(Optional) Activity 5.1 - Molecular Modeling for Methane Burning (40 min)

Tell students that if they can explain what happens when ethanol burns, they can also explain what happens when many other materials burn. That is because the same rules apply: matter and energy change in similar ways. Tell students that they will practice with another material: a natural gas called methane (CH₄).

Show students slides 3-6 to observe a methane flame at the macroscopic and atomic-molecular scale.

• Pose the question: “What’s the hidden chemical change when methane burns?” Explain to students that both methane and O₂ enter the flame at the bottom.
• Show slides 4 and 5 to contrast the molecules at the bottom and top of a flame.

Show slide 7 to remind students that explaining chemical changes always involves answering the Three Questions. Divide students into pairs or small groups and have them practice answering the Three Questions in a new context. Remind them that the same rules about matter and energy apply for methane and ethanol.

This is an optional step. If you feel that your students can explain methane burning and answer the Three Questions, skip to activity 5.2.

Divide the class into pairs and give each pair a molecular model kit, a set of Forms of Energy Cards, and Molecular Models 11 x 17 Placemat. Pass out one copy of 5.1 Molecular Models for Methane Burning Worksheet to each student.

• Use slide 8 to show instructions to construct oxygen and methane molecules. Students can also follow instructions in Part B of their worksheet.
• Use slide 9 to instruct students to compare their own molecules with the picture on the slide.
• Use slide 10 shows an important message: after students create their reactant molecules, make sure they put away all unused pieces of their molecule kits. This helps reinforce that the matter and energy in the reactants are conserved through the chemical change, and that only the materials from the reactants are used to build the products.

Accommodation: Do this optional activity. Arrange the molecules along with students so they have a step-by-step model of what the molecules should look like.

When they have completed their reactants, tell students to complete the table about matter and energy in their worksheet for the reactants.

Tell students to follow the instructions the worksheet to construct their products.

• Show slide 11 of the PPT and have students re-arrange the atoms to make molecules of CO₂ and H₂O. To do this, they will need to move their molecules from the reactants side to the products side of the placemat. Explain to students that atoms last forever, so they should not add or subtract atoms when they change the reactant molecule into product molecules.
• Show students Slide 12 to compare the products and a comparison between reactants and products.
• Show students Slide 13 to overview the entire process.

When they have completed their reactants, tell students to complete the table in their worksheet for their products.

Have students verify that the number of atoms before and after remained constant: Atoms last forever! Tell students that this means that the number of atoms before and after the reaction does not change.

Show slides 14-19 in the PPT to help students make connections between what is happening in the animation and the molecular models they made.

• For each slide, focus on different atoms and forms of energy and how they change. The animation draws attention to where they atoms begin and end in the reaction.

Tell students that now that they have represented a chemical equation using molecular models and in animations, they will represent chemical change by writing the chemical equation.

• Show Slide 20 of the presentation to guide students through the process of writing a balanced chemical equation for the combustion of ethanol. Tell students that these rules apply to all chemical reactions.
• Tell students to write their equations in Part C of their worksheet.
• Have students write their own chemical equations before comparing them with the one on Slide 21.