Limiting Reactants - S'mores Lab (Marcie Samayoa)

Title: S’mores Limiting Reactant Lab

Principle(s) Investigated: Students will investigate how limiting reactants affect the production of products in a chemical reaction.

Standards :

Introduction:

Stoichiometry is the area of chemistry that allows us to determine the relative quantities of reactants and products in a balanced chemical equation. The law of conservation of mass states that the mass of all participants in a chemical reaction must remain constant over time. Matter is neither created nor destroyed in a chemical reaction, and this principle lays the foundation for stoichiometry.

We have already become familiar with stoichiometry. We are going to build on this concept by learning about limiting reactants. Instead of directly working with chemical equations, we are going to use a food analogy to introduce limiting reactants.

Pre-Lab:

Watch the YouTube video on EdPuzzle to get an introduction to limiting and excess reactants:

Limiting Reactants Video

Assigned Group Roles

DATA SHEET

Materials:

For each group:

Procedure:

The following balanced equation shows you how to make a s’more. You will use this balanced equation in your calculations. All data and calculations should be put into the data sheet:

G= graham crackers

C= chocolate piece

M= marshmallows

2 G + 2 C + 1 M --> 1 G₂C₂M

1. Have the MATERIALS MANAGER obtain a plastic bag containing your “reactants” (graham crackers, chocolate bars, and marshmallows), paper towels, and gloves. Based on the balancedequation given to you, determine what your product is going to be made of. Each s’more must be exactly that same. You will NOT use all the ingredients.
2. Count how many graham crackers, marshmallows, and chocolates are in your group’s bag. DO NOT TAKE IT OUT OF THE BAG. Have the ACCOUNTABILITY MANAGER record it in the data sheet.
3. Predict how many s'mores you can make with your reactants and have the ACCOUNTABILITY MANAGER record it in the data sheet.
4. Place the ingredients on a paper towel and perform the reaction (make as many s’mores as you can using ONLY the materials in your bag and making all of them according the equation). Have the ACCOUNTABILITY MANAGER record it in the data sheet.
5. Answer the question provided for you in lab and check in with the teacher before moving on to number 6. Have the ACCOUNTABILITY MANAGER record the answers in the data sheet.
6. Using wood splint, roast the marshmallow over the Bunsen burner until golden brown.
7. Quickly place it onto graham cracker with the chocolate
8. Wait for loss of heat energy.
9. Eat the product and create a chemical decomposition reaction in your mouth.
10. The MATERIALS MANAGER ensure that everything is neat and put back into place after the lab is done.

* Throughout the lab, PROJECT DIRECTOR ensures everyone is on task and TIME MANAGER takes into account time

Questions & Answers:

1. Let’s say the recipe for s’mores changes and each s’more gets only one chocolate instead of two. Is the ingredient you listed as the limiting reactant in the lab still the limiting reactant? Explain.

2. One group of students had 40 graham crackers, 10 marshmallows, and 16 pieces of chocolate in their bag. They claimed that marshmallows were the limiting reactant because there is less marshmallows than there is graham crackers and chocolate. Determine if this group’s reasoning is right or wrong and justify your answer using complete sentences. In your answer, make sure you include the following academic vocabulary words: limiting reactants and excess reactants.

3. How might the knowledge of limiting reactants be important in a plant where chemicals are used to make a product to sell?

Student prior knowledge:

Students should know how to balance chemical equations, identify different types of chemical reactions, and make gram to gram stoichiometry conversions.

Explanation:

This unit discusses stoichiometry and limiting reactions. In this curriculum, there are YouTube videos that explain what limiting and excess reactants are. It also consists of a s’mores lab thatallows students to develop a deeper understanding of the lesson’s learning target. Students demonstrate competency through hands-on activities and data analysis.

This unit was conducted in my classroom which consists of students who are behind grade-level in reading and mathematics. Unfortunately, this makes science comprehension extremelydifficult. Especially since science involves unfamiliar words, complex combinations of text and numbers, and considerable amount of information to decode and organize. Using food items such asmarshmallows, chocolate, and graham crackers to make s’mores allows students to intimately interact with content that they encounter on a daily basis. This is especially helpful since students whofeel connected with the content are more likely to engage with complex scientific concepts such as stoichiometry.

In the classroom, the learning target is aligned to the Next Generation Science Standards and are written in student friendly language so that they can take the material that they havecomprehended and self-assess their ability to understand the learning targets.

Potential Misconceptions:

Students often choose the reactant with the least amount as the limiting reactant. To correct this misconception, the bags of “reactants” will have a reactant with the least amount that results NOT to be the limiting reactant. The lab also asks a question that addresses this misconception.

Students may also be confused with limiting reactant and limiting reagent being used interchangeably. Hence, word walls and flashcard creation tools are used to help remedy these issues.

Applications to Everyday Life:

Stoichiometry is also used in ecology and it is known as ecological stoichiometry. Ecological stoichiometry examines the stoichiometric relationship between the nutritional demands of a species and the food available to the species. One popular example is algal blooms. Algal blooms often result when one limiting reagent is supplied to the environment, either naturally, or through human activities. The limiting reagent that prevents uncontrolled algae growth is often phosphorous, and it may be in low concentrations because phosphate mineral sources are insoluble. However, increased amount of phosphorous has been dumped into ocean water due to human activity. This causes algal blooms which can exhaust the oxygen in water required for other oceanic species to thrive.

Photographs:

Continued:

After students conduct this lab, students will be taught how to apply the concepts learned in the s’mores lab to chemical equations. They will be reminded that just like ingredients can run out while making s’mores, certain chemicals can run out as well before using all of the reactants that are available. To determine the limiting reactant in a chemical equation, we must conduct stoichiometry calculations to observe which reactant produces the least amount of moles of a product.

Directions:

1. Balance the chemical equation.

2. The value that is given in the word problem is to be placed on the top left of the “picket fence.”

3. Convert it into grams by using molar mass.

4. Use the mole ratio to find the number of moles of product that will be produced if using that specific reactant.

5. Repeat steps 1-4 for the second reactant.

6. The reactant that gives you the least amount of moles of product will be your limiting reactant.

Sample Questions:

H₂ + O₂ --> H₂O

If you start with 10 g of H₂ and 5 g of O₂, determine the limiting reagent.

Answer:

Fe + O₂ --> Fe₂O₃

If you start with 10 g of Fe and 5 g of O₂, determine the limiting reagent.

Sources:

www.chemmybear.com/lv2007/smorelab.doc

http://www.sciencedirect.com/science/article/pii/S1877343512000644