This might be the perfect lab. Kids love to light things on fire. Kids love junk food. I know, let's light junk food on fire! I have used this lab as an introduction to thermochemistry. It gives them something to talk about as well as being competitive in the lab. Please see the Resources Page for a handout.
I feel like I should have a disclaimer for this lab. Students will be lighting chips on fire. I have ventilation over each lab station in my room. If you do not have good ventilation, this might not be the right lab for you, as burning chips do produce smoke. If you are not comfortable managing your class with fire, then this lab is definitely not for you. Common sense will allow students to enjoy this lab and be safe.
The Big Idea
Students create there own system to measure the heat released by burning a chip, They will then compare that value to the nutritional facts on the package. They will need to be taught the difference between calories and kilocalories. Since the entire chip won't burn, I have students compare the energy released per gram of chip burned. The takeaway for kids is that there is a lot of energy in food, it is calculable, and it's a lot of fun to watch junk food burn!
Background for Students
I spend a portion of a period before this lab explaining the concept of specific heat capacity, endo and exothermic reactions, and where energy in chemical reactions comes from. They will need to be able to understand what a calorie is and how to convert to kilocalories. I also spend some time connecting the reaction for cellular respiration to the reaction for combustion. It is neat to see their eyes open wide when they first connect the idea that the cellular respiration reaction they learned last year looks just like a combustion reaction they learned this year! It creates good conversation in the class.
Methods
I prefer to allow students to create their own apparatus for testing this concept. Their job is to calculate the number of kilocalories released per gram of chip burned. Then they compare their results to the values published on the nutritional facts on the label. I have them communicate their results as a percent yield; their kcal/g versus the kcal/g from the label.
I give them limited items and they must create the best system possible. You will be impressed by the variety you see. I do give them basic pointers on the fact that the reaction needs oxygen and also needs to let smoke rise. All of this needs to be accomplished while minimizing heat loss.
I try to give them minimal instruction and let them problem solve. I give them free access to my ring stands, wire stands, tall-awkward-I-don't-know-what-they-are stands. They get one large sheet of aluminum foil (12"x12"). I used to just set the roll of foil out and let them go to town, but they were using six feet at a time.
They also get a small square of foil (4"x4"). The small squares are used for a couple of reasons. The students make a stand to hold the chip vertically. This helps with oxygen access. The foil also catches the ash and grease that come off the flaming chip. That way, when they weigh before and after, they are getting a more complete picture of the mass after. I have them weigh the chip and foil together every time.
I also guide them in how much water to use. Though the instructions allow the students free reign, before the lab starts I guide them through the process. I light one chip on fire in front of the class so they can see how much energy is being released. I ask them what would happen if they used one mL of water. "It would boil away." What would happen if you used one liter of water? "The temperature wouldn't change much." I tell them that anything under 20 mL of water will start to boil and we don't currently know how to calculate that energy. (But we will by the end of the unit!)
Benefits
I have the kids bring in their favorite chips. It connects them personally to the lab, something that doesn't always happen. They joke and tease and then want to try other people's chips. They spend their time fully engaged because they wonder what the next one will do! This lab gives you an opportunity to jump off into any number of thermodynamic conversations. It is a decent, simple anchoring phenomena. You can lead into calorimetry, heat exchange, enthalpy, bond enthalpy, and on and on....
Challenges
Your room will smell like burned chips! You will have to help them use the heat equation and ensure that they understand why we are measuring this lab in terms of kcal/g burned. They might get frustrated that they lost 80% of the energy that was supposed to be produced.
All in all, I love this lab as a way to grab their attention, apply it to something that is already familiar, and then let their brains work to solve problems in real time. Hope it helps you!
Looking to engage your students in stoichiometry? Try Up & Atom! It is a strategic game designed to help students quickly convert among moles, atoms, and mass.
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