Balancing Chemical Reaction Equations Using Models

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SWBAT balance basic chemical reaction equations using models to demonstrate conservation of matter.

Big Idea

Atoms are neither created nor destroyed during a chemical reaction; bonds are broken in reactants and the reactant atoms are rearranged to form products with new bonds.

Why This Lesson?

In this lesson, students are introduced to the idea of balancing chemical reaction equations and conservation of matter by using molecular models.  Students are developing and using models (SEP 2) in order to fully understand conservation of matter--in order for a chemical reaction to make products, the atoms needed must have come from somewhere.  They are building the reactant and product molecules, and then determining if they have balanced the numbers of each type of atom.  In building additional molecules (reactants and products) as needed to balance the reaction, students engage in using mathematics and computational thinking (SEP 5).

One Performance Expectation asks that students be able to use mathematical representations to support the claim that atoms, and therefore  mass, are conserved during a chemical reaction (HS-PS1-7).  This lesson directly supports students in meeting that particular PE.


5 minutes

While I take attendance, students do a warm-up activity in their composition Warm-Up/Reflection books.  I use warm-ups to either probe for students' prior knowledge about the day's upcoming lesson or to have them bring to mind and review what they should have learned previously.  (To read more about Warm Up and Reflection Books, please see the attached resource.) 

Today's Warm-Up: "What happens to reacting molecules during a chemical reaction?"

In this case, the warm-up is asking students to think about what is going on during a chemical reaction on the molecular/atomic level.  The warm-up allows students to think creatively about what might be happening that they cannot directly observe.  Until this point, students have only observed what is tangible evidence of a chemical reaction.  It is also preparing students to envision reactions on the molecular level.

If time permits, I walk around with a self-inking stamp to stamp the completed warm-ups indicating participation, but not necessarily accuracy.  On days when there is too much business keeping, I do not stamp.  Students have been told that warm-ups are occasionally immediately checked and other times not.  At the end of each unit, Warm-Up/Reflection Books are collected and spot-checked.  


10 minutes

I pass out ((file)) to students.  I explain that we are using candy today to represent atoms, and with these different types of "atoms" we will be building molecules.  I explain the steps of today's modeling activity while modeling how to do the example reaction myself.  As I am modeling the reaction, I use a document camera and LCD projector to project on the whiteboard so that all students can see what I am doing.

The example equation that I display is:


I also display a smaller version of what is set up on their tables.  They have a large sheet of construction paper to build their models on.  I use a simple piece of paper.  I explain that there are two sides of a reaction equation, as we saw in the day's previous lesson.  I remind students that we call the left side reactants and the right side products.  I show students how to divide the paper in half by drawing a line down the middle and labeling at the top "reactants" and "products" on the correct sides of the line.

I ask students how to build an M2 molecule.  They should understand that two marshmallows would be needed to make that one molecule.  I connect the two marshmallows using frosting as glue.  I explain that for our purposes, we are not concerned about where the bonding occurs right now, we are just modeling the correct ratios of different atoms.  I build an M2 molecule and place it on the reactant side of the paper.  

I continue with an S2 molecule.  Students should indicate that two Skittles are needed to make one S2 molecule.  I glue the Skittles together using frosting and place it on the reactants side of the paper.  

I ask students what we need to make the product molecule.  As they respond, I create an M2S molecule by gluing two marshmallows and one Skittle together with frosting.  Again, I point out that it doesn't matter how they arrange the atoms within this molecule as long as there are only two marshmallows and one Skittle all attached together.

Then I ask if this reaction is balanced.  This is the first time I am using the term "balanced" in this context.  Students will probably not know what this means.  I follow up with these questions (and these expected student answers):

  • How many marshmallows are on the left? (2)
  • How many marshmallows are on the right?  (2)
  • Is that balanced? **I hold my hands up with two fingers raised on the left and two fingers raised on the right to cue the notion that both sides have equal amounts and are therefore balanced.**  (Yes)
  • How many Skittle are on the left?  (2)
  • How many Skittles are on the right?  (1)
  • Is that balanced?  **Again, I hold my hands up, this time with two fingers on the left and only one on the right to cue the notion that both sides are NOT equal and are therefore NOT balanced.  (No)
  • How many more Skittles do we need on the right?  (one more)
  • Can I just add a Skittle to the right?  Not unless the equation says there is a 1 S molecule as a product.  Is there a 1 S molecule as a product?  (No)
  • How can we balance this?  **I pause to allow students to ponder what to do.  (We can add another M2S molecule.)  **I build the M2S molecule and add it to the product side.
  • Are the Skittles balanced now?  (Yes)
  • Are the marshmallows balanced?  (No--there are two in the reactants and four in the products.)
  • How can we balance them?  (Add another M2 molecule to the reactant side.)


I explain that in order for a chemical reaction equation to be balanced, the numbers of each type of atom in the reactants has to be equal to the numbers of each type of atom in the products.  I tell students that they will be working to model the rest of the equations on their handout.  I tell them that as they finish each reaction, they need to call me over to look at the models and verify that they are balanced.  Then, I stamp in the designated area of their handouts and they move on to the next equation.  I tell them that if they choose to eat the models as they finish reactions, I will not stop them.

To finish the balancing, I model counting how many M2 molecules there are--two--and write a number two in the area for that coefficient.  Then I ask students to tell me what other coefficients should go in the equation (one S2 and two M2S).  I tell them the equation is now balanced.

In order to differentiate between honors level and general level students, I have two additional reactions that honors groups will model.

Modeling Reactions

35 minutes

Students work in small groups to build their models and balance the reactions.  As they finish building, I check their work and their models, then stamp when they have the right answer before they move onto the next equation.

If students are having difficulty, I ask questions to prompt them back on track.  Students continue working in their small groups on the modeling task and accompanying questions until there are about 7 minutes left of the period.  Students clean up their group work areas and get ready for their reflection prompt.


Student Reflection

5 minutes

In student's Warm-Up/Reflection Books, students should spend about 3-5 minutes writing a response to the day's reflection prompt.  Prompts are designed to either help students focus on key learning goals from the day's lesson or to prompt deeper thinking.  The responses also allow me to see if there are any students who are missing the mark in terms of understanding.  The collection of responses in the composition books can also show a progression (or lack thereof) for individual students. 

Today's Reflection Prompt:  "In your own words, how do we know a chemical reaction equation is balanced?"

Desired student responses should indicate that:

  • The numbers of each type of atom on each side of the reaction must be equal.