Lesson Plan: Modeling the Melting of Ice FOR THE TEACHER

Transcription

1 Lesson Plan: Modeling the Melting of Ice FOR THE TEACHER Instructional Notes and Answers In our exploration of the tiles, we ended with a macroscopic model of thermal energy transfer as well as a model that explained our observations in terms of the movement and interaction between the particles. Instructional notes are bold. Student answers are red. Submitted by ACS High School Professional Development Team Washington, D.C. 1. Now, we will think about the process of the melting of the ice. Using the model that you came up with previously, describe how the particles will be moving in the ice as kinetic energy continues to be transferred from the tile to the ice. Student responses will vary. 2. If the particles are behaving in the way that you have described above, how will the temperature of the ice change as it melts? Student responses will vary. Many students will expect the temperature of the ice to increase. 3. Test your prediction by measuring the temperature of the ice as it melts using an IR thermometer. Record your observations here. The temperature remains the same. NOTE: Try this experiment with your particular IR thermometers. Students should aim the thermometer directly at the ice while it is melting. Reading the temperature of the resulting liquid water or the block may provide inconsistent data. Some discussion of the accuracy of the equipment may be warranted as well. Students should find that, within the limits of the experiment, the temperature of the ice remains unchanged. 4. Do your observations support your model? Explain how your observations fit with your model and modify your model to better incorporate any observations that contradict it. Be prepared to share your group s model with the class. The key observation is that the transfer of energy into the ice cube is not resulting in an increase in temperature. This suggests that at the particulate level it is not the kinetic energy, related to motion, that is changing. NOTE: Small group questions: What other aspects of the particles behavior are changing? How might energy be involved in that change? A large group or group-to-group sharing should not necessarily lead to class consensus at this point, but simply provide an opportunity for students to hear each other s ideas. To facilitate class discussions, you may wish to have students build these models on a large piece of paper or whiteboard. Student responses will vary. See the Resources section at the end of this document for examples and ideas of how this situation may be modeled. Differences between groups will provide excellent material for larger group discussions. Resources for facilitating small and large group discussions are provided in the resources section at the end of this document. American Association of Chemistry Teachers

2 The instructor s role is to facilitate and classroom discussion that allows for students to hear each other s ideas, participate in productive science dialogue and clarify their own understanding of the concepts. Guide student discussion towards a consensus of how energy is transferred at the iceparticle interface. 5. Suppose a piece of ice is taken out of a very cold freezer (-40 o C) and placed into a beaker. Then it is placed on a hot plate and heated, transferring energy to the ice at a constant rate. The temperature is taken until the ice has melted. What would the graph of Temperature vs Time look like? Make your prediction on the graph provided below. Student responses will vary, but many students will simply draw a straight line. Some may have more sophisticated ideas in light of the IR thermometer results. NOTE: At this point, you may wish to have students actually do this as an experiment and collect their own data. An alternative may be to present the phenomenon as a demo. The following question provides a premade graph of lab results, but you may choose to have students use their own data and graphs. Your instructor will provide you with a graph of Temperature vs Time. American Association of Chemistry Teachers

