Lesson 10 of 10
Objective: SWBAT balance simple unbalanced chemical equations.
In the previous lesson, students determined how to tell if a reaction was balanced. Today they will learn how to balance it.
This is the heart of HS-PS1-7, using mathematical reasoning (SP5) to show that matter is conserved in a chemical reaction. By balancing the chemical equation, students are correcting the model of the reaction (SP2) to align with the Law of Conservation of Matter. This also represents the middle school level of the Energy and Matter Cross Cutting Concept.
Students will be presented first the mathematical process to balance an equation, then shown a visual modelling method that can help support the mathematics. Our district has decided to keep balancing simple: students will not have to balance combustion reactions, and they will not have to do much with polyatomic ions either.
This restriction is part of our transition to the more conceptual side of the NGSS. We didn't want the mathematics to dominate our teaching, we wanted to ensure the students understood that balancing equations is how we find the way a reaction satisfies the law of conservation of matter.
When the bell rings, I ask students to get out their worksheet from the end of yesterday. We go over the answers, asking for students to explain how they knew a particular equation was unbalanced. Students do a good job of isolating a particular element they found to be out of balance.
They might have struggled a little on 9 and 10, not being sure what to do with the coefficient. Some students incorrectly applied it only to the first element, some distributed it throughout the formula correctly.
Students may have made some errors in the difference between a coefficient and a subscript, so if they did, we fix them now. I remind them that "sub" means under, so the subscript is written below the element symbols.
I ask a student from each table to come up and get a table copy of the Notes Word Bank and a Modeling Balanced Equations paper for each person at the table. For why I provided the word bank, see the reflection attached to this section.
As we go through the notes part, I am asking students to fill them in as we go. I pause for students to think about each term. As this is the last time we are talking about conservation of matter, I expect students to be able to get all those terms filled in.
As we move forward, I provide more information quicker to help us get through, but still count on students to provide the terms "coefficient", "reactants" and "products".
This student had products and reactants in the correct spot, but switched them. She also made two later errors.
We then go through the first steps, counting the number of atoms of each element, and figuring out who is unbalanced. Students quickly recognize the oxygen as the unbalancer. I have students box together parts of a compound, so they treat them the same.
Next we connect to algebra, and show the students why they had to learn the Least Common Multiple their freshman year. We use the idea of the LCM to figure out what to multiply each side by to bring oxygen back into balance.
Now we add the coefficient of 2 to the potassium chlorate, which unbalances the potassium and the chlorine. We repeat the LCM step with those elements and finish the process, writing the final equation.
We move on to the back of the page, and I demonstrate for students how to visually balance the equations by drawing as well. You can see an example in this screencast.
Now I ask students to work with a partner and to use their preferred method to balance the level 1 problems. I pass out mini-whiteboards and markers for students who want to draw out the equations.
As students are ready to begin their practice. I explain that I want them to go in order, that the practice gets harder with each level. I tell them they can check answers with a partner, but-- just like the day before, struggle is natural and to be worked through. I encourage students to help each other, and to ask me if they get stuck.
I point out that if they get stuck, to not stop working, but to go ahead and label how many of each element in are in the rest of the problems so they can have some productive time. I tell them that their goal for the day is only to finish the level 1 problems.
While students are working, I circulate the room. 95% of my students choose not to draw out the atoms, instead relying purely on the math. When some students get stuck, I make them draw out the elements before I will help them, to see if it gets them unstuck. For about half of my students who are struggling, the visuals do the trick and help them connect the math with the molecules.
The most common errors I see are choosing the wrong LCM and splitting formulas up. For example, some students just multiplying the numbers together rather than determine the LCM.
Many students split the formulas when writing the final answer, as seen below.
To combat that, I have them write the original equation with lines to hold the coefficients like below. It helps the process, but students still find ways to alter the formulas.
Students will be working at these for the next three days. Some students will struggle, so I'll introduce the vertical method seen in the work above to track their balancing.
I remind students that it is all about finding a method that works for them, whether it is drawing the atoms, writing the numbers across and working down the page, or writing the elements vertically and working the math horizontally.
As students obtain mastery, I ask them to start helping their classmates. I also provide additional challenge practice if they are willing to take it on.