Balancing Chemical Equations

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Objective

Students will be able to explain the law of conservation of mass and they will begin to be able to balance chemical equations.

Big Idea

Atoms are not destroyed or created in chemical reactions; they are rearranged. This means that a reaction must have the same amount of atoms of each element on both sides of a chemical equation.

Introduction

In this lesson students learn two things. First, they begin to learn how to balance chemical equations. Certainly, they are taught how to balance chemical equations, but the learning will come after students have had a chance to practice this skill. Some students learn the skill more quickly than others, but practice is the great equalizer.

Second, they learn the skill that I call articulation. By articulation, I mean that I want students to be able to write about how to do something. Balancing chemical equations is a skill that some students can do but they cannot explain how to do it. My point to the class today is that if you can articulate how to do something, you will actually be more likely to truly learn the skill because putting what you know into words uses a different set of skills and a different part of the brain. By articulating, a person makes more connections between neurons in the brain that are associated with the skill.

This lesson aligns to the NGSS Disciplinary Core Idea of HS-PS1-7. Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction. Most of the focus today is on the skill of balancing reactions, but I do note that the reason for having a balanced chemical reaction is that chemical reactions rearrange atoms and so whatever number of atoms you start with you end with as well.

Similarly, it aligns to the NGSS Crosscutting Concept of Energy: The total amount of energy and matter in closed systems is conserved.

The lesson aligns to the NGSS Practice of the Scientist of Using mathematics and computational thinking because it uses mathematical representations of phenomena to support claims. In this case the claim is the law of conservation of mass. It also aligns to the Practice of Obtaining, evaluating, and communicating information because students are asked to put a significant amount of effort into articulating the procedure they use for balancing equations.

In terms of prior knowledge or skills, students should have a basic understanding of how coefficients and subscripts are used to describe atoms of each element, as described in this lesson.

There are no special materials needed for this lesson.

 

 

Do Now/Activator

10 minutes

Do Now: I begin class by asking students to read a section of their text book, similar to the content found on this webpage, and answering these questions:

  1. What is the relationship between a chemical reaction and a chemical equation?
  2. What are products and reactants?
  3. Use these three words to describe chemistry: atoms, compounds, and reactions.
  4. What do the symbols + and à mean in a chemical equation?
  5. What is the Law of Conservation of Mass?
  6. How does the Law of Conservation of Mass relate to balancing chemical equations?
  7. What is an unbalanced chemical equation? How does it break the Law of Conservation of Mass?
  8. Use the words subscript and coefficient to explain how to balance chemical equations.
  9. What number do you never write as a coefficient?

I reason that this is a good way to start class because it will tune students into the material that we will work with today.

Activator: I give students a chance to discuss their answers at their lab tables before cold-calling students to share out answers:

  1. A chemical reaction is represented by a chemical equation.
  2. Products are the chemicals you end up with and reactants are the chemicals with which you start.
  3. Chemistry is the study of how atoms and compounds rearrange in chemical reactions.
  4. The symbol + separates reactants from one another and products form one another, while the symbol à means produces.
  5. The Law of Conservation of Mass states that atoms do not get destroyed in chemical reactions, only rearranged.
  6. The Law of Conservation of Mass is symbolized by a balanced chemical equation.
  7. An unbalanced chemical equation has unequal numbers of atoms for each element on the reactant and product side. It breaks the Law of Conservation of Mass because it shows the creation or destruction of atoms from a chemical reaction.
  8. Subscripts cannot be changed when balancing equations but coefficients can.
  9. The number you never write as a coefficient (or a subscript) is the number 1.

I have chosen this approach because it breaks up the content that I need to teach. Students can be active teachers during this part of the lesson, which will reduce the amount of time I am the only voice in the room; when I am modeling how to balance a chemical reaction with a think-out-loud, this will be a new activity and helps students to focus more than if I had taught all the other material using a lecture format.

 

Mini-lesson and Guided Practice

15 minutes

Mini-lesson: Projecting my work with a document camera, I start with the chemical equation for the burning of methane: CH4 + O2 --> CO2 + H2O.

