While the applications of limiting reagent calculations might feel obvious to you and I, they aren’t to the students. Often, the students just want to memorize the method in order to get the answer in order to get you off their back. But the concept of limiting reagent is what drives chemistry! It has so many applications to industry and the real world. Wednesday, I sat down to design a new lab where students could manipulate limiting reagent in order to actually accomplish a goal. The result was a ton of fun and the kids started to see the real applications of their calculations. Here’s how I did it.
Jet Boat Design
I used the good old baking soda and vinegar reaction to produce carbon dioxide. I had these old dropper bottles in the lab that I used to make a “boat”. I pulled the dropper tip out of the bottle and then poked a small hole with a dissecting probe on the “shoulder” of the bottle under the cap. On the opposite side of the bottle I drew a large arrow. The arrow showed the kids which side to put up as well as which way the boat travels.
Students would measure a certain volume of vinegar into the bottle and then a certain amount of baking soda into the lid. If they slowly put the lid on while holding the bottle at an angle, the reaction wouldn’t start until they shook the boat. Once they shake it, the carbon dioxide forms and starts shooting the gas and extra vinegar out the hole propelling it down the water trough.
I made a water trough from a piece of metal drip edge cut to 1.8 m long. I hot glued some plastic ends on the drip edge making a trough (yes I was VERY busy on my prep period). I drew a goal area (two lines about .25m apart) with marker that were 1.2 m from the start. The challenge in the lab was for the kids to manipulate the limiting reagent (sodium bicarbonate) to make the boat stop in the goal zone. Here is a video of what that looks like.
The Good
I loved the engagement that I got from the kids. I also loved the number of stoichiometric calculations that they made, but with a purpose. You can only do so many practice problems in class before they are just practice problems. But this time we were solving a problem and being competitive.
The students could see quantifiably, how limiting reagent effected a process in real time. Put less baking soda in, the boat doesn’t go far. I know that seems silly, but honestly many of our students have never actually visualized the impact of limiting reagent.
The Bad
Vinegar isn’t pure acetic acid. In order to do a limiting reagent problem with my first-year chem students, I had to make conversion factor that isn’t standard. I calculated that in 1 mL of vinegar there is 0.00083 mol of acetic acid. This felt a little artificial and the kids were slow to embrace the unorthodox conversion factor.
As the reaction takes place, vinegar is spraying out of the hole. This is why I made baking soda the limiting reagent, but the calculations lack precision because the vinegar wasn’t actually present for the whole reaction. The really particular kids may notice this and argue about the reaction (none did for me, but I was waiting for it).
The Ugly
In my teacher brain I had developed this great application. Hear me out. Using sodium bicarbonate as the limiting reagent, they would calculate the average liters of carbon dioxide needed to land the boat in the goal zone. Then we would have “The Ultimate Challenge” in which I changed the chemicals to sodium carbonate and vinegar. They could convert from the liters of carbon dioxide back into the new reactants to find the perfect recipe and then land the boat in the goal zone on their very first try because they were applying good science. But instead, they discovered due to the new molar ratio between acetic acid and the sodium carbonate, that they would need 16 mL of vinegar. Which is great, except that our boats only hold ten!
This meant that the deep application of limiting reagent was impossible with the setup. I will try rewriting the lab to make that portion work better. But that is for another day when I feel overzealous about creating a new lab design from scratch.
Will I Use it Again?
Absolutely. The kids were manipulating reactions in order to change outcomes. And then doing the math to prove it. That is a huge win toward kids understanding the purpose of limiting reagents. You can download the writeup in its current form here. Please change it and make it better (and then send me a copy!).
I figure if chemistry teachers aren’t trying new and exciting things, then nobody is. Take a risk. Try something new. Write your own lab. Will you fail sometimes? Sure. But other times you will be off to the races! (See what I did there? Jet boat races… sheesh).
Check out this email that I just received from a customer talking about Up & Atom.
"We played with our Up & Atom cards for the first time, and wow! It was slow at first, but the conversations and collaboration was amazing to be a part of... At the beginning of the period were groans and statements like, "I don't want to do moles again!" which moved to, "Oh, I get it now!" and, "Can we play this again tomorrow?" So, thank you!!" -Erica C.
Want to join in a new way of engaging students? Try Stoich Decks curriculum-centric games today!
Do you have sample student data? Also are they using the 22.4 L/1 mole conversion for volume of CO2 or are they using PV=nRT. Thanks so much for this idea!