Cue the ominous music. Stoichiometry is coming! Every year, chemistry teachers of all shapes and sizes await the coming of stoichiometry. Some of us waiting with anticipation. Mostly because I like to watch math accomplish things. Please don't tell your math friends my little joke that I slip into the beginning of school each year.
Science is just math, but with a purpose!
And some of you await the coming of stoichiometry with fear and trepidation. The students will be confused. They don't know why they are doing what they are doing. They get so bogged down in dimensional analysis that they don't know which way is up. I am going to offer a couple of tips on the way that I help my students, particularly the "visual and kinesthetic learners". It starts in August.
1. The Metric-English Doorways
During the first unit in chemistry where we begin to discuss units and measurement, I introduce a skill that will pay off in stoichiometry. I am going to assume that most of you teach some form of dimensional analysis to use while converting units. Obviously, the metric system is superior, but I still teach how to navigate between the measuring systems. And this is where it gets visual. On the inside of my door, I post a sign that says "2.54 cm". And another that says "454 g". They are on the side of the door facing into the classroom. Obviously, we all teach metric in our science classrooms, so I make sure the students know that in this room, we speak "metric". Once you walk out that door, we speak English. And so on the outside of my door I post "1 inch" and "1 pound". Here is an example problem that we might see early in the year.
Convert 431 feet into mm.
I ask the students where we are at. Are we starting in the hall where we speak English or in the science room where we speak metric? Clearly, 431 feet is English. Therefore, we are somewhere down the hall. Now before we can walk into the metric system, we need to walk to the doorway. So while converting from feet, we must first go to inches. Once we have converted to inches, we are now standing just outside the door. Still in the English world. In order to enter the room we have to use the doorway. On the outside it says 1 inch. And on the inside it says 2.54 cm. Therefore the conversion factor is 2.54cm / 1in. NOW we are standing in the classroom where metric is spoken (phew we made it). However, the question asked us to convert to mm. So I ask the kids, by walking through the door are we in mm? No. Therefore, we have to walk somewhere else. To mm.
Utilizing the visual and spatial cue of walking through the metric-English doorway really seems to help kids recognize why we are doing the dimensional analysis patterns. (By the way, if you would like a nice clean metric conversion chart, click on the link to sign up for Shorter Than A Shih Tzu game. The Stoich Steps are included in that game and are really helpful.)
2. The Mole Kingdom
Fast forward to early stoichiometry. Students are just learning about the mole. It is baffling and confusing (though it really shouldn't be). We do the rocket lab to learn about moles. Then we play Up & Atom to start to estimate mole values in our heads as well as converting among moles, particles and mass. Then we start converting for real. And the kids, once again, lose track of where they are at. Years ago, one of my students who had gone off to college came back and told me about this idea her professor had used; The Mole Kingdom. She explained it to me. She drew it on the board. I listened politely. And then promptly blew off the idea as too fluffy or gimmicky for me. And just this year, I brought the idea back and connected it to the doorway idea. And the kids actually used it! It helped their understanding. The students in my lower caliber chemistry class really appreciated it. Soon, you will be able to download a poster of it on this website, Until then, you will have to put up with my sketch.
There are four areas on the map. The only place connecting all the lands is the Mole Kingdom. It sits on an island. There are bridges from Particle Land to the Mole Kingdom and from Gram Land to the Mole Kingdom. There is also a chain that connects to a hot air balloon that we call Gas Land. The only way to move from one land to another is by passing through the mole kingdom utilizing the bridges.
Here is an example problem:
How many water molecules are contained in 36 g of water?
I ask the students what land are we in? 36 grams would fall in Gram Land (duh.) Therefore, in order to get to Particle Land, we must pass through the Mole Kingdom. Since we have already played Up & Atom, we know how to convert to from grams to moles (by dividing by the molecular mass). Now we are in the Mole Kingdom. Since our goal is to get to molecules, we must get out to Particle Land. Again, students have already been trained on how to convert moles to molecules (by multiplying moles by N). Each of the elementary steps is not anything earth shattering or hard. But part of the challenge of starting stoichiometry is the layering of skills. This map gives them a visual representation of "where they are". It feels a lot like the doorway from the beginning of the year. They know where they are and where they have to get. Now they can more easily and visually develop a plan. In my example, I didn't mention Gas Land. I use Gas Land for converting using molar volume of gases at STP. (22.4L / mol @ STP)
Even if you don't use these exact strategies, please consider how to help your students visualize mathematical plans to convert units. If they see it, they can do it.
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