Lesson Plan: Particle Modeling of Hand Warmers FOR THE TEACHER Instructional Notes and Answers Typical student responses are shown in red and instructional notes are bold. The reusable hand warmers we’re considering contain a supersaturated aqueous solution of the ionic compound sodium acetate (CH3COONa). The main species present in this solution can be represented as shown below:
Submitted by ACS High School Professional Development Team Washington, D.C.
1. Draw a particulate representation of the supersaturated solution of sodium acetate that is present before the hand warmer is activated. Describe in writing what your image seeks to represent.
NOTE: Students should use their current particulate model to describe the situation. Allow time for students to compare their models with other students or small groups. As suggested above, at this point the particles should be represented as simple shapes without any specific internal structure. 2. Activate the hand warmer by bending the metal disk inside the plastic packet several times. Observe and feel any changes that may occur. Describe in writing all changes that you observed. Students should observe a rapid crystallization which results in a significant increase in temperature. NOTE: Provide a hand warmer for each small group. 3. Draw a particulate representation of the contents of the hand warmer after bending the metal disk. Describe in writing what your image seeks to represent.
American Association of Chemistry Teachers
|1-292
1
NOTE: Encourage students to use their particulate model to explain what they have observed. How did the particles arrange themselves when the solid formed? What does the change in temperature suggest about the behavior of the particles? Allow time for students to compare their models with other students or small groups. 4. Using the two particulate representations before and after the activation, build an explanation for any changes in temperature that you perceived in the system. Your explanation should be based on the properties and behaviors of the molecules and ions that comprise the system. Student answers will vary. NOTE: Encourage students to consider how their particulate model of phase changes (including energy) may be similar to this new situation. Group questions: How did you represent the increase in temperature in your model? How did the kinetic energy of the particles change when the hand warmer was activated? Where did the kinetic energy come from? Questions 5-7 below may also be used during this time to help direct student work. Have student share their models with the rest of the class and lead a larger discussion on the merits and limitations of the ideas presented. Carefully facilitate this discussion. Ensure that students show respect for one another’s ideas but encourage them to press other students to clearly explain their models. Students should be challenged to back up their claims with evidence. Avoid judging the ideas presented by students, but press them to clearly justify their reasoning. Build a public record of common ideas expressed by different groups that could help you build a “class-wide” consensus model that explains the behavior of reusable hand warmers. It is important to ask students to express their ideas about how changes in particle rearrangements lead to changes in potential and kinetic energy in the system. As students work through answering these questions for themselves they must understand how these concepts are distinct, yet related to each other. Press students to explain how “temperature” and “heat” are represented in their models, and how their model explains changes in temperature and transfer of thermal energy (heat). 5. Describe how the kinetic energy and potential energy of the molecules and ions in the system change when the hand warmer is activated. Liquid becomes a solid because energy is released. NOTE: As previously mentioned, this system is complex and the dynamics of the solvent-solute interactions are beyond the intended scope of this activity. Student models should suggest that the liquid → solid transition resulted in a decrease in potential energy (or “phase energy”). That energy was not “lost”, but instead translated into an increase in kinetic energy of the particles in the hand warmer. 6. Using bar graphs similar to those used in Parts 1 and 2 qualitatively represent the kinetic energy and the potential energy of the particles before and after the activation.
American Association of Chemistry Teachers
|1-292
2
NOTE: The use of this question may depend on time, students’ prior knowledge and the intended outcomes of the lesson. There are many approaches to the graphical representation of energy. The following outlines one approach you may choose to use. See questions 3 in Part 1 and 10 in Part 2 for more detail on this specific type of graphical representation. Depending on the consensus models built by the students, the kinetic and potential energies of the water and the sodium acetate may be considered separately, or simply combined as the “solution.” The following representation suggests the latter. If the system is just considered the “solution” then energy does not move from one substance to another, but simply changes form. An important discussion is the distinction between the increased temperature of the hand warmer and the resulting “warming” of one’s hand. Students should be lead to consider the hand warming graphically, as well. The following is a suggested model. This model suggests that the hand is part of the surroundings. Removing the hand from the system allows for a visual representation of the energy transfer (heat) between the system and the surroundings. Students should be pushed to explain why the hand warmer itself decreases in temperature over time. 7. How do you think the particles in the hand warmer behaved similar to the melting ice we considered previously? How do you think the two situations are different? Student answers will vary.
