I’m doing an experiment. With my students. I introduced the concept of limiting reagent before they even know how to balance equations. I was impressed with the results so far. The goal is for them to start to visualize the physical limits of reactions as they start to form and balance reactions. My hope is that they will have a closer to connection to the reality of matter in- matter out by the time we get to stoichiometry. I will outline what I did.
Here is the link to the lab sheet. It's simple!
Intro the Lab
I rewrote a simple limiting reagent lab that reacts powdered zinc with copper (II) sulfate. In one block period, the kids can complete the reaction, decant the excess aqueous ions, dry the copper and determine the mass produced. Because it is simple and compact, they get to see the entire process in one sitting, which helped.
Veteran Teaching Tip: Feel free to rewrite everything. Every year. Make it fit your needs and the needs of your kids. Just because it worked for you last year, doesn’t mean it’s perfect for this year.
Veteran Teaching Tip #2 As soon as you see an error or change you would like to make, change the file on your computer right away. No, you won’t remember what you wanted to change next year.
I slowly introduced the chemical reaction that we would be using. I showed the reactants on the board. Then I made the kids predict the products. They haven’t been trained at all for this. I am using this lab as the intro. They figured out copper metal pretty easily. And them seeing that zinc nitrate was also made came a little slower. But once we put it on the board and I asked how they knew that they got good answers.
Those are the only ions left.
We didn’t want to break the nitrogen from the oxygen in nitrate.
They are just swapping places.
I gave a brief reminder of the states. Where will the solid be? Where will the aqueous compound be? What will the aqueous compound look like? That way, the students had a mental picture of what would be happening in the reaction.
Make the Lab Competitive
Then I made it competitive. I told one half of the room that they would be getting .01 mol zinc and .011 mol of copper (II) sulfate. To the other side of the room I gave .01 mol zinc and .022 mol of copper sulfate. I told the group B side it is because I like them better. Now, we are going to compete to see which side of the room can produce more copper! The group A side groaned. Group B smirked. I smiled a sly smile because I had them right where I wanted them.
Then they followed the instructions for the lab and completed the reaction.
Veteran Teaching Tip #3: Students may not ask a lab question until they have consulted the lab map (read about those here) and read the instructions. I was reminded of this when a student came up and asked, “Which beaker am I supposed to weigh?”. The first words on the lab map were, “Clean, Dry and Weigh the Smaller Beaker”. I pointed to the board to remind them that there is no learned helplessness happening here. They were embarrassed.
Share Results
As the groups finished, they came to the board and wrote the mass of copper that they produced under Group A or Group B. Shockingly, the mass of copper produced was not significantly different. That allowed me to start asking why.
This is the point where I was the most surprised. They picked it up pretty easily from the balanced equation. They said that there is a ratio that has to be followed. Exactly. A molar ratio. My favorite answer of the day for why the two groups tied was one from a student after we had talked about the charges of the metals changing. He identified the limiting reagent based on the number of electrons that were limited by the amount of zinc. Hallelujah! He sees redox limiting reagent. It makes my teacher heart so happy. And he wasn’t even in my honors class.
My experiment isn’t over. In fact, it has just started, but I am hopeful that by starting with a competitive limiting reagent lab, the concepts of conservation of mass and molar ratios will be easier to grasp in the coming weeks. Fingers crossed.
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