Balancing Chemical Reactions--Part 2

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Objective

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

Big Idea

Atoms of one element may be present in multiple reactants and/or reactants and must be accounted for accurately when balancing a chemical reaction equation.

Why This Lesson?

In the lesson immediately previous to this one, students worked to balance reactions using mathematics and computational thinking (SEP 5) without using models.  Today's lesson builds upon this skill and challenges students to balance more complicated chemical reaction equations, including those with the same element present in multiple reactants and/or products.

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: "Balance the following chemical reaction equation:

In this case, the warm-up is asking students to think about what they learned during the previous lesson about balancing chemical reaction equations--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 only needed to balance equations that had elements that appeared in one of the reactants and one of the products. Today, I want students to apply what they already know to a step-by-step process used to balance chemical reaction equations where some elements appear in more than one reactant and/or product.  This warm-up asks students to balance such a type of equation without having received specific instruction as to how.

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 gave my students a challenging equation to balance as a warm-up prompt.  I want my students to discover that this particular reaction is challenging to balance because there is one element (in this case oxygen) that appears in both reactants AND both products.

I use this reaction 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 practice sheet to my students.  I have a combined class that has both general level and honors level students.  For this activity, I want my honors students to feel challenged without making my general students feel overwhelmed.  I do this by assigning two different handouts.  The honors students have far more challenging equations to balance (Balancing Chemical Reactions Practice 2 HONORS) and the general students have a handout with a few challenging ones with more of the easier ones to solve (Balancing Chemical Reactions Practice 2).  

For example, this equation is found only on the Honors handout:

I consider this equation challenging because nitrogen is present in two of the reactants.  Some students struggle with balancing back and forth with changing coefficients multiple times.  I do put a couple of those types on the general handout, but not as many as I include for the honors students (who generally need a challenge to keep them interested).

Independent Work

35 minutes

In my combined level class, I sit on the side with my general level students so that I can work with them in small groups as needed.  My honors level students bring me their work as they finish so that I can do a quick spot check.  Honors students who finish early with accuracy are encouraged to help other students who need it.  Surprisingly, most of these students share their expertise with no prompting since we build a collaborative culture from the beginning of the school year.  I make sure to say thank you personally to students who are helping ("Thank you, Matthew, for helping Elizabeth understand.") because I want to encourage the behavior and, for my students, a simple acknowledgement goes a long way.

I allow students who need more time to finish the work at home, but I do require that all work is shown so that students cannot simply copy someone else's coefficients to fill in the blanks.

The keys for each assignment are available here:

Balancing Chemical Reactions Practice 2 KEY Page 1

Balancing Chemical Reactions Practice 2 KEY Page 2

Balancing Chemical Reactions Practice 2 HONORS Key Page 1

Balancing Chemical Reactions Practice 2 HONORS Key Page 2

 

Because I did not grade for accuracy in the previous day's lesson (Balancing Chemical Reaction Equations, Part 1), I do grade for accuracy in today's lesson.  The assignment is collected at the end of the period, whether student's have completed it or not.  I then have one of my student aides scan the assignments for incorrect answers and write the correct coefficients above any incorrect ones.  I assign the final grade for the work later when I go back through the work.  There are several students that work slower and may not reach completion, but who are near completion and have perfect papers.  I give those students full credit (a 4).  Other students will power through the entire assignment, but will make mistakes.  I would rather these students slow down and get correct answers instead, and I give those students 3's.  Students who make minimal effort and who do not demonstrate progression towards mastery in their work, or who do not show their process, may get 2's.  Finally, no effort is awarded a 1.

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 makes balancing chemical reaction equations that have the same element in more than one reactant or product different from those that don't?"

Desired student responses should indicate that:

  • Changing coefficients will effect different compound amounts of that element, and not the entire amount of that element for the whole side
  • Amounts of each element could be listed underneath the individual compounds to aid in balancing