Name______________________________
Period___________
Date_______________
What is a limiting reactant?
A delicious treat known as a S’more is constructed from the following ingredients:
2 graham crackers squares
1 chocolate bar
1 marshmallow
Suppose we find that these ingredients are available only in full packages, each of which contains one
dozen of the item. The packages of ingredients have the following weights:
graham crackers squares
chocolate bars
marshmallows
200.0 g
145.0 g
75.0 g
Each group will build S’mores out of the package of ingredients that you receive. Build as many S’mores
as you can, but don’t eat the S’mores yet.
Questions: (You may use your S’mores to help you visualize these problems)
1. Using G for the graham crackers, C for the chocolate bars, and M for the marshmallows, write a
balanced equation that would represent the production of S’mores from the starting materials.
2. Based on the information given about the three ingredients, which weighs the most? Which weighs
the least?
3. If we have 12 graham crackers (one package), how many chocolate bars and how many
marshmallows do we need to make S’mores with all the graham crackers?
4. How many S’mores would we be able to make?
5. Suppose we have one package of each of the ingredients. (Show work)
How many S’mores can we make?
Will any of the ingredients be left over?
How much?
6.
A)
B)
C)
Based on what is in your bag of ingredients….
How many s’mores can you make?
What is (are) the limiting substance(s)?
What is (are) the excess substance
7. Is it correct to say that if we start with 4 lb each of G, C, and M, we should end up with 3 x 4 = 12 lb
of S’mores? Explain
Now let’s apply the same concepts to a chemical situation: SHOW WORK!!!
Ammonia (NH3) can be formed from the elements N2 and H2 , as shown below.
N2
+
3 H2 -------------- >
2 NH3
1. How many moles of ammonia can be made from 1.0 mole of N 2 and 3.0 moles of H2?
2. Suppose we had 3.0 moles each of the N2 and H2 available to react. Which of the reactants would be
the limiting reactant? Hint: calculate moles of NH3 produced by each and see which makes less.
3. A) How many moles of ammonia could we make?
B) Would any of the reactants be left over?
C) How many moles would be left?
4. Using the same equation above…
A) What mass of ammonia could we make from 100.0 grams each of N 2 and H2?
B) What is the limiting reactant?
Now that you have finished….bring your paper to get checked and to make your s’more 
reactant and an excess reactant. They will also be able to determine the limiting and
excess reactants in future problems.
S’mores - What is a limiting reactant?
Teacher Notes
This lab requires S’mores material for students. It can be run in partners or groups.
If a brand-new, never-been-used-for-chemicals Bunsen burner is used, the S’mores
can be heated and consumed. If you have no brand-new Bunsen burner available,
the S’mores should be eaten cold or warmed in a food-only microwave. This activity
can be used as an introduction to limiting reactants, letting the students determine
their own definition for a limiting reactant, or as a supplement during lecture/notes
on the topic.
Materials
Enough Graham crackers, marshmallows, and chocolate for all students; brand-new
Bunsen burner for heating the S’mores; student worksheets
Safety Concerns
S’mores should only be heated over a never-used-for-chemicals Bunsen burner or in
a food-only microwave.
Real-World Connections
Limiting reactants are used when cooking food (hot dogs are sold in packages of 10,
while hot dog buns are sold in packages of 8), making sandwiches, making
hamburgers, etc.; when the Mentos and Diet Coke reaction is too small, more
Mentos (the limiting reactant) should be added. Chemists determine the limiting
reactant to maximize the reactions when using very expensive chemicals (cancer
and AIDS drugs).
Sources
Lab Idea taken from:
www.sci.ccny.cuny.edu/~chemwksp/Smores.doc
Procedure/Description of Lesson
See the following 2 pages. 25
Assessment Ideas
A snowball quiz could be given the next day. A regular quiz is given; students
answer the quiz questions on a half-sheet of paper, but do not write their name on
it. The class is then divided in half and told to stand on opposite sides of the room.
The quiz is crumpled into a snowball and a snowball fight ensues. When time is
called (usually a few seconds), students grab one piece of paper from the floor;
answers are given and problems worked out for the students. Students are able to
learn from their own mistakes and the mistakes of the other students that they are
grading, but there is no point value associated with this assessment. Students have
a good grasp of where they are with the material and what they need to work on,
but do not have to worry about being penalized if they do not fully grasp the
concept at that moment.26
Name______________________________
Hour___________ Date_______________
What is a limiting reactant?
A delicious treat known as a S’more is constructed from the following ingredients:
2 graham crackers
1 chocolate bar
1 marshmallow
Suppose we find that these ingredients are available only in full packages, each of which
contains one dozen of the item. The packages of ingredients have the following weights:
graham crackers 200.0 g
chocolate bars 145.0 g
marshmallows 75.0 g
Each group will build S’mores out of the packages of ingredients that you receive. Build as
many S’mores as you can, but don’t eat the S’mores yet.
Questions: (You may use your S’mores to help you visualize these problems)
1. Using G for the graham crackers, C for the chocolate bars, and M for the marshmallows,
write an equation that would represent the production of S’mores from the starting
materials.
2. Based on the information given about the three ingredients, which weighs the most?
Which weighs the least? Explain your reasoning.
3. If we have 12 graham crackers (one package), how many chocolate bars and how many
marshmallows do we need to make S’mores with all the graham crackers? Explain.
4. How many S’mores would we be able to make?
5. Suppose we have one package of each of the ingredients. How many S’mores can we
make? Will any of the ingredients be left over? How much?
6. If we make S’mores from the materials described in #5, which ingredient limits the
amount of S’mores you can make—i.e., which ingredient will you run out of first? (This item
is known to chemists as the limiting reactant because it is the reactant that limits the
amount of the final product that can be made) 27
7. How many dozen S’mores will you have made?
8. Suppose you had 500.0 grams of graham crackers, 500.0 grams of chocolate bars, and
500.0 grams of marshmallows.
a. Which item do you have the most of? Which item do you have the least
of? Explain your reasoning.
b. If you attempt to make S’mores with these amounts of ingredients, what
item will you run out of first?
c. How many S’mores can you make?
d. How many grams of each left-over item will you have?
9. Is it correct to say that if we start with 4 lb each of G, C, and M, we should end up with
3 x 4 = 12 lb of S’mores? If not, why not?
Now let’s apply the same concepts to a chemical situation:
Ammonia (NH3) can be formed from the elements N2 and H2, as shown below.
N2 + 3 H2 -------------- > 2 NH3
1. How many moles of ammonia can be made from 1.0 mole of N2 and 3.0 moles of H2?
2. Suppose we had 3.0 moles each of the N2 and H2 available to react. Which of the
reactants would be the limiting reactant?
3. How many moles of ammonia could we make? Would any of the reactants be left over?
How many moles?
4. How many moles of ammonia could we make from 1.0 mole each of N2 and H2?
5. What mass of ammonia could we make from 100.0 grams each of N2 and H2?
Retrieved from: http://www.oakland.k12.mi.us/Portals/0/Learning/Stoichiometry.pdf