American Association of Chemistry Teachers
|1-292
3
NOTE: This question may be used at various times. You may wish to direct small groups to consider it while they are building their models for question 4. You may wish to use it during large group discussion to help guide the conversation. You may wish to use it as a post-discussion question to circle students back to previous experiences. In our simplified consideration of the hand warmer, there was a liquid to solid transition, the kinetic energy of the solution increased and that kinetic energy was exchanged with the surroundings. In the ice-block scenario, there was a transfer of kinetic energy to the ice, which resulted in a solid to liquid transition. 8. The models you’ve built suggest a different form of potential energy, commonly known as chemical potential energy. How could this form of energy be defined? Build a detailed explanation of how chemical potential energy in the supersaturated solution of sodium acetate is transformed into kinetic (thermal) energy as a result of the activation.
Student answers will vary. NOTE: This question allows for the summary of the particulate model so far as well as some prediction and extension. The crystallization of the sodium acetate serves as a bridge between particle behaviors in physical changes (which we have considered) and chemical changes. While changes are often categorized as physical or chemical, the nature of the particle interactions is the same, only differing in scale. 9. The packaging provides the following directions for reuse: “Fully immerse the hand warmer in a pot of boiling water for 20 minutes until all of the crystals in the pack have disappeared.” Does your model predict this phenomenon? Explain.
Student answers will vary. NOTE: This question allows for the application of the particulate model so far. Students should be able to model the reverse process through particle drawings, word descriptions and energy graphs. The pot of boiling water may be considered the surroundings, with a high kinetic energy. That energy is transferred to the hand warmer, resulting in an increase of both the kinetic and potential (or phase) energy of the hand warmer. This results in a solid to liquid transition in the hand warmer. When the hand warmer is removed from the boiling water, it is allowed to cool and remains liquid. Thus, the kinetic energy of the hand warmer is lost to the surroundings, but the added potential (or phase) energy is preserved.
American Association of Chemistry Teachers
|1-292
4
Additional Resources: To learn more about energy in physical and chemical changes: ACS. Energy Foundations for High School Chemistry. http://highschoolenergy.acs.org/content/hsef/en.html ACS. Exothermic, endothermic, & chemical change. http://highschoolenergy.acs.org/content/hsef/en/how-can-energy-change/exothermicendothermic-chemical-change.html To learn more about the chemistry of reusable hand warmers: http://home.howstuffworks.com/question290.htm http://www.heatinasnap.net/FAQ.html#22 To learn more about students’ misconceptions about energy in physical and chemical processes: http://www.rsc.org/images/Misconceptions_update_tcm18-188603.pdf http://pubs.acs.org/doi/abs/10.1021/ed081p523 http://pubs.acs.org/doi/abs/10.1021/ed069p191?journalCode=jceda8 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3671656/ To learn more about facilitating small and large group discussions: http://inquiryproject.terc.edu/shared/pd/TalkScience_Primer.pdf https://www.nsta.org/store/product_detail.aspx?id=10.2505/9780873537452 To learn more about how to build an understanding of the particulate-level behavior of particles in a solid, liquid and gas: https://www.gvsu.edu/targetinquiry/tidocuments-home.htm (register for the site and find the “Putting the World in a Box” activity) http://modelinginstruction.org/teachers/resources/chemistry-core-units/ There are a variety of interactive computer simulations that illustrate how the particulate and molecular models of matter can be used to explain physical and chemical changes: http://phet.colorado.edu/en/simulation/gas-properties http://phet.colorado.edu/en/simulation/states-of-matter http://phet.colorado.edu/en/simulation/states-of-matter-basics http://www.cbc.arizona.edu/tpp/chemthink/resources/U1_M2/partmodel4.html http://mw.concord.org/modeler/ (Click on “Showcase” at the top menu on this page, and then “Click” “Chemistry” on the next page to display chemical models. These models illustrate a variety of physical and chemical processes). http://lab.concord.org/embeddable.html#interactives/sam/phase-change/6-phase-changescaused-by-energy-input.json http://lab.concord.org/embeddable.html#interactives/student/stateofmatter/latentheat2.json http://lab.concord.org/embeddable.html#interactives/student/stateofmatter/latentheat3.json
American Association of Chemistry Teachers
|1-292
5
