Modeling Chemistry: Rearranging Units 2 and 3

The current curriculum resources for Modeling Instruction in Chemistry present the following as units 2 and 3:

Unit 2: Energy – Particles in Motion

Unit 3: Energy & States of Matter

This blog post will make a case for switching these two units around in order to lead to greater student understanding of both the particle model and the energy model. First, I’m going to briefly describe each activity in the units as written and why they are important for model development. Then I will describe suggested modifications for the rearrangement and explain why I think they are beneficial. 

Current Sequence

Unit 2: Energy – Particles in Motion

  • Demonstration/discussion – diffusion of gases
    • There are a number of different ways to carry out this demo, using perfume, popped popcorn, peeled oranges, or burning candles, for example. Through observation and discussion, students conclude that particles are moving. This trait is added to the particle model.
  • Demonstration/discussion – diffusion of liquids in hot and cold liquids
    • Food coloring is dropped simultaneously into containers with hot and cold water. Through observation and discussion, students conclude that warmer particles (of the same substance) move faster. This trait is added to the particle model. This is an opportunity to discuss energy, and temperature can be established as average kinetic energy of the particles.
  • Observe the three states of matter (Eureka 1-3)
    • Through observation and discussion, students compare the three states of matter, particularly for the characteristics of rigidity, fluidity, and density.
  • Define temperature (Eureka 4-5)
    • Through observation and discussion, students understand that a heated substance expands. Further discussion leads to students understanding how this characteristic can be used to explain how thermometers work. A discussion of how particles transfer energy through collision can help tie together the ideas of temperature as average kinetic energy and how thermal expansion is used in thermometers.
      • Worksheet 1 – Temperature and Motion of Particles
  • Discussion – what is pressure and how is it measured
    • Through discussion and observation, students conclude that gas pressure is caused by the collision of particles with the surfaces of objects they encounter.
      • Worksheet 2 – Measuring Pressure
  • PVTn labs
    • Students collect data to determine how variables affect gas pressure. Experimentation and discussion leads to students understanding the relationships between pressure and volume, pressure and temperature, and pressure and the amount of particles.
      • Worksheet 3 – PTVn Problems
  • Discussion – theories vs laws, KMT
    • Through discussion, students understand how theories and laws are used and expressed in science. Emphasis is placed on how laws make predictions, while theories offer explanations. The tenants of the Kinetic Molecular Theory are discussed through the context of the evidence students gathered throughout this unit. Students come to a consensus about multiple representations (graphs, equations, particle diagrams) of the theories and laws from this unit.

Unit 3: Energy and States of Matter

  • Discuss heat (energy) and temperature (Eureka 6)
    • Through discussion and observation, students understand that temperature is related to the speed of particles, while energy is related to both the speed of particles and the mass of the particles.
      • Energy Reading and Study Guide
  • Icy Hot Lab
    • Students take a beaker of ice and transfer energy in at a constant rate until the contents of the beaker have been boiling for several minutes. Temperature readings are recorded every thirty seconds and a graph of the data is produced. Discussion of the lab further leads to the understanding that temperature and energy are not the same thing, as there are sections of the graph where energy was added to the contents of the beaker, but the temperature did not change. Through discussion, students understand that energy is stored in the motion of the particles (thermal energy) and in the arrangement of particles because of the attraction between them (phase energy). Students draw particle diagrams for different sections of the graph showing the changes the particles are going through according to the macroscopic observable evidence (either a phase change or a temperature change).
  • Introduce energy bar charts
    • Through discussion based on evidence gathered in the experiment, students develop a representation of energy storage and transfer. Emphasis is placed on energy conservation, as well as the macroscopic evidence that is tied to the changes in energy storage.
      • Worksheets 1 and 2: Energy Bar Charts
  • Thought experiment – quantitative energy treatment
    • Through discussion, students understand that changes in energy of a substance are proportional to the mass of the substance and to the change in temperature of the substance. They also understand that different substances transfer energy at different rates, and the idea of specific heat capacity as a constant of proportionality is developed. For phase changes, students determine that the energy needed is proportional to the mass of the substance, and is related to a constant of proportionality for each substance (heat of vaporization or heat of fusion).
      • Worksheets 3 and 4: Quantitative Energy Problems
  • Optional – calorimetry lab
    • I like to have students do a calorimetry lab at some point in this unit so that we can discuss interactions between systems and how we go about calculating the amount of energy transferred between two systems.
      • Worksheet 5 – Calorimetry Problems

Suggested Sequence

Unit 2: Energy and States of Matter

Much of this unit will go unchanged, as we still need all the same model development. However, because we have not yet established any ideas of particle motion, that must be added to the model first.

  • Demo – particle behavior
    • This can be done exactly as the first two activities of the old unit 2 have been done. By the end, students must understand that particles are in motion, and that warmer particles move faster.
  • Temperature vs Energy Lab
    • This is the same lab as the Icy Hot Lab, however I call it the Temperature vs Energy Lab to emphasize what students will be learning. In this new sequence, we have not established any definition of temperature yet, so I can pose the question to students, “Are temperature and energy the same thing?” Evidence from this lab supports that they are indeed not the same thing. This creates the need to develop a way to talk about changes energy causes that are not related to temperature.
  • Introduce energy bar charts
  • Thought experiment – quantitative energy treatment
  • Optional – calorimetry lab

Unit 3: Models of Particle Motion

For this unit, I am suggesting more significant changes because students are now constructing knowledge with a solid understanding of the energy model.

