There’s a Murphy’s Law-style saying that goes, “There’s never time to do it right, but there’s always time to do it over.” When it comes to teaching a high school lab science course, I would say that the opposite is true: “There’s always time to do it right, but there’s never time to do it over.” If you don’t succeed the first time, the teacher tells you what was supposed to happen and you move on to the next topic. (To be fair, the Mass. frameworks have so much that we have to cover that this approach is pretty much necessary in first-year chem classes.)
My Chem II class (mixed AP and college prep) is studying thermochemistry and thermodynamics. When we got to calorimetry, they loved the term “bomb calorimeter” (which for my non-chemist readers means a calorimeter designed for measuring the heat produced by a chemical reaction–usually involving combustion of a hydrocarbon). So I offered that the lab for the topic could be to build and test one, which they eagerly pounced on.
Here is a diagram of a bomb calorimeter, from Britannica.com:
Our calorimeter was a simplified design, using a styrofoam coffee cup and an empty soda can. We put water in the styrofoam cup with a thermometer in the water, and we put the can into that. The kids made it work from there.
The reaction was combustion of ethanol:
C2H5OH + 3 O2 –> 2 CO2 + 3 H2O
I had them work through the calculations–use PV=nRT to find the moles of air in the soda can, multiply that by 0.21 to get moles of oxygen in the can, and then use stoichiometry to calculate the moles of ethanol needed to react with it. (I recommended that they use excess ethanol, so that the known volume of oxygen would be limiting.)
Then I had them calculate ΔHr/mol for the reaction and multiply that by the number of moles of ethanol predicted to react. Finally, I had them use q=mCpΔT to calculate the predicted temperature rise of the water.
The first time they tried it, the hot air from combustion rose, so the top of the can got hot, but the water didn’t heat noticeably. So one of them got some clay from an art teacher and rebuilt the calorimeter with the soda can and water sealed with clay, and inverted it.
The second time, they forgot to mass the water in the calorimeter. They wanted to stop at that point, but I had them continue doing the rest of the experiment anyway, as a shake-down.
On the third try, they made everything work. We had predicted a ΔT of about 3°C, and we actually got a ΔT of about 1.5°C.
The kids really enjoyed the experiment, and they probably learned at least as much during the debugging process as they did during the first attempt. One of them commented that she had never in any of her previous science classes had the opportunity to re-do an experiment that hadn’t worked the first time.
As one of my classroom signs proclaims, “You can’t learn effectively without the freedom to make mistakes.”
