Summary

In this lesson, students will use a climate change scenario to understand the role that polar bonds play in whether a molecule can be considered a greenhouse gas while learning the particle nature of matter-energy interactions.

Grade Level

High School

NGSS Alignment

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

• HS-PS2-6: Communicate scientific and technical information about why the molecular-level structure is important in the functioning of designed materials.
  • PS2.B: (Types of Interactions) Attraction and repulsion between electric charges at the atomic scale explain the structure, properties, and transformations of matter, as well as the contact forces between material objects.
• HS-PS3-2: Develop and use models to illustrate that energy at the macroscopic scale can be accounted for as a combination of energy associated with the motions of particles (objects) and energy associated with the relative positions of particles (objects).
  • PS3.A: (Definitions of Energy)
    • Energy is a quantitative property of a system that depends on the motion and interactions of matter and radiation within that system. That there is a single quantity called energy is due to the fact that a system’s total energy is conserved, even as, within the system, energy is continually transferred from one object to another and between its various possible forms.
    • At the macroscopic scale, energy manifests itself in multiple ways, such as in motion, sound, light, and thermal energy.
    • These relationships are better understood at the microscopic scale, at which all of the different manifestations of energy can be modeled as a combination of energy associated with the motion of particles and energy associated with the configuration (relative position of the particles). In some cases the relative position energy can be thought of as stored in fields (which mediate interactions between particles). This last concept includes radiation, a phenomenon in which energy stored in fields moves across space.
• HS-PS3-5: Develop and use a model of two objects interacting through electric or magnetic fields to illustrate the forces between objects and the changes in energy of the objects due to the interaction.
• HS-PS4-4: Evaluate the validity and reliability of claims in published materials of the effects that different frequencies of electromagnetic radiation have when absorbed by matter.
  • PS4.B: (Electromagnetic Radiation) When light or longer wavelength electromagnetic radiation is absorbed in matter, it is generally converted into thermal energy (heat). Shorter wavelength electromagnetic radiation (ultraviolet, X-rays, gamma rays) can ionize atoms and cause damage to living cells.
• HS-ESS2-4: Use a model to describe how variations in the flow of energy into and out of Earth’s systems result in changes in climate.
  • ESS2.D: (Weather and Climate)
    • The foundation for Earth’s global climate systems is the electromagnetic radiation from the sun, as well as its reflection, absorption, storage, and redistribution among the atmosphere, ocean, and land systems, and this energy’s re-radiation into space.
• Scientific and Engineering Practices:
  • Developing and Using Models
  • Analyzing and Interpreting Data
  • Constructing Explanations and Designing Solutions
  • Obtaining, Evaluating, and Communicating Information

Objectives

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

• Explain how carbon dioxide can affect the Earth’s temperature.
• Analyze data in graphical form to make predictions and to explain processes.
• Identify both polar bonds and polar molecules, then describe how the polarity of the molecule can shift with the molecule’s natural vibrational motions.

Chemistry Topics

This lesson supports students’ understanding of:

• Energy
• Heat
• Heat Transfer
• Radiation
• Light
• Particle Nature (interaction of light and matter)
• Molecular Structure
• Polar bonds and Dipole Moments

Time:

Teacher Preparation: 5 minutes (gather supplies for the demo)
Lesson: Two 45-minute classes

• Demo and discussion: ~10 minutes
• Optional lessons on energy foundations: 20 minutes, possible homework assignment (11 minutes of video, plus any appropriate summarizer or follow-up questions)
• Warm-up and energy review: 20 minutes, including discussion of answers
• Part I: ~15-20 minutes
• Part II: ~15-20 minutes
• Part III: ~15-20 minutes (if summarizing question is assigned as homework)

Materials

• Demo:
  • Clear containers (for water and other colored substances)
  • Concentrated white light source (ex: pen light)
  • Color LED or laser light sources
• (Optional) Videos from ACS Energy Foundations for High School Chemistry
  • What is Energy? (~5 minutes)
  • Temperature and Energy (Why do Puddles Evaporate?) (~5 minutes)
• Simulations from ACS Chemistry in Context
  • Collisional Heating in the Atmosphere
  • CFCs in the Atmosphere
• (Optional) Simulation from PhET
  • Molecule Polarity
• Short YouTube Videos (~1 min each)
  • CO₂ Vibration Modes, dipole Moments and IR (45 seconds)
  • Molecular Level Explanation: IR (~ 1 minute)

Safety

• Be careful to aim any laser light sources away from students.

Teacher Notes

• This lesson is designed to be completed as three consecutive parts. However, depending on the level of your students, and the class time available, a specific section could be selected and used separately as an individual activity. 
• The student handouts are available for teachers as a single file that includes all student handouts compiled, as well as four individual files separated as Warm-Up Section and Energy Review; Part I; Part II; Part III.
• An answer key document is provided for teacher reference.