  • Demo – heating solids and liquids
    • In unit 2, students established that transferring energy into a system of particles can cause the particles to speed up (thermal energy) or cause the particles to change their arrangement (phase energy). Here we can use several demos to show thermal expansion – the slight change of arrangement of particles as their temperature increases.
      • Bimetal barScreen Shot 2017-07-27 at 8.11.31 PM
      • Ring and ballScreen Shot 2017-07-27 at 8.10.22 PM.png
      • Density chamberScreen Shot 2017-07-27 at 8.09.57 PM
      • Worksheet 1 – Thermal Expansion
        • Sample questionScreen Shot 2017-07-27 at 8.08.39 PM.png
  • Volume vs Temperature Lab
    • Now that students have seen that solids and liquids both expand as their temperature is changed, we can ask whether gases do as well, and what specific relationship might exist. This leads to an experiment. Students collect data on the volume of gas in a flask at different temperatures. Through discussion, they see that increasing the temperature increases the volume. Understanding of the Kelvin temperature can be reached through this experiment, and students see that the volume of a gas is proportional to its absolute temperature.
      • Video of experiment courtesy of Seth Furlow
        • https://youtu.be/AHOdGtTtvgg
        • In carrying out this experiment, I have each group collect two data points, the volume of gas in a boiling water bath, and the volume of gas in a cooler water bath, as demonstrated in the video. In order to extrapolate a more meaningful trend than what can be seen from two data points, I have students put their flask in different temperature water baths, and then we combine all data on one graph. It should be noted that the change in volume is small, so large differences in the temperatures of the water baths are needed. I use baths that are near fifty and zero degrees Celsius.
      • Worksheet 2 – Volume vs Temperature
  • Discussion – pressure
    • To begin this conversation, I return to the previous experiment and ask students why water entered the flask when it was placed in a cooler water bath. Through discussion, students develop the idea of atmospheric pressure, and they understand that the greater pressure outside the flask pushed water into the flask until the two pressures were the same. Because the pressure was constant, we were able to determine the relationship between volume and temperature. Further discussion leads to what pressure is (particle collisions with surfaces, like what was established in the old unit 2), and we can now ask what variables may affect gas pressure and how they affect it.
      • Worksheet 3 – Measuring Pressure
  • PTVn Labs
    • These labs are carried out and discussed the same way they were in the old unit 2, though the students already know about the Kelvin temperature scale.
      • Worksheet 4 – PTVn Problems
  • Discussion: theories vs laws, KMT
    • This discussion can be carried out much the same way it was in the old unit 2. However, because the students already have a firm grasp on energy, there are opportunities to ask more challenging questions. (This should also be determined based on who your students are. This is certainly best done with students who have already had Physics and understand the relationship between kinetic energy, mass, and velocity.)
      • Worksheet 5 – Applications of KMT
      • Sample questionsScreen Shot 2017-07-27 at 8.09.20 PMScreen Shot 2017-07-27 at 8.09.26 PM
      • Building this foundation now helps students better understand Avogadro’s Hypothesis, which is coming up in unit 4.

Benefits of This Sequence

  • Energy is introduced even earlier, and with a deeper consideration, which can only be an advantage in later units of chemistry when we rely on the model we have built.
  • One of the most pervasive misconceptions surrounding energy in Chemistry is that temperature and energy (when it is called heat) are the same thing. With this new sequence, the very first thing we do to introduce energy is ask if this is true, and then answer that question with solid evidence.
  • The current unit two, Energy – Particles in Motion, can sometimes feel a bit disjointed. We do establish that particles are in motion, and we begin to have an energy conversation, but then we switch gears to talk about pressure before finishing that conversation. For students, there is no apparent reason why we are talking about pressure, or why we started the unit talking about all states of matter but are now focusing on just gases. This sequence presents a more coherent storyline for the unit about particle motion. The flow from thermal expansion to gas pressure, using a Charles’ Law lab, feels much smoother.
  • With the Icy Hot/Temperature vs Energy Lab, students develop the idea that energy causes either a change in motion or a change in position. However, they see with thermal expansion that both changes can occur in a substance at the same time. With the old sequence, thermal expansion is discussed before this lab, and then this lab can seem to contradict the idea of thermal expansion. With this new sequence, the lab happens first, and then later when we discuss thermal expansion, students can refine their understanding of energy.
  • Students can gain a deeper understanding of the Kinetic Molecular Theory if they build that understanding on a solid energy foundation.
  • In unit 4, when discussing types of substances, we rely heavily on Avogadro’s Hypothesis as evidence for the Law of Definite Proportions. The students’ understanding of KMT and the gas laws in unit 3, rather than unit 2, again makes for stronger connections in the overall storyline.
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