Balancing Equations

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Objective

SWBAT determine the number of atoms and molecules in an equation in order to balance equations to model the Law of Conservation of Mass.

Big Idea

In this differentiated lesson, students review key concepts regarding the Law of Conservation of Mass and use manipulatives and technology to assist in balancing equations.

Engage

5 minutes

When the students enter the room, they get their Chromebooks and log into Socrative to respond to the prompt:

When do we use subscripts and coefficients in science?  What does a subscript tell us?  What does a coefficient tell us?

After a few minutes, I ask students to share their thoughts with their group members.  This gives them an opportunity to discuss and formulate their responses prior to sharing with the whole group.  When the students have had a chance to converse, I ask for volunteers to share their thoughts.  During this time I ask for students to support their ideas and the ideas of their classmates by giving examples.

Explain

10 minutes

After our discussion, I ask the students to complete a brief quiz to check their understanding of subscripts and coefficients.  Once the students have finished with the quiz, I go through the questions, asking students to share their answers and the reasoning behind them.

I ask the students to close their Chromebooks in order to ensure their attention to the lesson.  I ask the students to remind me of the definition of the Law of Conservation of Mass.  As a follow up question I ask them how the law relates to subscripts and coefficients.  The students briefly reviewed balancing equations the year before, so some of them will be able to answer the question.  I explain to the students that we will practice balancing equations as a way to describe some of the chemical reactions that we have completed and will complete in the lab and as a way to model the Law of Conservation of Mass.

I begin by writing a balanced equation on the board, then I show the students how to create a list of the elements in both the reactants and the products to see if the same number of atoms are present on each side.

I then have the students walk me through a couple of sample equations on the board. 

Explore

20 minutes

Once I have a strong sense of who is and is not understanding the lesson, I divide the students into three groups.

I get one group started on a list of equations to balance.  I have them begin with a worksheet, so they can make a list of the number and types of atoms on each side of the equation.  This will help the process of balancing equations become more clear to them and soon they will no longer need to write the chart.  Once these students finish the brief worksheet, I check over it and have them move on to working on balancing equations on an interactive website (this website does contain advertisements) on their Chromebooks.  Once they have finished with that interactive, they will move on to another.

The next group of students works on the same initial worksheet, but I sit and work with this group using beads to represent atoms.  The beads do not work for some students, so I also have sets of cards with element symbols on them.  I use the beads and the cards to model the first couple of questions for this group of students.  As the students begin to understand the concept, I move them to another group where they will work on interactive websites that provide immediate feedback in response to their answers.  I continue using models with this group of students until they are all able to work independently.  The use of these manipulatives as models and the use of written equations meets NGSS MS-PS1-5 - Develop and use a model to describe how the total number of atoms does not change in a chemical reaction and thus mass is conserved.  The assessment boundary for the standard does not include balancing equations, but in my district this is something that students are expected to know prior to entering high school.

Once this group of students are working independently, I begin working with the first group of students to explain how parentheses are used in chemical equations.  When the students demonstrate understanding of this concept, I have them work on a more difficult set of equations.

This video provides demonstrations of the websites used in the lesson.

Wrap up

5 minutes

At the end of the lesson, I have the students help me create an anchor chart for balancing chemical equations.  This chart is placed on the wall and a digital copy is placed on our classroom.google.com.