I ask if the equation is balanced, and some students see that it is not, while others are unsure. For those students, I show how to keep track of atoms on both sides of the equation using a table. Students have to count how many atoms of each element exist on each side of the equation. The skill I really emphasize is to add up all the atoms for each element on each side. Oxygen is particularly challenging for some students. They may need to write down how many oxygen atoms are in each compound on the reactant side and then add those numbers together, especially once we start adding coefficients to the mix.

Element

Reactant

Product

C

1

1

H

4

2

O

2

3

 

This reaction is not balanced because it does not contain the same number of atoms for each element on both sides of the equation. I explain as I think out loud that I will try to balance the elements that are in only 1 compound on each side first because this is a good strategy. Seeing C is already balanced, I will move on to balance H. I must add a coefficient so that the H’s are balanced. The question I have to ask is “What times 2 will give me 4?” I remind students that coefficients are multiplied by subscripts to get the total number of atoms for an element. They answer that 2 x 2 = 4. Now my table looks like this:

Element

Reactant

Product

C

1

1

H

4

4

O

2

4

My reaction now looks like this: CH4 + O2 --> CO2 + 2 H2O. I note that when I put the 2 in front of the H2O this not only changes the H but also the O. I remind them that the O is in 2 places, and so I have to figure out how many O’s are in each compound and then total these amounts. I then notice that everything is balanced except for the oxygen atoms. This is an easy fix because O is alone on the reactant side. My final table and equation look like this:

Element

Reactant

Product

C

1

1

H

4

4

O

4

4

CH4 + 2 O2 --> CO2 + 2 H2O

Guided Practice: I then ask students to try to balance an equation for themselves:

The problem I give them is H2 + O2 --> H2O which as a balanced chemical equation looks like this:

2 H2 + O2 --> 2 H2O

I chose this approach so that students can gauge their own comfort level with the procedure and discover questions or misconceptions they have about the procedure. 

 

Application

25 minutes

Student Activity: All students at this point are asked to complete problems a-m in class from this balancing chemical equations problem set. I selected these problems because I do not want to overwhelm students but I want to give them enough to practice. I also want to give time and space for the articulation exercise that follows.

During this portion of the class about two thirds of the class works independently or in small groups, and about a third of students begin this section of class working with me for more guidance. I use small white dry erase boards with this group because then they have plenty of space to write out their work, and erasing coefficients is quite easy with this tool. 

I think it is important to let students decide how they want to learn. Some are more comfortable in trying to figure it out for themselves, while others want more support. They are the best judge of how they learn best, so I try to let them decide. If a student does not get help and does not seem to make any traction, I check in with the student to make sure he or she is feeling ok, and sometimes that is all it takes for the student to then reach out to me or a peer.

I want students doing this work because practice is the key to this skill. It is rather like many things in life; a person can learn the skill, but practice leads to mastery.

Catch and Release Opportunities: After most students have gotten traction as evidenced by correctly balancing several problems, I stop them to talk about the need for articulation. I explain that articulation is the term I am using for the process of putting their ideas about how to balance equations into words. I explain that if you can articulate how to do something, you will actually be more likely to truly learn the skill because putting what you know into words uses a different set of skills and a different part of the brain.

I also not that by articulating, a person makes more connections between neurons in the brain that are associated with the skill. I note that while this may feel awkward at first, by practicing articulation students will become more adept at analyzing and expressing what they are doing, and that this is a life skill as much as it is a chemistry skill.

Here is one student’s attempt at written articulation for how to balance chemical equations and a video in which the same student articulates how to balance chemical equations verbally. I believe that making students think about their work to the point that they can explain it to others is a key measure of deeper learning.

 

Debrief

10 minutes

To wrap this lesson up I ask a student to show their work for problems a-m. I note that there should come a time in the near future when they should not need an answer key because if they are checking their work, paying attention to detail, and doing their math right, that balancing chemical equations is a self-correcting process; you either have an equal amount of atoms for each element on both sides of the equation, or you do not.

Some students have completed problems a-m, while others have not. I believe this is a skill that will take more than one class period to master, and so I envision providing more practice problems for students in the future.

I also spend a little time asking students to discuss why articulation is important. Here is video of some students explaining their opinions on the subject.