HS-PS1-7: Stoichiometry

During my methods class tonight, a student asked how one could use inquiry-based instruction to teach something like stoichiometry. So, we ran through a little simulation of how I think about teaching stoichiometry using inquiry.  Inquiry is not necessarily limited to investigations with the hands. Inquiry is more about helping kids make conclusions and engage in trying to figure out aspects of the natural world.  Below is a very simple outline of how stoichiometry might be approached via inquiry, but the actual instruction typically took several class periods with kids.

First, I ask students, “What is water made of?” Students know that there is hydrogen and oxygen from previous courses or other sources. Then, I ask, “How are these atoms put together to make water?” Now students tell me to draw an oxygen atom with two hydrogen atoms connected via lines.  (At this point, I might explore molecular geometry using balloons, but that is for another post). Then I tell kids (yes, we do tell kids things in inquiry when the need the information and/or are ready for the information), “If I run electricity through water, oxygen and hydrogen are produced”.  I immediately follow this up with, “What will the hydrogen and oxygen look like?” or “How could I represent the hydrogen and oxygen?” Students sometimes simply want to draw a O and and H and call it good. Here, I have to remind them of their knowledge of valence electrons (we should teach bonding and valence electrons before getting into the more abstract stoichiometry). So I ask, “What would the valence electrons of these atoms be?” Once students draw (or tell me how to draw) the valence electrons, I note/ask, “this is a problem that the valence shells are not full, how are we going to fix this problem?”

Kids sometimes struggle to simply bond the hydrogen to another hydrogen and the oxygen to another oxygen, so I ask them to consider what would happen if we had another water that had been broken apart and draw more H and O atoms for them to work with. Typically, students figure out that they can connect two hydrogen atoms, but sometimes I do have to give them that piece of information. Once they’ve connected two hydrogens, I ask, “How can we use this strategy on the oxygens?” and students quickly connect two oxygen atoms via a double bond.

Now we turn our attention to the equation. I ask, “How could we show what is happening to water from start to finish?” Students often (because of notations we’ve used previously) note that we could use an arrow.  Then I ask the students to explain what should go on both sides of the arrow.  Students tell me to write H2 + O2 <– H2O.  Then I ask, “What is wrong with this equation?  If students struggle, I note/ask, “We know that mathematical equations are typically set as equal to each other on each side of the equals sign. Why are the two sides of this equation not equal?”  Student note that there are more oxygen atoms on the left than the right, so I ask “how are we going to fix this problem?”

Students sometimes want to add a single oxygen atom to the right side of the equation so we revisit the issue of having only one oxygen atom with respect to valence electrons. If students want to add O2 to the right side, other students quickly note that this simply reverses our problem.  While at least one student typically suggests superscripts, I sometimes have to write a two in front of the H2O and then ask, “How does this solve our oxygen problem?” and then ask “What further changes do we have to make now?” to clue students into the need to add a 2 to the H2 on the left.  From here, we revisit conservation of mass when I ask, “What does it make sense the the number and kind of atoms on the left would be the same as the number and kind of atoms on the right?”

Notice how I give students a lot of information during this discussion, but the new information is always followed up with a question to ask students to reflect on the new information or to use the new information in some way.  While we do want students to investigate phenomena directly, sometimes inquiry is a matter of making kid use a new piece of information to solve the next problem.

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2 Responses to HS-PS1-7: Stoichiometry

  1. David says:

    Stoichiometry certainly can be taught through inquiry. Interestingly, when I first started on my journey away from lecture many years ago, stoichiometry was the concept I most believed I would not be able to teach through inquiry. It took me longer to move away from the projector screen on that one than any other. When I finally did, I was shocked at how much better inquiry worked than lecture. I now don’t explicitly even teach stoichiometry. The students sort of “find it” while we are learning about the formation of compounds. Stoichiometry used to be the biggest hurdle for my students to get over. Now it’s like the one given that I know everyone will understand. The key has been spending an agonizing amount of time developing a particle model of matter prior to even beginning to explore chemical change, using the ideas from Modeling Chemistry, which I highly recommend.

    I do wonder about this statement: “we should teach bonding and valence electrons before getting into the more abstract stoichiometry”

    There are many ways students can be guided into an understanding of stoichiometry from observable macroscopic events. On the other hand, no one has ever seen a valance electron. In my mind that makes the latter the more abstract concept, so I never teach those ideas in the order you’re suggesting.

  2. jerridkruse says:

    I completely agree, an overstatement on my part.

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