Day 4 was a half day.
After discussing the Gabel article, we reviewed the topic of pressure (in student mode), and brainstormed what factors might affect pressure on the particle level. While some ideas were impossible to test (weather, elevation), we could test temperature, number of particles, and volume. The instructor introduced the use of the equipment (Vernier Gas Pressure Sensor & LabQuest), and we performed the pressure vs. volume experiment.
The approach to gas laws in modeling is very similar to the approach I’ve used for the past two years (based on a JChemED article by Bopegedera, An Inquiry-Based Chemistry Laboratory Promoting Student Discovery of Gas Laws). We whiteboarded our data for P vs. V, and reviewed the relationship that we observed between mass and volume from Unit 1 (What happens as one variable increases? If you double the volume, what would happen to the mass? What did we call this relationship?). We then compared it to what we observed in the pressure vs. volume graph (If you double the volume, does the pressure double? What does happen? Does this make sense on the particle level?). We looked at several boards, and looked at the relationship between different data points. We observed that the pressure halved when you doubled the volume, and defined this relationship as inversely proportional.
Next we studied pressure vs. temperature, and again whiteboarded the data. We discussed whether the relationship between pressure and temperature was more similar to mass vs. volume or pressure vs. volume (direct or inverse), but noticed that this data set did not come close to passing through the origin. We discussed what was happening at the particle level at our lowest temperature, and that if there was still pressure, the particles were still moving. More questioning (If the particles stopped colliding, what would that mean (no pressure), how can we find out how cold this is? (x-intercept). We then derived absolute zero based on our data, and discussed the kelvin unit of temperature.
Next, a pair of groups whiteboarded particle diagrams that explained what we observed in each of the three experiments (P vs T, P vs V, and P vs number of particles, not tested at workshop). We then discussed similarities/differences between the two groups representations of each.
Again, the previous exposure to data interpretation in the density and volume labs made the discussion of new relationships rich and meaningful.
Theories vs. Laws
We briefly discussed theories vs. laws. Laws were defined as having predictive power (like equations, graphs), while theories have explanatory power. Models can have both predictive power and explanatory power. Also discussed ways to clear up common misconception that theories –> laws (“theories don’t graduate and go to law school!”)
Kinetic Molecular Theory
Rather than giving students the list of the properties of the kinetic molecular theory, students are asked what behaviors in our model account for the observations that we observe. They should be able to derive most if not all on their own with careful questioning.
The placement of these gas laws this early in the curriculum made some participants uncomfortable, but it is certainly intentional. The behavior of gases can be easily explained with our basic model. You don’t need to know anything about atomic structure, nomenclature, or stoichiometry to understand these empirical relationships, and historically, were the earliest concepts discovered. Gases will be revisited once we refine our models to include moles and such.
We finish up Unit 2 on Day 5, and begin Unit 3. Stay tuned!