MU Modeling Chemistry Workshop Day 5b

By | August 15, 2011

Energy is an integral part of the modeling curriculum. We began to talk about it a bit in unit two, and will extend the model in unit 3.

We started with a discussion of heat vs. temperature (which included viewing a Eureka video):

Next, we did a lab (Icy Hot), where we heated a sample of ice at a constant rate until it boiled. Very straight forward heating curve lab (especially using a Vernier temperature probe), but the discussion portion was very helpful. (My logger pro file)

The most important part of this lab is that students record observations during the lab. Since the computers is doing most of the work (set up to record a temperature twice a minute), students can really focus on observing what’s happening in the beaker.

Afterwards the discussion focuses students in on what’s happening at various points of the graph.

Since heat (energy) is being supplied at a constant rate,

  • Why doesn’t the temperature of the water increase for ~ 14 minutes? Where is that energy going? What do you observe during this time (still ice present)
  • What do you observe when the temperature begins to increase (liquid water)? Where is that energy going?
  • What do you observe when the temperature levels off (beginning to boil/boiling)? Where is that energy going?

Students struggle with explaining/accepting this first plateau (want to blame it on human error, or “it takes time for the energy to get to the water”), but you can note that the beaker is obviously getting hot, and a physical change is observed).

Since we use temperature as a measure of how fast particles are moving, and the temperature does not increase on the plateaus, then the energy has to be used for something else. In our model, the energy is forcing the separate particles apart from each other (which we observe as a change from solid to liquid). Once separate particles are no longer held together by these attractions, then the energy supplied can make those particles move faster and faster (the rapid increase in temperature).  Once we reach ~100 degrees, it levels off again, which means the temperature is no longer changing, so the energy must be going towards pushing the particles apart again.

Students conclude this activity by sketching their heating curves on whiteboards and drawing a particle diagram for 5 different points on the graph.

Because energy can have two different uses, we want to distinguish between the two. For thermal energy (energy that contributes to a change in temperature, we use E_{th}. For phase energy (energy that contributes to a change in phase), we use E_{ph} (Later, we’ll use chemical energy, E_{ch}). Its all energy, E, the same energy, but its being used for different purposes (different accounts).

The modeling approach gets away from the common approach (energy takes different “forms:” potential, kinetic, etc. ), as it is easy for students to assume that there are different kinds of energy, and that energy changes. While it makes sense to most teachers who have an understanding of energy, it can be confusing to students. (“How does the hotplate know when to stop supplying potential energy, and start supplying kinetic energy?”). Instead, modeling uses the metaphor of energy as a substance, as something that can be stored and transferred into and out of different accounts.  We use energy bar charts (LOL charts) to describe the transfer of energy from one account to another, as well as energy flow into and out of the system:

LOL Chart. Get it?

For a example, we could complete an energy bar diagram for the first plateau of the Icy Hot graph (where the temperature didn’t change, but the ice melted) would look like:

You will notice:

  • Energy in the thermal (E_{th}) account does not change from initial to final (1 bar in each, though the size is arbitrary)
  • Energy in the phase  (E_{ph}) account increases (from low energy solid to higher energy liquid).  (solids = 1 bar, liquids = 2 bars, gases = 4 bars)
  • The chemical account is left empty. No chemical change is happening. (though I question why/whether it should be left empty).
  • The energy doesn’t come from nowhere. In the “O” portion, we identify the system (ice) and show the flow of energy into or out of the system. In this case, 1 bar of energy enters the system (from the hotplate).

So, for the the portion of the Icy Hot graph where the temperature of the liquid water does change, the energy bar diagram might look like:

where the initial “L” is the same as the final “L” from before. The added bars of energy goes into the thermal (E_{th}) account, since an increase in temperature is observed. No change is observed in the phase, so the phase account will still have two bars of energy.

For the final plateau, we observed no change in temperature, but a change to gas phase, so the energy bar diagram might look like:

LOL charts are also useful for situations where external energy isn’t being added to a system. For instance, when you leave a hot mug of tea on a table for some time, it will cool down:

Here, we depict two energy bars from the thermal account leaving the system.

The bar charts are a useful way to qualitatively describe what the energy is doing, and connecting it to a particle description matter.

We did some reading about energy, and practiced drawing bar charts for different scenarios. It does take practice to get it (moreso for teachers!) , but writing this post has helped it make more sense to me!

More energy on Day 6!


1 Comment

Carolyn Rost on March 21, 2012 at 8:52 am.

Which Chemistry text do you suggest. We need to update with eBook. Thanks, Carolyn

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