of all the different forms in the system will not change, no matter what kinds of gradual or violent changes actually occur within the system. But energy does tend to leak across boundaries. In particular, transformations of energy usually result in producing some energy in the form of heat, which leaks away by radiation or conduction (such as from engines, electrical wires, hot-water tanks, our bodies, and stereo systems). (Science for All Americans, p. 50.)

Different energy levels are associated with different configurations of atoms in molecules. Some changes in configuration require additional energy, whereas other changes release energy. For example, heat energy has to be supplied to start a charcoal fire (by evaporating some carbon atoms away from others in the charcoal); however, when oxygen molecules combine with the carbon atoms into the lower-energy configuration of a carbon dioxide molecule, much more energy is released as heat and light. Or a chlorophyll molecule can be excited to a higher-energy configuration by sunlight; the chlorophyll in turn excites molecules of carbon dioxide and water so they can link, through several steps, into the higher-energy configuration of a molecule of sugar (plus some regenerated oxygen). Later, the sugar molecule may subsequently interact with oxygen to yield carbon dioxide and water molecules again, transferring the extra energy from sunlight to still other molecules. (Science for All Americans, p. 51.)

Energy conservation is a difficult concept for students to grasp because it is counter-intuitive to their everyday experiences with heat, sound, light, and other forms of energy. It is important, therefore, to go through each step of this lesson carefully so that students understand that the energy released from hydrocarbon combustion comes directly from the stored energies within the bonds. As a follow-up, the energy that is released from the combustion of hydrocarbons also is not lost; rather, it is used to make our cars run, radios produce sound, bulbs light, and so forth.

In order for students to do this lesson, as well as the other lessons in this series, they need to have prerequisite knowledge of the basics of atoms and their structure. Basic information about atoms can be found at The Atom. Students also should be comfortable drawing molecular structures and determining stoichiometrically correct chemical equations. Because organic chemistry usually concludes a year-long, high-school chemistry course, the lesson also includes some concepts of thermodynamics, such as bond energies, and endothermic and exothermic reactions. These bond energies incorporate units in joules and moles so students should be comfortable using calculations involving moles. Although the term “entropy” is not explicitly mentioned here, it is appropriate to incorporate the second law of thermodynamics into the lesson, particularly as it applies to bond energies. Most advanced chemistry textbooks and curricula include mathematical methods of evaluating the entropy change in a reaction, which in turn is used to calculate the amount of energy that is available for work from a reaction (i.e., Gibbs free energy).

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Before beginning the main portion of this lesson, it is important to determine any misconceptions that students may have about the necessity of oxygen to support combustion reactions. To figure out what students already know about fuel needing oxygen, light a small candle, such as a tea light, as a demonstration activity. Ask students: “What might happen to the candle if we cover it with a glass?” (Accept all ideas and hypotheses.) Place a small, clear drinking glass or cylinder over the tea candle and allow students to make observations. The flame will extinguish as all the oxygen is utilized.

Ask students:

  • Why did the flame extinguish?
    (In order to burn, oxygen is needed.)
  • Where is the oxygen?
    (It is in the air.)
  • When the glass was placed on top of the flame, it still burned for some time before extinguishing. Why?
    (All the oxygen in the air around the flame, within the glass, was utilized. Once that was completely used, there was no more oxygen and the flame extinguished.)

Re-cap for students that oxygen was used by the candle. Each time a substance combines with oxygen, oxidation occurs. Sometimes, substances react with oxygen quickly, and at other times, very slowly over time. Ask students for some examples of substances that react with oxygen quickly and slowly. Students may suggest iron reacting with oxygen to form rust or wood reacting with oxygen in order to burn and release energy. Have students think about what would happen to iron or wood on the moon. Without any atmospheric oxygen, would these items rust or burn? (No.) Tell students that the role of the candle activity was to confirm that burning requires the presence of oxygen. This oxidation plays an important role in how we obtain energy from fossil fuels.


Have students re-make their methane models, as described in the first lesson of this series. Review some principles of the first lesson with students by asking them:

  • Is this compound organic or inorganic?
    (It is organic.)
  • Why?
    (It is composed of carbon.)
  • Specifically, what type of organic compound is methane?
    (It is a hydrocarbon.)

Tell students that hydrocarbons go through a special type of oxidation called combustion. When hydrocarbons burn, the reaction produces carbon dioxide and water.