It will be useful to clarify the following points throughout the lesson:

• The term “spectrum” is a graphical term and usually refers to a plot of intensity against a range of values. In this activity, the “range” will always be a portion of the electromagnetic spectrum, using a wave property (wavelength, frequency, or energy) as the value for the x-axis.
• Use the warm-up questions to make sure students recognize that “carbon” can mean many things and that when discussing the atmosphere and global warming, it usually means carbon dioxide (CO₂).
• A major concept that may need clarification is that a single photon can interact with a single molecule. This is shown throughout the simulations, and it is worth pointing out that the little squiggles that represent a photon will “disappear” upon interaction with the molecule, while other photons that don’t interact will simply continue moving past. The disappearance means that the molecule now contains the energy that was previously a photon and this makes the molecule “do something.” This drives home the common definition of energy as “the ability to do work.” When the molecule gains energy, it does something with the energy.
  • Part II, question 2 shows different things that can happen to a molecule when different amounts of energy are absorbed.
  • It will be useful to reinforce that any time something “happens” energy is exchanged, so, if energy is absorbed, then something must happen.
    • Example: A bond vibrates. That bond is a result of electrostatic attractions, so a very simplified explanation is that it must absorb energy to stretch, then release energy to compress. If a molecule absorbs a photon with a certain amount of energy, it may increase the vibration of the bond or it may make it “stretch” so much that it breaks the bond.

Lesson

• Optional: Review Energy Fundamentals. You can use the ACS videos noted in the Materials section to either teach or review concepts of energy in preparation for this lesson.
• Students need to be familiar with the following:
  • Energy manifests in various forms.
  • All matter above absolute zero has energy.
  • Matter is continually exchanging energy with its surroundings through radiation, collisions, and heat transfer.

Demo (~10-15 min)

• Using any combination of colored solutions, colored light sources, white light source, and plain tap water, shine the light sources through the various solutions and through water.
• Ask students to observe and predict or explain what is happening.
• The major goal for the demo is to help students understand that molecules absorb light in specific ways, and that there are examples of that everywhere. This will prime their minds for exploring the particle-level interactions throughout this lesson.

Introduce the activity (~15 min)

• Students complete the Warm-Up and Energy Review Questions on their own.
• Review answers with the class and discuss, as appropriate.

Parts I and II: Student Independent or Collaborative Work (~30-40 minutes, including discussion of answers)

• Option 1:
• Assign Parts I and II as homework.
• Following day, give students 5-10 minutes to compare and discuss answers, then share out and discuss as appropriate.
• Option 2:
  • Use collaborative groups to complete Parts I and II.
  • Share out and discuss after each part.
• Note: pie chart graphic is available on Wikimedia Commons.

Part III: Student Collaborative Work

• Teacher Background: The following are notes about this type of spectrum in case it is new to you! It is not necessary for students to understand all of this detail, as long as they understand the simulation.
  • Wavenumber is used in IR spectroscopy because it is directly proportional to energy. It is the inverse of wavelength in cm, so its unit is cm^-1.
  • The blackbody curve is the idealized spectrum of Earth’s radiation, based on its average temperature. This is the range of photons that are continually sent out from the Earth (because it is an object that has a temperature higher than absolute zero).
  • The “Laboratory Spectra” display is a typical IR spectrum for each gas. IR spectra are typically run in transmittance mode. This means that the graph shows relative amount of each wavelength that has been transmitted through the sample. So if it does not get absorbed, the trace will be high on the y-axis, representing 100% transmittance, or a relative intensity of 1.0. This type of graph is opposite of what students are likely used to seeing, so it may be useful to point out that the downward peaks are representative of the sample molecule absorbing photons of those wavelengths.
  • The “Scaled Spectra” are lab spectra that are scaled to concentrations equivalent to those found in the atmosphere. This display option is NOT written into the student activity, but it can be used when reviewing their answers to Part III, Question 3.
    • Explained Example: The lab spectrum for CF₂Cl₂ shows strong absorption peaks within the range of Earth’s emission, so you’d expect that to be a very significant greenhouse gas. If you go to the “Scaled Spectra” display, you can see that those strong absorption peaks are effectively reduced to zero, due to the extremely low concentration of this gas in the atmosphere. Concentrations of CF₂Cl₂ are in the part per trillion (ppt) range, while CO₂ is in the ppb range.
  • The “Earth’s Scaled Emission Spectra” shows only the effect that each gas has on how much of Earth’s radiation gets sent (past the atmosphere) out to space. Each dip downward means that some portion of Earth’s radiation will get absorbed by that molecule and never make it past the atmosphere. These spectra have been scaled to account for both atmospheric concentration and absorption efficiency for each gas.

Part III: The Lesson

• Project the simulation from Part III and review the points in “Understand the Display,” to ensure students understand the terminology and what the spectra are showing.
• Allow students to work through the first three questions.
  • Review answers.
• PhET simulation: Molecule Polarity
• Teacher-led discussion:
• Use the Real Molecules tab. Choose the Electrostatic Potential surface. Choose whether to show dipoles and other labels, based on what your students know or will learn.
• Choose carbon dioxide (CO₂) as the molecule and discuss with students what the colors mean. Confirm that this is a nonpolar molecule but that there are areas of partial charge due to its polar bonds.
• Select nitrogen (N₂) and oxygen (O₂) molecules and note their lack of polar bond. (Most abundant gases in the atmosphere, but NOT greenhouse gases.)
• Show either or both of the videos below to show how/why the vibration of a bond can affect the instantaneous dipole moment of a molecule.
  • CO₂ Vibration Modes, dipole Moments and IR (45 seconds), shows the changing polarity w/ dipole arrows
  • Molecular Level Explanation: IR (~ 1 minute), shows molecular motions – dipole motions implied
• Conclude with the explanation that IR radiation can only be absorbed by molecules that have an oscillating dipole moment. Explain this as having electron density that changes non-symmetrically as the molecule bends and twists and vibrates.
• Allow students to complete questions 4-6, then review answers.
• Assign Summarizing Questions for homework or for formative or summative assignment.