MU Modeling Chemistry Workshop Day 6a

On Day 6, we begin to take a quantitative look at energy. We used the LOL charts to describe the qualitative role of energy in phase and temperature changes.

While we can’t calculate the amount of energy in each of the accounts initially, or at the end, we can measure the energy entering or leaving the system, Q.

We used a few scenarios/mind exercises to get to the idea of specific heat:

Suppose you had two identical cubes of Substance A

If you wanted to increase the temperature of the first cube by 5K and the temperature of the second cube by 10 K, how would the energy needed compare? (since everything else is identical, it will take twice as much energy to heat the second cube by twice the amount).

Consider two cubes of Substance A that are different sizes:

If you wanted to heat increase the temperature of both cubes by the same amount, how would the amount of energy necessary compare? (Since the second cube is twice as massive, will require twice as much energy to heat increase the temperature by the same amount as the first cube)

Finally, if you had two cubes of equal mass, but of different substances, and wanted to heat them increase their temperatures by the same amount,

how would the amount of energy required compare? There’s not enough information! So specific heat is introduced as a way to take the nature of the substance into account . If we assign arbitrary specific heat (C) values to each:

You can use cancelling units to help determine just how much energy will be needed in each case.

2\frac{J}{gK}\times{50 g}\times{10K}= 1000 J needed to heat substance B

Compared to Substance A, which will need half as much energy. You can introduce Q=mc\Delta T equation here, or just use the unit reasoning shown above. If I use the equation in my class, I’d rather have the students come up with it.

After this introduction, we whiteboarded some problems from two worksheets from the unit, making intentional mistakes so that we can observe how the discussion is led. The idea is to keep students as far from “plug and chug” as possible, and not divorce the equation (if used) from the meaning.

Since we did not observe temperature change during phase changes, it should be clear that we will need something else to determine energy for those situations (H_{fusion} and H_{vaporization}). For evaporation and condensation, it is useful to go back to the LOL charts:

Note that energy isn’t entering or leaving the system, but energy is transferred from the thermal account to the phase account to go from liquid to gas phase. This is why our bodies don’t have to reach the boiling point of water for our sweat to evaporate.

Doesn’t the LOL chart perfectly explain why we call it “evaporative cooling?”

We then discussed what we liked and didn’t like about unit 3, and the modeling approach to energy. The first day of bar charts, there was a HUGE learning curve among all of the participants, but by the second day (and through the subsequent units) we all became much more comfortable with it. I’m going into a great deal of detail on energy, primarily because its helping to cement the concept in my own mind (and because I think its really cool!).

It would make sense to include a specific heat lab, and a heat of fusion lab in this unit, so time permitting, I hope we’ll do those.

Next, we started Unit 4. It makes sense to split posts, based on the content, so stay tuned for Day 6b!

3 thoughts on “MU Modeling Chemistry Workshop Day 6a”

  1. In the second and third examples for building the specific heat idea you use the language “if you wanted to heat the two samples by the same amount” to mean raise the two example by the same temperature. You then indicate that it’ll take different amounts of energy due to the larger mass or different internal structure of the different substances.
    I think this presents certain linguistic issues in students still confounding heat and temperature unless you explicitly say “raise the temperature by the same amount.”
    Just a thought!

    1. Ha! Of course you’re right. I frequently made this mistake during the workshop. That’s one problem with looking back at month-old notes, I suppose. I’ll fix that.

  2. This is so helpful – thank you! I was confused with the LOL diagrams and you really cleared it up for me (as I set out to teach these this year).

Leave a Reply

Your email address will not be published. Required fields are marked *