Summary

In this lesson, students will learn about air pollution and some steps toward mitigating it. First, they will burn a candle and measure its mass and the concentration of CO₂ over time. Students will discuss which data set they have more confidence in and why and then use stoichiometry to predict outcomes. Next, students explore incomplete combustion in a model-based worksheet that shows how a lack of O₂ in the burning of fuels can produce air pollution. Students work together to interpret the models, define terms, and draw conclusions. Lastly, students work in groups using Lego models to illustrate how a catalytic converter works. They race “Nature” against catalysts “Palladium,” “Platinum,” and “Rhodium” to see what breaks down air pollution molecules fastest.

Grade Level

High School

NGSS Alignment

This lesson will help prepare your students to meet the performance expectations in the following standards:

• HS-PS1-7: Use mathematical representation to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
• HS-ETS1-3: Evaluate a solution to a complex real-world problem based on prioritized criteria and trade-offs that account for a range of constraints, including cost, safety, reliability, and aesthetics as well as possible social, cultural, and environmental impacts.
• Scientific and Engineering Practices:
  • Using Mathematics and Computational Thinking
  • Developing and Using Models
  • Analyzing and Interpreting Data

Objectives

By the end of this lesson, students should be able to:

• Analyze combustion lab data to support an argument.
• Calculate the theoretical amount of CO₂ produced from their combustion lab.
• Interpret models of incomplete combustion.
• Support a claim about which fuels are more likely to produce more air pollution.
• Model the function of a catalytic converter.

Chemistry Topics

This lesson supports students’ understanding of:

• Chemical Reactions
• Combustion
• Stoichiometry
• Limiting Reactants
• Catalysts and Rates of Reactions

Time:

Teacher Preparation: 30 minutes
Lesson: 150-180 minutes total (over the course of 3-4 class periods)

• Introduction:
  • Warmup activity about air pollution: 10-15 minutes
  • Introduction to the consequences of too much CO₂ (videos or other): 10 minutes
• Candle Combustion lab:
  • Review prelab question and instructions: 10 minutes
  • Complete lab procedures: 15 minutes
  • Graph and analyze data: 15-20 minutes
  • Discussion about results: 10 minutes
• Incomplete Combustion modeling worksheet:
  • Pairs of students complete the worksheet, pausing for class discussions: 40-50 minutes
• Catalytic Converter activity:
  • Introduce catalytic converters (video or other): 10 minutes
  • Groups of 4 complete the catalytic converter simulation activity: 30 minutes
  • Class discussion about strengths and limitations of the models: 10 minutes

Materials

• For each lab pair or group in the combustion lab:
  • Small candle
  • Small ball of clay (to help the candle stay upright)
  • Lighter or matches (to light candle)
  • Weight boat
  • Balance
  • Vernier CO₂ Gas Sensors (or similar) for measuring ppm of CO₂ (Note: they do not have to work well!)
  • Stopwatch (or students could use a phone, wall clock, etc.)
  • Goggles
• For each group for the Catalytic Converter group simulation (4 students per group):
  • Name tags labeled “Nature,” “Palladium,” “Rhodium,” “Platinum”
  • 1 bin of Legos (about lunch box sized). Smaller if you separate them into different color groups.
  • 1 pair of gloves (knitted winter gloves work well)

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Always use caution around open flames. Keep flames away from flammable substances.
• Always be aware of an open flame. Do not reach over it, tie back hair, and secure loose clothing.
• Open flames can cause burns. Liquid wax is hot and can burn the skin.
• An operational fire extinguisher should be in the classroom.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them on how to clean up their materials and dispose of any chemicals.

Teacher Notes

• This lesson was developed as part of the AACT Chemistry and Sustainability content writing team. It connects with the following UN Sustainable Development Goals:
  • Goal 3: Good Health and Well-Being – Ensure healthy lives and promote well-being for all at all ages.
  • Goal 7: Affordable and Clean Energy – Ensure access to affordable, reliable, sustainable, and modern energy for all.
  • Goal 13: Climate Action – Take urgent action to combat climate change and its impacts.
• Prior to starting this lesson, students should be familiar with chemical formulas, ball-and-stick models, balanced equations, moles, and stoichiometric calculations.

Day 1: Introduction and Candle Combustion Lab

• As a warmup activity, ask students what comes to mind when you say the phrase “air pollution.” Have them do a Think-Pair-Share, Snowball Fight, or other active learning technique to share their thoughts with the class.
• Introduce a combustion reaction to students and explain to them that measuring CO₂ is important since it is the main molecule contributing to climate change. There are two videos on increasing CO₂ levels and where the CO₂ comes from that you could show to engage students.
• In the candle combustion lab, students will combust paraffin wax and try to measure both the amount/mass of wax burned and the amount of CO₂ produced. They will light their candle and record both pieces of data every minute for 10 min.
  • For the purposes of this lab, C₂₅H₅₂ is used as the formula for paraffin wax. It is actually a mixture of many hydrocarbons, usually ranging from C₂₀ to C₃₀ carbon chains.
• Be sure demonstrate for students how to operate the CO₂ sensors if they haven’t used one before. If you use the Vernier probe, the User Manual may be a helpful resource.
• It is valuable for them to make their own graph but plan on them needing to revise or restart as necessary as they get a better feel for their data. Students who need additional support would benefit from pre-made axes or setting up the axes together as a class. Alternatively, a graphing program such as Excel could be used to create the graph. You could also assign the Graphing Simulation to review proper graphing techniques before conducting this lab activity.
• The CO₂ meters will likely have a lot more variation or not measure accurately. This is part of the point of this activity! See the answer key, available in the sidebar, for sample data and graphs.
• When students have two reasonable graphs, have them discuss which data set they trust more and why (analysis question #2). Facilitate a class discussion asking students to share their interpretations. Some likely observations:
  • The C₂₅H₅₂ mass decreases at a steady rate.
  • The CO₂ measurements don’t change or change erratically.
  • CO₂ is hard to measure since it’s a gas and it disperses into the air pretty quickly. Solids are easier to measure since the particles are contained and don’t spread apart like gases.
• If students already know how to do mass-to-mass stoichiometry calculations, have them complete the extension question on the student handout after the discussion about reliable data (otherwise, you can delete it before giving the worksheet to students). Instead of trying to measure the CO₂ directly, they will use the more reliable measurements of the mass of wax burned to calculate the amount of CO₂ produced. This could be assigned as homework if time is limited.

