Balancing Chemical Reactions--Part 1

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Objective

SWBAT balance simple chemical reaction equations using a charting method learned in class.

Big Idea

The numbers of atoms of each element on the left side of the chemical reaction equation must equal the numbers on the right side; Subscripts cannot be changed to balance.

Why This Lesson?

In the lesson immediately previous to this one, students were introduced to the idea of balancing chemical reaction equations and conservation of matter by using molecular models.  This lesson asks students to take their previous experience using the molecular models to balance chemical reaction equations and be able to balance reactions now using mathematics and computational thinking (SEP 5) without the models.

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.


Warm-Up

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: "How do we know a chemical reaction equation is balanced?"

In this case, the warm-up is asking students to think about what they learned during the previous lesson about conservation of matter--that the number of atoms of each element must be equal on both the left side of the equation and the right side.  Previously, students used molecular models to help visually balance chemical reaction equations.  Today, I want students to apply what they already know to a step-by-step process used to balance chemical reaction equations without using molecular models.  This warm-up asks students to articulate what those steps might be.

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.  

Introduction to the Activity

10 minutes

I explain to students that today we are going to look at chemical reaction equations in terms of real chemical reactions, using chemical formulas, as opposed to the modeling we did in the prior lesson (Balancing Chemical Reaction Equations Using Models).

I use a simple equation as my example and explain how to balance it using a charting method that I demonstrate on the white board.  Here is an example of the method I teach:

Then, I hand out the Balancing Chemical Reactions Practice 1 sheet to my students.  I ask them to solve the first one by following the steps as I tell them to.  These are my directions:

1. Separate the reactants from the products.

2. List the elements that appear on the reactant side, then copy that list (in the same order) to the product side.

3. Write the number of atoms that appear on the reactant side for each element.

4. Do the same for the product side.

5. Choose an element to balance, making sure that you apply the coefficient to any elements that are also bonded in that particular molecule.

6. Continue balancing until your numbers of atoms for each element are the same on the reactant side and the product side.

 

I ask students who want to continue balancing on their own to keep working and to help each other if they get stuck.  I then walk through completing the next equation for the students who do not feel confident and who are not understanding right away.  Usually, students will catch on, but those who need additional help are the ones I will work with separately at a lab station as a large group.

Independent Work

35 minutes

Students continue working on the handout and I work with the students who need more help.  Once I have my group of students receiving extra help working sufficiently on their own, I circulate to see if the rest of the class is still working well independently.  At this point, I am able to see which students really excel and who might be able to explain how they are doing the work.  

I ask students who are flying through to tell me what they are doing and I might follow up with questions like "why?" or "why wouldn't you put a 2 there?" so that I can gauge if they would be able to help the struggling students.

After finding students capable of explaining and completing the work accurately, I pair them up with the struggling students to continue working.  This allows star students a chance to build their understanding by having to articulate what they are doing as opposed to just following the steps and coming up with solutions.  It also allows the struggling students one on one help from a peer.

Here is a sample of student work that clearly shows the charting work:

 

This is the first time that students are balancing chemical reactions with paper and pencil so I do not want to overwhelm them with too many to do, but I also want them to have plenty of practice and enough work to last the class period.  These eleven equations that I chose are basic chemical reactions for balancing--unique elements appear in only one reactant and one product, eliminating the issue of finding one element in more than one molecule on one side of the reaction (which complicates the coefficient solving process).  I do want students to be able to solve any reaction, however, and we go over the process for solving those more complicated equations in the following day's lesson: Balancing Chemical Reactions, Part 2.

I grade today's work on effort and completion.  At the end of this independent work time, I spend two minutes projecting the answers (with my work shown so that students can compare my process to theirs and hopefully identify where any incorrect answers of their differed) and quickly reading off coefficients.  I ask for a show of hands for who got each one correct as we go through (1-The coefficients are 1, 1, 2, 1 as we read across.  2-Who got it right? Raise your hand!  3-Check your work against mine.  Do you see the discrepancy?  4-Then I move on to the next one.) the worksheet.

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:  "What is happening during a chemical reaction that explains why we end up with the same number of atoms of each element on the products side as the reactants side?"

Desired student responses should indicate that:

  • The numbers of each type of atom on each side of the reaction must be equal because a chemical reaction is the rearrangement of atoms that already exist in the reactants.
  • Chemical reactions occur when bonds of reactants are broken, atoms are rearranged, and new bonds are formed between those same atoms to make the products.

 

I was very proud that my students were able to communicate understanding of conservation of matter through their responses to the reflection prompt.  During the activity, students were very focused on the mechanics of equation problem solving, so I felt it was important to tie this balancing process back to conservation of matter so that students do not forget the overall picture and get too wrapped up in the solving algorithm.  While some students were still trying to meet proficiency in solving by the end of the period, I did take solace in knowing that the big idea had taken hold.  Students will be able to continue practice in the next lesson, Balancing Chemical Reactions, Part 2.