Have students construct an oxygen (O2) model using gummy bears and toothpicks. Ask students to react their methane compound with the oxygen to produce carbon dioxide and water by rearranging the marshmallows, gummy bears, raisins, and toothpicks. Students will realize that they need more than one oxygen to utilize all the hydrogen atoms to produce water. Tell students that they began with a methane molecule and then broke it into carbon dioxide and water pieces. Ask them: “Do you think this process of burning released energy or absorbed energy?” Allow students to think about this question. To help them understand that energy is released in this reaction, ask two students to volunteer. One student represents carbon in a methane compound and the other represents one of methane’s three hydrogens. Ask the two students to hold hands and pull back from each other. Tell the class that the joining of hands represents a covalent bond. Ask students:

  • What might happen if we break this bond between the carbon and hydrogen?
    (The two students will pull in opposite directions.)
  • What will happen to all the energy that is currently stored in this bond?
    (It will be released.)
  • What are some ways in which energy is released in a chemical reaction?
    (They are light, heat, fire, and sound.)

To reinforce that energy is released when the Carbon-Hydrogen bond is broken, ask the two students to hold hands again while they stand next to each other, without any pulling. Ask the class again about the energy that will be released if the bond is broken. (Less energy is released because there was less stored energy in the bond to begin with.) Tell students that knowledge of the amount of energy released from hydrocarbons is important because methane is the main component of natural gas. When methane goes through the process of combustion (i.e., burning) in a clean manner, the only products are carbon dioxide, water, and energy. This is known as complete combustion.

Review with students:

  • In a combustion reaction with hydrocarbons, like methane, what does the hydrocarbon react with?
    (It reacts with oxygen molecules.)
  • What are the products?
    (They are carbon dioxide and water.)
  • What else is produced?
    (Energy is produced from the breaking of hydrocarbon bonds.)
  • In what forms might this energy take place?
    (It might take the form of light, heat, sound, or fire.)

Students should use their Combustion of Fossil Fuels student esheet to go to and read Combustion of Fossil Fuels. Students can answer questions about the reading on the Combustion of Fossil Fuels student sheet. You can find answers to the questions on the Combusion of Fossil Fuels teacher sheet.

Discuss the reading together and clearly review the Combustion Reaction Energy from Bond Energies image. The image can be given to students as a handout or displayed as an overhead. Go through each step of the combustion process, ensuring that students understand the difference between positive and negative energies, endothermic and exothermic energies, and how the net total energy is determined.

In order for students to understand how many bonds are being broken at a time, write the chemical equation for methane combustion in addition to structural drawings of the four compounds involved in methane combustion. This will allow students to actually count the number of bonds being broken in a single molecule. Students then account for the total number of that molecule in the reaction.

To do this, provide students with the bond energy chart found at the top of Energy from Fossil Fuels. Ask students:

  • How much energy is in a single C-H bond?
    (410 kJ/mole.)
  • How many C-H bonds are present in methane?
    (Four C-H bonds are present.)
  • What is the total amount of energy needed to break four C-H bonds?
    (410 kj/mole multiplied by four is 1640 kJ/mole needed. Tell students that the 1644 kJ/mole given in the Combustion Reaction Energy from Bond Energies for methane is approximate.)


Have students determine the bond energies for the combustion of ethane and propane. To do this, have students first determine the correct chemical equation for ethane. Ask students to complete Part III of the Combustion of Fossil Fuels student sheet. As you review the answers with students, draw the structural formula for all of the molecules. This will make it easier for students to count bonds and multiply by the total number of molecules present. For example, ethane has six C-H bonds and one C-C bond. This means that the total amount of energy to break six C-H bonds is 410 kJ/mole multiplied by six, which is 2460 kJ/mole. The total amount of energy to break the C-C bond is 347 kJ/mole. Thus, the total amount of energy needed to break apart all the bonds in a single ethane molecule is +2807 kJ/mole. However, the balanced equation requires two ethane molecules so, in fact, the total amount of energy is +5614 kJ/mole.


Follow this lesson with the next two lessons in the chemistry of petroleum series:

The Science NetLinks lesson, Carbon: Structure Matters, provides a good review of carbon.

An excellent extension for this lesson is to discuss the conservation of energy. Energy can be transferred by collisions and movements in chemical reactions, but it is never destroyed.

Have students investigate complete and incomplete forms of combustion. The online resource Combustion of Hydrocarbons compares the two forms of combustion for methane.

Funder Info
American Petroleum Institute
This content was created with support from the American Petroleum Institute.

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