Day 2: Incomplete Combustion model-based worksheet

• Model-based worksheets inspire collaboration as students evaluate drawings to learn new concepts. This worksheet teaches them about air pollution and limiting reactants, and it reinforces the concepts of balanced equations and the law of conservation of mass.
• Have students work in pairs with two different colored pencils and take turns reading and writing as they discuss the worksheet. Circle the room to answer questions and check on the work. Occasionally pause the class to discuss the various interpretations of a key question, or have groups call you over to discuss their answers to those questions before they move on (recommended for questions 6, 10, 11, and 18).
  • One way to recognize students who may not readily volunteer to share their ideas with the whole class is to write down the names of students who have made interesting or insightful interpretations on a notecard while circling the class, then call on those students to share as a way of diversifying the conversation. That way a wider variety of student responses can be celebrated, rather than just the usual hand-raisers.
• If students have difficulty with question 9 (identifying unburned fuel in an incomplete combustion), drag a finger along the octane molecule to help them see it burning along the way, oxygen breaking bonds until the oxygen runs out.
• Groups will likely work at different paces. Students who finish the activity more quickly can work on the extension questions.

Day 3: Catalytic Converters: chemical catalysts that clean the air

• Explain to students that there are some types of technology that can reduce air pollution and that an important one is catalytic converters. Catalytic converters are car parts that further react with the exhaust from an engine to produce substances that are less harmful. You could show them the Catalytic Converter video that is part of the Chemistry of Cars Resource Collection in the AACT resource library. It is less than four minutes long and provides a good overview of how catalytic converters work, discussing different redox reactions that take place and the roles of platinum, palladium, and rhodium. It could also be assigned in advance as homework.
  • Note that this video distinguishes between the different reactions that platinum, palladium, and rhodium catalyze, but this simulation activity does not. Only the hydrocarbons are broken down in the activity, which would be facilitated by platinum or palladium catalysts, whereas the rhodium would catalyze reactions involving nitrogen oxides. Only the hydrocarbons are included in this activity so students can focus on one reaction. This could be a point of discussion in the penultimate question on the student handout about the strengths and limitations of the Lego model.
• A catalyst is a substance that speeds up a chemical reaction but is not used up in the chemical reaction. In this activity, the students themselves will simulate catalysts inside the catalytic converter.
• Divide students into groups of 4 and give each group a tub of Legos. Explain to them that car engines produce several types of air pollutants such as carbon monoxide (CO), hydrocarbons (which are also called volatile organic compounds, or VOCs), and nitrogen oxides (NO_x). Give out 1 worksheet per group. Have students be car engines and use their Legos to make 8 C₂H₄ pollutant molecules. They will need to decide which color of Legos represents each element. You do not have to distinguish between the shape or size of Legos, only colors, so not all of the C₂H₄ will look identical. (If you want, you could call the different shapes/sizes different isotopes, then the molecules with the same atom types but varying isotopes would be called isotopologues!)
  • This is another way in which these models are not completely accurate. The main purpose of this activity is to model the rate of breaking bonds and making new bonds, not the geometry of the molecules. This could be another point that is raised when discussing the limitations of the model.

• Each group should identify one student to be “Nature.” Explain that “Nature” will try to break apart the pollutant molecules and turn them into CO₂ and H₂O as in the reaction, but that he or she will be wearing gloves while doing it. Have the other students time “Nature” as they react as many of the pollutant molecules as they can in 30 sec. (Note: “Nature” must complete a full reaction before moving on to the next pollutant molecule. In other words, no just breaking all the pollutants apart before putting the products together.)
  • While it may be tempting to use molecular modeling kits instead of Legos, these may be too easy for “Nature” to manipulate with gloves on. Legos are more challenging and will make the point better that the catalysts speed up the reaction.

• Assign the other 3 students to be “Palladium,” “Platinum,” and “Rhodium.” (You can give them name tags or sticky notes if you want.) Explain that these three metals are the catalysts that are inside of the catalytic converter and that speed up the reactions. Tell students that after the catalyst helps perform a reaction it can be used again to perform another reaction. The catalyst also does not get included in either the reactants or the products.
• Have students complete the worksheet and activity together.
• Don’t worry if the students’ first trial doesn’t go perfectly. Circle the room and watch and have discussions – don’t forget to laugh with them! It usually only takes one repetition before they get data that illustrates the big idea.
• The last two questions would be great to discuss as a whole class, as well as in their pairs. These questions can help them to think deeply about what the models are trying to represent and how they succeed (or do not) in doing so.