3 6. In the circles below, draw particle models for the water at each section of the graph. 7. Using the model you ve developed so far, what can you say about the kinetic energy of the particles during each portion of the graph? During each portion of the graph, how is the behavior of the particles changing as energy is added? Student drawings should demonstrate a gradual decrease in structure from the ordered solid to the random liquid. Students may also show the relative motion of particles with cartoon-like action lines. The energy gained by the ice from B to C is resulting in a rearrangement of particles and not a change in their speed (kinetic energy). NOTE: The graph can be accessed by clicking the link above. 8. Some of the information from the graph suggests that the added energy does not only result in a change in the movement (kinetic energy) of the particles. What other particle changes are being caused by the addition of energy? Student responses will vary, but should center on the phase change occurring. Their IR thermometer observations should beg the question: If temperature (kinetic energy) isn t changing, what aspect of the particle s behavior is changing? 9. How do the particles of a substance change during the phase change? What role do you think energy plays in the change? Student responses will vary. Students may mention particle spacing, arrangement and orientation. 10. As a group, return to the model of the melting ice from question 1. Modify your model to incorporate any new understandings of the role energy plays in phase changes. The amount of detail expected from student models will depend on course outcomes, student ability and available time. NOTE: You may choose to have a large group discussion at this point to work towards a class consensus. Depending on course outcomes, you may wish to leave the concept more vague; possibly phase energy which is simply energy that is gained or lost when particles rearrange into a new state of matter. You may wish to further explore electrostatic attractions and the resulting interparticle forces that are the underpinnings of potential energy. You may wish to consider in more detail how the arrangement and orientation of the particles changes in each process as well. 11. Now that we ve come to a consensus as a class, let s think about how we can model the changes in energy visually and graphically. American Association of Chemistry Teachers

4 a. What objects are exchanging energy in this situation? List the objects that are exchanging energy and connect objects that are exchanging energy with a line. Air, ice, tile NOTE: The development of a schema is a precursor to the graphical representation below. Here, students need to define all of the objects that may be involved in an exchange of energy. They connect the objects that are exchanging energy with lines. b. The diagram on the right (known as a schema) represents some of the key objects in this situation. The dashed circle separates the system (objects we re studying) from the surroundings (everything else). Draw arrows at the end of each solid line indicating the direction of energy transfer between objects. NOTE: Defining the system and surroundings allows students to consider whether the total energy of the objects in question is changing. This is an important step towards a more quantitative consideration of energy. A sample schema is provided on the right. Students may choose to include other objects (i.e. the table the tile is resting on). While the exchange of energy with the air does occur, we will not consider it in the models used in these materials. Since the focus is on the changes that are occurring within the ice, we will consider only the ice in the system. Moving the tile outside the system means that the energy coming from the tile to the ice will be coming from the surroundings, increasing the overall energy of the system. 12. Use the chart below to represent the initial distribution of energy and how the energy flows at each stage in the process. A B C D American Association of Chemistry Teachers

5 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. If this is the student's first use of such a diagram, they may need some guidance. Small and large group discussions of these graphs will circle back to the particle model and will help the students begin to think about how to quantify energy. The bar graphs presented below are not intended to be accurate quantitative representations. Depending on time, student ability and the course outcomes, you may wish to consider a more quantitative approach with your students. The Energy Flow circle represents the separation between the system and the surroundings. Any object that is considered part of the system is placed inside the circle. Any object that is not part of the system is placed outside the circle. The block is still transferring kinetic energy to the water. In this case, the energy is going towards the rearrangement of particles. Thus, there is an increase in potential energy amongst the water molecules. Note that the kinetic energy of the water remains unchanged. Student should construct these graphs (either individually or in small groups) and then share their ideas with a larger group or whole class. Small group questions: Did the ice have any kinetic energy before it was placed on the block? Did the ice have potential (or phase ) energy before it began melting? How do you know if the added energy shows up as kinetic or potential energy in the water? How do you know whether the PE of the water has increased or decreased? How does the total energy of the water change? 13. Now, let s see if we can apply our model to the reverse process. Consider liquid water being placed into a cold freezer. How will the behavior of the particles change over time? How will energy be exchanged over time? Use the diagrams provided below to construct your model of the process. American Association of Chemistry Teachers

6 NOTE: This question allows students to apply their model and is insightful formative assessment for the teacher. You may wish to not include the premade diagram and have students construct it themselves, depending on student abilities. Students should be able to determine that kinetic energy is being lost while the temperature is changing and potential energy is lost while the phase change occurs. An important question: Does the water need to increase or decrease in total energy for this phase change to occur? A challenging concept is that ice formation (which students cite as happening when it is cold ) actually releases energy to the surroundings. American Association of Chemistry Teachers