MU Chemistry Modeling Workshop: Day 4

By | July 13, 2011

Day 4 was a half day.

Pressure Labs

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!


5 Comments

Tracie on July 13, 2011 at 7:49 pm.

Not having taught chemistry in forever, I didn’t know anything was out of place when I took the class :) It makes perfect sense to me since the motion of gas particles seem to be more obvious and easier to describe. The idea works well when you leave the ideal gas law out of it until later. My kids picked right up on it once they remembered “those terrible graphs we made.”

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gasstationwithoutpumps on July 14, 2011 at 12:16 am.

I was a bit bothered by this distinction between a “theory” and a “law” (I use the terms interchangeably, with the choice of which to use based mainly on historical usage). Newton’s Laws are no more predictive than Einstein’s Theory of Relativity.

Distinguishing between a hypothesis (the prediction made from a model of the outcome of an experiment) and theory (a general rule use for building models) is good, but the distinction described here doesn’t seem particularly useful.

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Ms. Bethea on July 14, 2011 at 6:48 am.

I agree with you, and discussed this in particular for this unit reflection (will include in my Day 5 or 6 post). “Theory vs. law” is a new addition to the curriculum (not in 2008 materials), and its only discussed for about three sentences in the teacher’s guide. I think the goal is to eliminate the misconception that scientific laws are “true” and scientific theories are “not true yet.” But I agree that the purpose for the distinction described (predictive vs explanatory) is unclear.

I appreciate your comments! They’re very helpful as I’m grappling with how, if, or to what extent I’ll incorporate this into my curriculum. Keep ‘em coming!

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Zac on July 15, 2011 at 8:08 am.

I have not yet taken the Modeling Chemistry course, but have taken the Physics, but I have taught from the Chemistry Curriculum. I find it interesting you say people are uncomfortable with teaching gas laws this early in the curriculum. It would be even more interesting to be in the room when you talk about molecular and ionic bonds before even saying the word “electron” or talking about the fact that there are sub atomic particles that make up these particles that you have discussed up until this point.

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Ms. Bethea on July 15, 2011 at 9:26 am.

Our workshop materials discuss electrons before mentioning types of compounds.

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