We all have them, favorite labs. You know the one that you can’t wait to do again next year. The Rocket Lab is that one for me. I will explain how this useful lab can be used to introduce empirical formula or stoichiometry. Even after the unit is over, I find myself referencing that lab and the kids remember! I am sure that there are kits online but the method I am using came in honor of my chemistry teacher, Mr. Gantz. I remember loving the rocket lab in his class in high school. And like most high school kids, I didn’t take copious notes on how to set it up. By the time I decided to be a teacher, Mr. Gantz was unable to communicate with ALS. I went to visit him before he died. He could make some guttural sounds but couldn’t really talk or write. I told him that I was grateful for his influence and that I would figure out how to do his rocket lab in his honor. So, Mr. Gantz, now there will be schools all over hearing about your lab! Hats off to you.
The Concept
The rockets are actually just disposable pipet bulbs with the stem cut off, leaving about 0.5 cm of stem on the bulb. The rockets are filled with water and then use gas displacement to fill them with mixtures of hydrogen and oxygen gas. I make my labs competitive, so down the hall, I have stretched a tape measure and placed markers for the class record, the school record, and the “world record”. I keep the world records from year to year, so it is an all-time competition. The current world record is 9.80 m. Competition is very important in this lab.
The first goal of the lab is to get the correct ratio of hydrogen to oxygen. Every year, I am baffled that kids don’t write the balanced equation and see the 2:1 ratio. Or even just look at H2O and see the same ratio. But they try all sorts of various combinations. I tell them that they can try any combination, as long as they record the data.
The graph that we ultimately make on a shared data document is % hydrogen versus distance. Over the course of a class, they will create a beautiful bell curve, which generally peaks really close to 66% hydrogen. We will insert the quadratic line of best fit and calculate the best percentage of hydrogen.
The ”Aha” moment comes when they see that the records are almost always set at a 2:1 ratio, the class data suggests a 2:1 ratio, and the formula that they knew all along suggests a 2:1 ratio. This allows them to see a direct connection between moles, which they may still be learning, and the real world. I love this connection and what happens in their thinking. This allows me to use this concept in a number of different ways. I can use it to reinforce mole ratios in empirical formulas (which I will do this year), I can use it to reinforce balanced equations in stoichiometry, I can use it to explain limiting reagents. You can put it wherever you need!
The Set-up
Each group will need to make an oxygen generator and a hydrogen generator each in their own test tube. Have them make the hydrogen generator first, as that reaction happens much more slowly. I have 3 M hydrochloric acid available. Yes, that is high concentration and dangerous. But I have found that it needs to be in that approximate concentration to produce hydrogen fast enough. Plus, they only handle it twice; once to fill the test tube 1/3 full and once to empty it. Each group will add approximately 1/3 of a test tube full of HCl and then drop in a small piece of zinc (1cm x 1cm). If the reaction is going too slow for them, have them add another piece of zinc. Then they put a rubber stopper with a hole to plug it. This is where the lab starts to turn redneck. I have the kids put tape around a capillary tube until it seals in the hole in the stopper. It needs to be a tight fit. This give the gas a tiny tube to escape from the reaction chamber. Once the hydrogen generator is up and running, they can put it in a test tube rack and it will run for a long time.
The students will then make an oxygen generator. In the same sized test tube, they add 1/3 full of standard 3% hydrogen peroxide. To the hydrogen peroxide, they will add a tiny amount (1/2 of a pea) of potassium iodide (s). This will catalyze the hydrogen peroxide to break down creating oxygen. Cap the test tube with the same rubber stopper and capillary tube apparatus. Now they are ready to collect their gases.
The Competition
The students fill their rockets with hydrogen gas first, estimating the percentage of the volume filled with hydrogen. Yes, it could be more quantitative, but I have got the results I need from their estimations. Then they will fill the balance of the volume with oxygen, leaving a little plug of water to seal the rocket. Then carefully, holding the rocket vertical, they bring it to the launch pad in the hall. The launch pad consists of a metal test tube rack supporting (and I’m not making this up) a broken dental probe. The dental probe is metal and has a tapered tip that the pipet bulbs slide right on. Before the lab, I prepared all the rockets by taking a thumbtack and poking a small hole right above where the metal dental tool will be. The launch happens when I use (and I’m not making this up) a piezo electric striker that I tore out of an old barbecue grill. The spark shoots through the hole, passing through the hydrogen and oxygen on the way to the dental tool. This causes an explosion, and the rocket launches down the hall!
Kids cheer, or jeer, their launches. They go back into the class and record the percent hydrogen and the distance the rocket traveled on the shared document. Then they fill their rocket and launch again, this time with a new ratio. Once they get into the groove, data streams into the document, while the kids think they are just trying to beat their friends. Every time a class, school, or world record is broken, I make an official announcement to the class to up the intensity. They love it.
I spend one block period (90 minutes) collecting data. The next 50-minute class will be used to analyze and interpret the data that we gathered. From that point, I get to decide how to utilize the data they gathered to connect it to the content they are learning in class. It is a highly entertaining and effective lab when kids are just headed into stoichiometry. I hope you enjoy it, and I know that Mr. Gantz is looking down and smiling.
Looking to add more ways to have your students interact with the mole without taking notes? Try Up & Atom, the game that helps kids convert among atoms, grams, and moles in a strategic game. Kids enjoy it, and it gives a great foundation for stoichiometry in the future. Give it a try.
