Understanding Hydrocarbons

Remember how petroleum is formed? Marine organisms were buried and exposed to enormous amounts of pressure and heat. The substances in the organisms were chemically changed into new substances called hydrocarbons.

Petroleum is made up of hydrocarbons.

What exactly is a hydrocarbon? A substance made only of the elements hydrogen (H) and carbon (C).

Hydrocarbon Molecules A molecule is two or more atoms held together by chemical bonds. Examples of molecules: H O O C Carbon Dioxide Water

The smallest hydrocarbon molecule contains one carbon atom and four hydrogen atoms. Name: Methane Chemical Formula: CH4 Representation: C H The C’s and H’s represent atoms. Each “—” represents a chemical bond. Carbon always forms four chemical bonds. Show students how to interpret the chemical formula. The element symbols tell us which elements are in the substance (in this case, carbon (C) and hydrogen (H)). Subscripts tell us how many atoms of each element are in the substance. If there is no subscript, it means there is only one atom. For example, in methane (CH4) there is one atom of carbon and four atoms of hydrogen. Explain to students that the carbon atoms in a hydrocarbon always have four bonds (—) extending out from them.

Methane Model Fill in the chemical formula for methane on your hydrocarbon table. Write out the representation for a methane molecule on your paper. Using clay and toothpicks, build a model of a methane molecule. Inform students that you will verify that they have correctly assembled 3D methane molecules by stamping their worksheets next to the methane row of the table. Do not give students directions for building their models. Let them figure out the best way to do this on their own. Stamp or initial worksheets after students have successfully made methane models. Allow students time to complete making their two- and three-dimensional models of hydrocarbon for the next several slides, though it’s not necessary for students to make clay models of all the hydrocarbons in the chart. However, they should at least make the first four (through butane) to help them better understand the pattern. Note: See the “Background for Teachers” section for more information on the models.

Name: Ethane Chemical Formula: C2H6 Representation: The next hydrocarbon molecule has two carbon atoms and six hydrogen atoms. Name: Ethane Chemical Formula: C2H6 H H Representation: H C C H H H

Ethane Model Fill in the chemical formula for ethane on your hydrocarbon table. Write out the representation for an ethane molecule on your paper. Using clay and toothpicks, build a model of an ethane molecule. Verify that students have made accurate models of ethane molecules. Stamp worksheets next to the ethane row of the table.

Predictions Predict what the third hydrocarbon (propane) looks like. Write its chemical formula in your hydrocarbon table. Write out its representation on your paper. Using clay and toothpicks, build a model of a propane molecule. Stamp worksheets if students successfully write out the representation and build correct models. The representation should look like this: H H H | | | H — C — C — C — H H C

Predictions Predict what the fourth hydrocarbon (butane) looks like. Write its chemical formula in your hydrocarbon table. Write out its representation on your paper. Using clay and toothpicks, build a model of a butane molecule. Stamp worksheets if students successfully write out the representation and build correct models. The representation should look like this: H H H H | | | | H — C — C — C — C — H H C

Identifying Patterns Do you see a pattern? What is the repeating unit in hydrocarbons? Once students are done with their models, ask students to describe the pattern they see. Accept all answers. Ask students to find and describe the repeating unit in the representation of hydrocarbons.

Using the pattern you discovered, fill in the rest of the information on your hydrocarbon table. Have students try to find the pattern on their own and circle the unit in the octane representation, as instructed on the worksheet. Students should be able to describe a repeating H — C — H unit. Have a brief discussion to verify. Instruct students to finish filling in the hydrocarbon tables on their worksheets. Then have students answer the remaining questions on the first page of the worksheet. Note: Stop here and refer back to the “Lesson Plan” section for further discussion.

What is the difference between a liquid and a gas? Using the containers on your notes sheet, fill in what molecules of a liquid and a gas look like. Use small circles to represent molecules! Allow students time to think of differences and share their ideas. Then give students time to draw their models of liquids and gases on their worksheets. Ask students to recall when they have studied different states of matter and used models to represent the molecules of matter.

Liquid Gas

Gas Liquid In a gas, molecules are far apart and move very quickly. In a liquid, molecules are close together, but flow freely past each other. Liquid Gas

At what point does a liquid change to a gas (and a gas change to a liquid)? Boiling Point – The temperature at which a liquid changes to a gas and a gas condenses to a liquid. Give students time to respond to the question before uncovering the answer.

We can use the boiling point of liquids to separate them from each other in a process called distillation.

Draw a diagram of the distillation unit set up in your classroom. Include labels, and explain how the unit works by writing a brief description below the diagram. Briefly explain that you have water in the flask and will heat the water so that it comes to a boil. As the water heats, students should draw the setup. Allow students time to draw simple diagrams and label the components. Once the water boils and condensate begins to drip in the test tube, have a class discussion about what is happening. Then ask students to summarize what is happening by adding to their diagrams. Save the next slide for the end of class (after the Distillation Introduction activity).

Petroleum that is recovered from beneath the Earth’s surface is a mixture of hydrocarbons that are separated out by distillation. The distillation of petroleum takes place at refineries.