Chemical Reaction Equations--An Introduction

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Objective

SWBAT categorize objects in varying situations as the equivalent of reactants or products and identify those reactants as either limiting or excess.

Big Idea

Starting materials in scientific processes are called "reactants" and "products" are the result; reactants that run out limit the amount of product made, leftovers are called excess.

Why This Lesson?

In this lesson, students are introduced to the idea that we can express chemical reactions as equations and those parts of the equation have specific names.  In order to give students a frame of reference that they can understand, we start with familiar, real-world examples (not necessarily chemical reactions), like making a sandwich and gradually introduce chemical reactions using the same format. 

While there are no Performance Expectations PS1 (PEs) that directly assess whether or not students can identify parts of a chemical reaction, there are several that do directly assume that students know the terms reactants and products.  For example, HS-PS1-5 asks students to explain how concentration of reactants effects rate of reaction.  HS-PS1-7 asks students to prove mathematically conservation of mass during a chemical reaction, and in order to effectively teach students about this topic, I would naturally use the terms reactants and products.  I also want my students to use the academic vocabulary specific to describing chemical reactions and this is one way to introduce those terms, and have students practice using them, in describing processes that they already understand.

Students will be using recipes as their real-world tangible examples in order to model balancing chemical reactions, which engages students in Science and Engineering Practice 2.  They are also using mathematics and computational thinking, which is SEP 5.

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: "If you need 3 meatballs in every roll to make a meatball sandwich, and you have 72 rolls and 240 meatballs, how many sandwiches can you make?"

In this case, the warm-up is asking students to take a real-life situation and determine what "reactant" is going to run out first, therefore limiting the amount of "product" made without actually introducing the vocabulary.  It is also preparing students to participate in similar scenarios during which they will intuitively determine limiting reactants and excess reactants.  We will then apply that learning to analyzing chemical reactions, which students usually see as abstract but will be able to connect to real-life situations as a result of this lesson.

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.  

After students have had enough time to answer the warm-up question, we do debrief.  I give students two options: 72 and 80.  Then, I ask how many say 72 and how many say 80.  I already knew almost all of my students had answered either 72 or 80, so I deliberately chose those as the two options.  After taking that quick whole class poll, I ask (by name) one of my students who chose 80 why he or she chose that answer.  I ask another student (by name) who also chose 80 why.  Here, I am expecting students to answer with an explanation that 240 meatballs makes 80 sandwiches because 240 divided by 3 is 80.  Then, I ask (by name) a student who said there were only 72 why he or she chose that answer.  I am expecting that student to explain that we would run out of rolls first before meatballs.  Students generally understand the idea at this point.  This provides a perfect segue into my explanation of how to do the activity and the academic vocabulary of reactant, product, limiting reactant, and excess reactant.

Intro to the Activity

10 minutes

In order to introduce the lesson's activity, I first explain how to go about solving the warm-up question.  As I am doing this, I weave into the conversation four key academic vocabulary terms: reactants, products, limiting reactants, and excess reactants.  A brief explanation of how I do this is shown in this video:

 

Then, I pass out the activity entitled "Chemical Reaction Equations Introduction" to my students.  I explain that these questions are just like the one we did as our warm-up.  I allow students to work with their table groups (usually 2-3 students sit at each table), and I encourage them to ask for help if they need it or want clarification.

Individual/Small Group Work

35 minutes

As students are working independently (or with their table groups), I circulate to answer any questions.  If students are not in need of help, I will look over their shoulders to make sure that they are answering the questions correctly.  If they are not, I will ask pointed questions to get them back on the right track ("Are you sure about this one?" or "If this reactant could potentially make less product that the other reactant, what do we call it?").  

I noticed that students were having trouble making the connection between the academic vocabulary (limiting reactant versus excess reactant) and the real-world examples.  I did have to continually reinforce that whichever reactants are depleted first will limit the amount of product that can be made, and then explain that is why we call them "limiting reactants."  If there is extra left over, then those are "excess reactants."   

These examples of student work show a student strategy of determining how much product each reactant can form (as shown in their written notes near the questions) before determining which runs out first.  Almost all of my students adopted this approach, and all students performed quite well on this assignment (although most complained that it was making them hungry...).

Sample student work:

 

 

Once students are done, as long as time permits, I quickly go over the answers with the class shouting out the correct answers followed by my quick explanation as to why those would be the correct answers.  I found that we had time in one of my class periods to do this, but not in the other.  I graded the ones by hand that we did not have time to discuss so that students did get feedback.  This grade was recorded as a completion grade and not a content grade since students will be assessed on content when it is applied in subsequent lessons on balancing chemical reaction equations.

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:  "In your own words, explain the differences between limiting reactants and reactants in excess."

Desired student responses should include:

  • limiting reactants are those that limit the amount of product made
  • limiting reactants run out first
  • excess reactants do not determine how much product is made
  • excess reactants are those reactants that are left over

 

Sample student work: