# S'more Stoichiometry

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## Objective

Students will be able to use their knowledge of Stoichiometry to balance the reaction for S'mores and calculate percent yield for their S'mores.

#### Big Idea

In chemical reactions a limiting recant causes a reaction to stop while an excess reactant is leftover. Additionally one can calculate percent yield using the experimental value from performing a lab and the theoretical value.

## Introduction

In this lesson students have a chance to review for their unit 5 exam on Stoichiomery through doing a fun lab activity as well as doing a review study guide.  I love to do this lab with my students because they have a chance to make s'mores but are also reviewing Stoichiometry basics.

• This activity aligns with HS-PS1-7: Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
• This lesson aligns with NGSS Science and Engineering Practice 3: Planning and Carrying out Investigations because students are performing a lab.

For this lab activity there are several materials needed for each lab group including:

• 1 plate with 8 graham cracker halves, 1 chocolate bar, 5 marshmallows, and 4 skewers
• 1 Striker
• 1 Bunsen burner (should be cleaned from any toxic chemicals)

## Explain

10 minutes

To begin the lesson I explain to students what they will be doing in the lab.  I do this by:

1. I pass out the S'more Stoichiometry Lab Paper
2. I explain to students that they will be doing a fun lab but will be also reviewing for their Unit 5 exam.
3. I go over the Problem part of the lab with students and review the table at the top of the lab which gives the symbol for each reactant and the product S'mores.
4. I review safety with Bunsen burners and remind students that they must keep their goggles on when the Bunsen burner is lit.
5. I remind students that when they are finding the mass of each reactant and then the product in the end to make sure that they are using a napkin on the balance and that whomever is touching the ingredients to weigh them should use them in their individual S'mores.
6. I then break students into cooperative groups.  For more details about how I use cooperative groups see my Cooperative Lab Groups Reflection in Unit 1.
7. Finally I have student go up to their lab stations where I have the lab set up.  This  video explains how I set up each lab station with the appropriate materials for each groups.

## Elaborate

40 minutes

In this section of the lesson students are working on their labs with their groups.

• As students work on their labs I walk around the classroom to make sure that they are working well within their cooperative groups.  This means making sure that they are staying on task, recording information, and performing the proper procedures.
• As students work I also walk around and ask questions to make sure that they are understanding the answers to the questions.  Some examples of the questions I ask include:
1. How did you know which reactant was limiting?
2. How did you figure out the excess reactant?
3. How did you figure out your theoretical yield for your product?
• One of the most surprising parts of this lab is that many of my students have never made a S'mores before so I need to explain how you use graham crackers on the end and put the chocolate and roasted marshmallow in the middle.
• Some of the other confusions in the lab include:
1. How to figure out the number of moles of each reactant that the group starts with (for example each table has 5 moles of marshmallows).
2. The difference between limiting and excess reactants (some students switch the two).
3. When they find the mass of each reactant some of the groups want to weight more than one graham cracker or chocolate piece.
4. When they find their theoretical yield for their product some groups don't add up the mass for two graham crackers, one marshmallow and three chocolate pieces.
5. How to calculate percent yield.  In particular when students calculate percent yield they often get higher than 100% so think that they did the calculation incorrectly.

## Evaluate

After students have completed their S'mores lab I have them turn it in and have them start their Unit 5 Exam Review.

• The S'mores lab I grade for completion with a check (100%), check minus (75%), or check minus minus (50%) depending on completion.  Most students earn a check because they are working in groups and I am making sure that they are completing the lab as I walk around.  Here is one example of a student's completed lab.
• When students begin their unit 5 review I have them work either individually or with their group members.  I remind them that the point of the review is to help prepare for the exam so simply copying each others' papers will not help them.  I have students complete the review for homework which I stamp for completion and review with the following lesson.  Here is a copy of one student's filled in review.