Chemistry, more like cheMYSTERY to me! – Heat and Temperature

I fell a little bit behind in posting after I finished Unit 2 in class so now it’s time to play a little bit of catch-up!

Here is a quick recap of where we left off in our model so far…

  • Particles are always in motion
  • Temperature is a measure of the average speed of the particles
  • Pressure is a measure of the number of particles colliding with a surface
  • Pressure and volume are inversely proportional (Boyle’s Law)
  • Pressure and temperature in Kelvin are directly proportional (Gay-Lussac’s Law)
  • Pressure and number of particles are directly proportional (Avogadro’s Law)
  • Volume and Temperature in Kelvin are directly proportional (Charle’s Law)

Unit 2 set up the idea of energy that Unit 3 builds off of. Let talk about energy!

Ice Melting Blocks

I kick off Unit 3 with one of my favorite discrepant events, the ice melting blocks. You can purchase these two black blocks from Flinn. I have students pass the blocks around the room and make some observations. Students observe that one block feels heavier and colder. They infer that the blocks are made of two different materials. I then ask the question, “which block will melt an ice cube faster?” Almost every student will say, “the warm one.” I always get a few students who say, “the cold one because the opposite of what I think will happen always happens in this class!” I put a cube of ice on each block and you can guess what happens:

That is indeed the “cold” block on the right. After students get over their amazement, I have them whiteboard an explanation. This leads to a great conversation about energy transfer and whether “coldness” is transferred or “hotness” is transferred. We also talk about thermal conductivity, refrigerators and some other material science applications.

That demonstration sets us up for the Icy-Hot Lab

Icy Hot Lab

The Icy-Hot Lab (from the Modeling materials) is an incredibly simple lab but still absolutely worth doing. Every year, my students have the same 2 misconceptions coming into this unit: the temperature changes during a phase change and the bubbles in boiling water are made of air. The Icy-Hot lab allows me to address both of those misconceptions

The set-up is easy; students heat a beaker of ice until it all melts and then eventually boils. All the while they are measuring the temperature at a set interval (Vernier Lab Pros are great for this). I used Bunsen burners for the first time this year and got much better results than when I used hot plates in the past.

This lab produces a nice heating curve which gives way to a discussion about thermal energy and phase energy. Students must answer the question, “if temperature is not changing during the phase change, then what is?” When students get stuck here, I ask them to draw particle diagrams for a liquid and a gas at the same temperature. That makes it easy for students to see the difference is in the spacing of the particles. The energy is going into to separating the particles! Now, can they do it backwards?

Cooling Curve of Lauric Acid

I tried a new follow-up experiment this year, cooling lauric acid. Lauric acid has a melting point of about 110 °C so I kept a class set of test-tubes in a warm water bath all day and reused them every period.

During this lab, I uncovered a misconception I didn’t even know my students had. When the lauric acid began to solidify, I heard many students exclaim, “it’s freezing, but the test tube is still warm!”  They could not understand that the word “freezing” doesn’t necessarily mean cold. This lead to a great discussion about what “freezing” means and got us to definitions for the terms “endothermic” and “exothermic.” I also introduced energy bar charts (LOL charts as I like to call them) while whiteboarding this lab.

LOL Charts 

Energy bar charts are an awesome way to get students qualitatively representing energy transfers. I have students complete and whiteboard the energy bar chart worksheets from the Modeling materials.

For the second worksheet, I have students play the “Make a Mistake Game” from Kelly O’Shea. During this whiteboarding session, each group must purposefully make at least one intentional (and as many unintentional) mistakes as they want. It is then up to the class to find the mistakes and ask questions to correct them. As each group comes up to present, I assign a group still sitting as the main questioning group. Other people in the class can pipe in but I want to hear from as many students as possible.

Now that students have a qualitative representation of energy transfer, it is time to put a number on it.

