In this activity, students determine the number of graham crackers and chocolate pieces required to complete a “reaction” with a given quantity of marshmallows (the limiting reactant). They then use the same thought process with a problem involving a real chemical reaction.
This lesson will help prepare your students to meet the performance expectations in the following standards:
- HS-PS1-7: Use mathematical representation to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
- Scientific and Engineering Practices:
- Using Mathematics and Computational Thinking
- Developing and Using Models
By the end of this lesson, students should be able to:
- determine mass of product from a given mass of reactant.
- determine moles of product from a given amount of reactant.
- write and balance chemical reactions.
This lesson supports students’ understanding of
- Balancing equations
- Limiting reactant
- Dimensional analysis
Teacher Preparation: 10 minutes
Lesson: 30-40 minutes
For each group:
- Graham cracker
- Chocolate piece
- Food in the lab should be considered a chemical, not for consumption.
- At the beginning of the activity, give each student one marshmallow, graham cracker, and chocolate piece in order to determine their masses. (Keep in mind that for this activity, “one graham cracker” means one side of the s’more and “one piece of chocolate” means one square of a normal chocolate bar, which normally contains 12 pieces. Circled in the images below are what “one piece” means for this activity.) Instruct them to not eat the substances.
- After students have completed part II of the activity, you can reward students with supplies to make an edible s’mores. Make sure the food items have not been in the lab area. Food should be consumed outside of the lab area.
- Make sure the students understand that in part I, they are determining how many chocolate pieces and graham crackers needed to use up all of the marshmallows. For example, if they have 10 marshmallows, they can make 10 s’mores, so they would need 20 graham crackers and 30 chocolate pieces (because the chemical formula of a s’more is S2MmOr3).
- Since there is only one marshmallow per s’more, the calculations for limiting reactant are relatively straightforward – that is, 1 marshmallow = 1 s’more. If you wish to make the lesson a bit more advanced, you could ask students to start from graham crackers or chocolate pieces instead (though they end up doing all three calculations in the end anyway).
- For part II, students will need to be able to go from compound names to formulas to write the balanced equation. If they do not know how or if you just want to skip to the balancing part, you can provide the unbalanced equation or the formulas along with the names and just ask them to balance it.
For the Student
“Stoicheion” means element and “metron” means measure. The mass and quantity (mole) relationships among reactants and products in a reaction are found using the process of stoichiometry, the measurement of elements.
- If you are given one bag of large marshmallows, what is the maximum number of S’mores that can be made?
- How many boxes of graham crackers and how many chocolate bars are needed to make this many s’mores?
- Graham cracker
- Chocolate piece
PART I: S’MORES REACTIONS
- Obtain a marshmallow, graham cracker, and chocolate piece. Place each piece of s’more on the balance to determine its unit mass. Record in the table above.
- Calculate the unit mass of the S’more (S2MmOr3)below:
- a. Write a balanced chemical equation that explains how a s’more is made.
b. What does the equation tell you?
c. What do the coefficients represent?
d. Based on the masses in your table, if you had a bag of marshmallows with a mass of 454 g, how many marshmallows are in the bag?
e. How many units of graham crackers and chocolate pieces are needed to make the maximum number of s’mores with the number of marshmallows calculated above?
i. Number of graham crackers needed:
ii. Number of chocolate pieces needed:
f. When you go to the store, you cannot determine the exact number of graham crackers or chocolate pieces in a box or bar. The mass is easy to read, however. Using mass values, you can determine how much you need to buy.
i. If a box of graham crackers has a mass of 254 g, how many boxes do you need to get the number of graham crackers calculated above?
ii. If one chocolate bar has a mass of 49.5 g, how many bars do you need to buy?
PART II: CHEMICAL REACTIONS
Now you will transfer this process into the language of chemical reactions. If you added a 15.00 g piece of solid Cu to an aqueous solution of silver nitrate, the silver would be replaced in a single replacement reaction forming aqueous copper(II) nitrate and solid silver. How much silver is produced if 15.00 g of Cu is added to the solution of excess silver nitrate?
- Write and balance the chemical equation:
- Convert 15.00g Cu to moles Cu:
- Determine moles of Ag produced:
- Convert moles Ag to grams of Ag produced:
- If silver metal sells for $4.50/ounce, could you get rich from this lab? (How much would it be worth?) Conversion factor: (1 g = 0.0353 oz)