Gummy Bear Stoichiometry
Lesson 5 of 9
Objective: SWBAT utilize mass to mass stoichiometric calculations to determine the amount of potassium chlorate needed to fully combust a gummy bear.
After doing our Soda Stoichiometry, we spent an extra day just working on practicing the problem setup and mathematics. Then students learned how to do the mole bridge using mole ratios. Today's lesson finds students taking a quiz on the single step mole and mass conversions, and then putting it all together into full mass to mass stoichiometric calculations.
Again, I was seeking a hook to make the process more interesting. I decided to utilize the demonstration referred to as the Screaming Gummy Bear or the Gummy Bear Sacrifice as a pretext for the calculations. Today, we will teach students the process of doing stoichiometry under the guise of figuring out how much potassium chlorate we need to decompose to completely incinerate a single gummy bear.
The plan is to have students work on the calculations today and the first half of the next day, and to finish the calculations and perform the demonstration the following day.
Today begins the full implementation of HS-PS1-7 with students doing mass to mass conversions within a chemical reaction to prove the Law of Conservation of Mass. This lesson also utilizes Science and Engineering Practice 5, Using mathematical and computational thinking.
Moles to Mass Quiz
When students enter the room, I ask them to get out something to write with, a calculator, and a periodic table.
Before passing out the quiz, I explain that there are four problems for them to set up and solve. Students must show their work and units to get full credit. I also explain that for two of the problems, the molar mass was provided to save them time.
I then pass out the Moles and Mass Quiz and let students begin. As students finish, they come up and turn the quiz in and take our Mass to Mass Stoichiometry sheet, and I ask them to begin working on the molar masses at the top of the page.
The quiz key is available here. The most common error on the quiz is forgetting to put the units which causes them to set the conversion up wrong. The second most common is putting the wrong final units on their answer, as seen here.
Since students will finish the quizzes at different times, having a task they can work on while other students finish is important. Students have gotten very good at calculating molar masses, so this is a completely independent activity. By students doing these calculations ahead of time, we can start the next activity all together.
Students are finding the molar masses of fructose, oxygen, carbon dioxide, water, potassium chloride and potassium chlorate.
When everyone has finished the quiz, I ask students to share their molar mass values so students can all have the same starting information.
This student had time to calculate the first two molar masses, and gathered the rest from classmates.
Mass to Mass Calculations
I begin by having students share the molar masses they calculated for our six chemicals. I am projecting the worksheet, and record their answers so all students can copy them. I chose fructose since the gummy bears are made with high fructose corn syrup.
I explain that tomorrow, we will be incinerating the gummy bears. In some classes, students ask why and I explain that in the 1600s, it was common knowledge that bears were spirit animals of witches, and since witches got burned at the stake, that we must follow tradition and burn some evil gummy bears. Students chuckle, mumble about me being crazy, accept that it will likely be a cool demonstration, and get ready to work.
I inform the students that to burn the gummy bears we need a source of pure oxygen, so today we are going to calculate how much oxygen we need to burn a single gummy bear.
We look at the problem presented: "A single gummy bear has 1.118g of fructose in it. How much water is produced after fully burning all the sugar according to the equation above?"
I ask "What is our given information to start setting up our conversions?"
- "1.118g of fructose"
- "How do you know?"
- "It has a number, unit and chemical together"
- "Ok, let's put that for our given. <pause> What are we asked to find?"
- "How much water?"
- "How will we measure the water, what units?"
- "Then let's write 'grams of water' in the 'Asked for' part"
Now I direct them to the chart, I ask what's different about it. The students start to panic a bit and say "Its longer, so this must be way harder" I explain that they have done every part of this process already, today we are just putting the steps together.
We fill in the dimensional analysis like this:
I stress to students that they absolutely have to fill in their units each time, so that they know when to put grams on top or bottom, and when to change chemicals.
I use highlighters to show students how the units need to always end up diagonal of each other to cancel out, same as our single step reactions. I then cover the extra steps and show them how we have a one step mass-mole conversion, a one step mole bridge, and then a one step mole-mass conversion. The only difference is that we set them all up together.
We then repeat the process for the carbon dioxide and oxygen calculations. Each time I give students a little more lead time to work ahead of me and try to set up the next step. If students are getting overwhelmed by the process, I have them use a pair of post-it notes to cover the work before and after the step they are working on so they can treat each part as a one step problem. This helps about 10% of my students overcome their anxiety and build the confidence to tackle the full problem at once.
We then take our mass of oxygen needed, and bring it to the potassium chlorate reaction. I explain that now we need to figure out how much potassium chlorate to decompose to get our pure oxygen. Some students use the previous problems too much as a model, copying the chemicals from the fructose reaction. Once we clear up that issue, students can finish the last two problems. If they do not finish, I give them time to finish at the start of the following day.
The following day, we complete our calculations. Then we go back to the fume hood where I have a large test tube attached to a ring stand via a utility clamp. I show students the setup, and add in the amount of potassium chlorate they calculated. (I add a little extra when they're not looking to give me time to explain parts of the demo).
I then explain that to decompose the potassium chlorate, we need to heat it. I tell them that when the potassium chlorate is decomposing I will add in the gummy bear, or bears. I ask them to say goodbye to the evil little gummy bear witch, and go into the storeroom to heat the potassium chlorate.
Once the potassium chlorate is bubbling, I drop in the gummy bear(s) shut off the gas, close the hood window, and walk around the hood to stand with the students. As the gummy bear burns, it evolves a lot of light and steam, and the vigor of the reaction creates a moaning/screaming sound from the test tube. Students really enjoy the demonstration, and may ask some questions about the reaction. It is good to point out the steam as evidence of the written reaction producing water, as well as the purple characteristic color of potassium in the flame light.
This is a video of the demonstration. I strongly recommend only performing this in a fume hood. Other videos of this demonstration show it in an open classroom.
Once the demonstration is done, we finish the class practicing our mass to mass problems with another problem set. We will do two problem sets before moving on to the Save the Day guided inquiry lab.