Specific Heat of Copper Lab

I introduced specific heat this year using the balloon and flame demonstration. If you hold a flame under a balloon, it will immediately pop. If you fill a balloon with a little bit of water and do the same thing, the balloon remains intact. I asked my students to explain this phenomenon and they were able to conclude that water must have some special property that allows it to absorb more heat than air. I then ask students what else might affect the quantity of heat transferred into a system. After some probing, students come up with changes in temperature and mass which gives me the chance to introduce Q=mcΔT.

Then, students complete a pretty traditional specific heat of copper calorimetry lab. I am not in love with this lab and I think I am going to take it in another direction next year, just not sure where.

The data from this lab are usually pretty good so it does allow for us to discuss what this number with the crazy units means. I’m just not convinced that the students understand the math they are doing well enough to make it worthwhile.

Calorimetry Calculations

Now that student have been introduced to Q=mcΔT, they can use it to quantify heat transfer. I have students complete the worksheet from the Modeling materials which off simple asking for Q and then gets trickier when students have to solve for things like final temperature. Once students have temperature changes under their belts, we can get into phase changes.

Heat of Fusion Lab

I introduce the concept of “heat of fusion” with another traditional lab, but this one I like a lot. In this lab, students are challenged to determine how much energy it takes to melt ice by assuming that all the energy that goes into melting the ice comes from the warm water they are stirring it into. Students add ice to a styrofoam cup of warm water until no more ice can be melted. They then measure the change in temperature of the water and the volume of ice melted. Using those 2 numbers, students can calculate the quantity of heat that went into the ice and then use the mass of ice melted to figure out the heat of fusion. Again, students get pretty good data from this lab.

Heat of Fusion/Vaporization Calculations

I use the worksheet from the Modeling materials for heat of fusion/vaporization calculations and it is pretty short. To make whiteboarding more interesting, I came up with a new technique I dubbed the “gallery lot.” It is a hybrid between a gallery walk and a parking lot. Each group creates a whiteboard for a problem and props it up around the room. The rest of the class must tour the room and leave post-it notes on boards they have questions about. We then look at each board and give each group a chance to answer the questions posed to them.

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Combined Calorimetry Calculations

Once students have mastered calculating energy transferred during a temperature change and phase change individually, it is time to combine them! I approach this hurdle by having students break down the process for a simple situation: ice at -10°C is melted, heated to boiling and boiled away. First, students construct the heating curve. Then they figure out which equation they need to use for each region of the curve. Finally, they plug in the measurements and add up all the Qs.

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Students then complete the worksheet from the Modeling materials that goes with this topic and we whiteboard it the next day. I would like to say that these calculations are a breeze after that, but that would be a lie. I tell my students this is the top of Unit 3 mountain and they will get a little bit of a brain break in Unit 4. To get to the very top of the mountain, I give my students a Lord of the Rings inspired challenge problem where they must calculate the amount of energy needed to melt the gold statue in Desolation of Smaug.

With all of that covered, there is only one thing left…

Unit 3 Practicum

The main reason I keep the traditional specific heat lab I mentioned earlier around is for the practicum I am currently using. Using the same lab procedure from the specific heat lab, I challenge students to use copper shot heated to 100°C to raise the temperature of room temperature water to 25.0°C. Students need to rearrange their equation from the lab to solve for mass of copper.

I like the challenge of this practicum but I think it is too narrowly focused. Next year I think I will give students an energy transfer to measure and they will have to construct the LOL chart and temperature-time graph for the scenario and calculate the quantity of heat transferred.

Unit 3 is hefty! Let’s sum it all up with the model so far…

  • If temperature is a measure of the “hotness” of a system then heat is the quantity of “hotness”
  • Heat can go into a system (endothermic) or flow out of a system (exothermic)
  • Heat can be stored in 2 energy accounts: thermal and phase
  • A change in thermal energy means a change in particle speed and is shown by a slope on a temperature-time graph
  • A change in phase energy means a change in particle spacing and is shown by a plateau on a temperature-time graph
  • The quantity of heat transferred during a temperature change can be calculated using the mass, specific heat and change in temperature for the system
  • The quantity of heat transferred during a phase change can be calculated using the mass and heat of fusion or vaporization for the system

Don’t worry, we will cool it down a bit in Unit 4 (see what I did there :0).

 

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