There are a million different versions of the stoichiometry Alka Seltzer lab out there. And maybe I’m just a contrarian, but I have never done it! This year, I was in need of a simple limiting reagent lab and decided to try the Alka Seltzer lab. The problem is none of the labs online really focused on limiting reagent. So, I wrote my own. Here are the benefits (and costs) of doing the Alka Seltzer lab as a limiting reagent lab.
It creates a real world problem
When you take Alka Seltzer, you drop the tablets in water until the citric acid and the sodium bicarbonate finish reacting. You can show the kids the active ingredients, and it lists the sodium bicarbonate at 1,916 mg. But the problem is that you don’t actually take 1,916 mg! By the time it gets to your stomach, a significant amount of the sodium bicarbonate has been reacted and released as carbon dioxide. The question for the lab becomes, “How much antacid actually reaches your stomach?” By couching the question that way, the students feel like they are fighting the good fight against big pharma. They are standing up for the little guy! (No, I don’t teach my students in regular chemistry about the buffering activities of newly formed sodium citrate. I let them chase the sodium bicarbonate only.)
The limiting reagent is simple to observe
I tell the students that the citric acid is the limiting reagent in our first lab. Then, as a class, I ask how we could make the sodium bicarbonate the limiting reagent. Even though they are very new to limiting reagent, they easily recognize that we would need to add a lot more acid. This is when I explain why we will be using another beaker, but this time with vinegar. Drawings on the board before the lab give them a visual of how the two trials (water and vinegar) will differ. I love the fact that they are changing reactants and correctly predicting differentiated outcomes in the reactions.
The data collection is very simple
We happened to have a shortened schedule the day I did the lab, so we only had 80 minutes. The lab is so simple that explaining, data collection, and analysis fit into the shorter lab period.
The data collection is simple. How many grams of carbon dioxide are released? They figure out that they need the mass before and the mass after. Bam. Data.
Cleanup and safety are a breeze
Pour the medicine down the drain. I do like the fact that I had several students confirm that the chemicals should go down the drain before they poured them. I guess that shows they are paying attention! But yes, medicine and vinegar can go down the drain. There aren’t any particular safety hazards. (Though stabbing that foil wrapper under your fingernail isn’t the most pleasant. So I’ve heard.)
Calculations are simple
Once the students have the mass of carbon dioxide evolved from the reaction, the calculations back to grams of sodium bicarbonate are simple. The mole ratio is 1:1. Since my students are just being introduced to stoichiometry, this was a very appropriate level of math for them.
Applications are hard (the downside)
Once the students got to the application of their calculations is where many started to struggle. As I said, my students have had about one class period introducing limiting reagent. They aren’t skilled in that area at all. Even though they appeared to understand at the outset of the class the different limiting reagents between the water trial and the vinegar trial, once they had to write about it, they fell apart. There were many that did fine, but there were even more that couldn’t articulate or calculate how much antacid actually gets to their stomach. It will have to be something that we revisit.
Trying new things in chemistry is great
A student jokingly asked me after I announced that this was a new lab, “Mr. Koch, you would think after over twenty years of teaching this class, you would have it down by now!” It made me laugh. It also made me proud. I want to be innovative in my classroom. I want to do new things. I will hang on to what works, but I also want to find new and better ways to serve my students. Plus, we teach chemistry. What better place to explore and have fun than chemistry? It’s an honor to be searching for new knowledge along with the kids. So no, I don’t have it down yet. And that is just fine.
I have attached the simple writeup to the resources page. It is not completely ironed out. And that is OK too!
Want to try a new engagement strategy when you aren’t doing a lab? Try Stoich Decks games. All of my students learned about the mole through Up & Atom. I use it as an intro manipulative and then later as a strategic calculation game. Give it a shot.
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