Name: NAME:_______________________________________________
Limiting and Excess Reactants (R)
Is there enough of each chemical reactant to make a desired amount of product?
Why?
You have spent a lot of time studying the various types of reactions that can occur in chemistry.
You have also become experts in balancing chemical equations. One thing you will do in this
activity is practice stoichiometry skills while solving a problem with a familiar campfire snack.
Stoichiometry is the chemical term to describe calculations that allow us to find the amounts of
chemicals involved in a given reaction. We will also look at the idea of limiting reactants. Two
atoms or molecules must come together in just the right way in order for them to react. As a
result, it is virtually impossible to obtain 100% yield in a chemical reaction by combining the
reactants in exact proportions. In order to increase the odds that at least one reactant will react
completely, we often add more than is needed of another reactant. In this activity, you will look at
several situations where the process or reaction is stopped because one of the required
components has been used up.
Model 1 – Assembling a S’more
In this activity, you will use a recipe for S’mores as an analogy for a chemical equation in which
reactants and products are in set proportions to each other. You will be given varying amounts
of each reactant. One of these reactants will limit the number of S’mores you can produce. The
other reactants will be in excess. After working with this culinary “reaction,” you will be able to
identify the limiting and excess reactants in chemical reactions.
PROCEDURE:
1. Obtain a plastic bag of s’mores ingredients. Record your group number. ______ Record the
amount of each ingredient.
graham cracker
quarters_______
chocolate
squares________
marshmallows________
2. Write a balanced equation for the synthesis reaction using the chemical symbols given
in the recipe.
Recipe for 1 S’more (G2CsM)
2 graham cracker quarters (chemical symbol G)
1 chocolate square (chemical symbol Cs)
1 marshmallow (chemical symbol M)
Balanced equation:
3. Use the following recipe to perform a synthesis reaction using your ingredients. (i.e: cause
the reaction to go to completion by forming as many S’mores as possible).
4. How many products (S’MORES) were you able to form?
______________
*YOU MAY START ROASTING AND EATING BUT CONTINUE WITH ANSWERING
THE LAB QUESTIONS
Chem Is Try: Critical Thinking Questions
1. What was your limiting reactant (which ingredient limited how many s’mores your group
could make)?
2. Define, in your own words, what a limiting reactant is.
3. Which starting materials were in excess?
4. Define, in your own words, what an excess reactant is.
5. Suppose you had a bag of 11 graham crackers, 12 chocolate squares, and 7 marshmallows.
a. How many s’mores can you make? This is known as your theoretical yield, or
amount of product that should be produced given the quantity of reactants. Explain
your reasoning.
b. What (if anything) will be left over, and how much of that item will there be?
Explain your reasoning.
c. What item limits the number of s’mores you can make? Explain your reasoning.
Model 2- Manufacturing a S’more
Molar Masses of the Substances:
Substance
Molar mass of 1 piece of Chocolate (Cs)
Molar Mass (mass of 1 piece or 1 mole)
3.69 g/mole
Molar mass of a Marshmallow (M)
Molar mass of a half Graham cracker
(G)
Molar mass of a S’more (G2MCs)
7.34 g/mole
7.75 g/mole
You figure this out!
Chem Is Try: Critical Thinking Questions
6. Calculate the Molar mass of the S’more (G2MCs) below and put in the table above:
7. If you have 4 moles (pieces) of M, how many grams of M do you have? Show your work.
8. If you have 30 grams of Cs, approximately how many moles (pieces) do you have? Show
your work.
9. If you have 33.21 grams of Cs,
a. How many moles of M and G will be necessary to make the maximum number of
S’mores possible? Show your work.
b. How many grams of S’mores can be made? Show your work.
Model 3 – Assembling & Manufacturing a Race Car
Chem Is Try: Critical Thinking Questions
10. How many of each race car part are needed to construct one complete race car?
Body (B)
Cylinder (Cy)
Engine (E)
Tire (Tr)
11. How many of each part would be needed to construct three complete race cars? Show
your work.
Body (B)
Cylinder (Cy)
Engine (E)
Tire (Tr)
12. Assuming that you have 15 cylinders and an unlimited supply of the remaining parts:
a. How many complete race cars can you make? Show your work.
b. How many of each remaining part would be needed to make this number of cars?
Show your work.
13. Count the number of each race car part present in Container A of Model 3.
Body (B)
Cylinder (Cy)
Engine (E)
Tire (Tr)
14. Complete Model 3 by determining the number of cars that can be made from the parts in
Container A. What excess parts do you have? How many of each part? Draw the cars
next to Container A in Model 3.
15. A student says, “I can see that we have three car bodies in Container A, so we should be
able to build three complete race cars.” Explain why this student is incorrect in this case.
16. The number of complete cars you predicted that could be built is called the
theoretical yield. Theoretical yield is defined as the maximum amount of product
produced during ideal conditions in a chemical reaction. It is what we mathematically
think will be produced. What is the theoretical yield of cars in this scenario?
17. Fill in the table below with the maximum number of complete race cars that can be built
from each container of parts (A–E), and indicate which part limits the number of cars that
can be built. Divide the work evenly among group members. Space is provided below the
table for each group member to show their work. Have each group member describe
to the group how they determined the maximum number of complete cars for
their container. Container A from Model 3 is already completed as an example.
1 B + 3 Cy + 4 Tr + 1 E = 1 car
18. You are planning activities for a younger sibling’s birthday party. You decide that each of
the 10 partygoers will build a toy race car to take home. You have almost enough toy race
car parts to build 100 cars, except you only have 10 car bodies and 46 tires.
a. Which component is the limiting reactant? Explain how you came to this conclusion.
Show any math you did.
b. Do you relax or do you run to the toy store to buy more car pieces? Why?
19. Look back at questions 20 and 21 with your group. Is the component with the
smallest number of pieces always the one that limits production? Explain your
group’s reasoning.
Model 4 – Limiting Reactant Mathematical
Below are two examples of mathematical calculations that could be performed to find the
limiting reactant when given 8 moles of H2 and 6 moles of O2.
Balanced Equation: 2H2 + O2 2H2O
Example #1
Use example #1 to solve to find the limiting reactant. Show all work.
20. Iron and oxygen combine to produce iron (III) oxide.
Write a balanced equation below, and then complete the columns of the data table.
balanced equation: 4Fe + 3O2  2Fe2O3
Have
(Given amounts)
Show work
30 moles iron
25 moles oxygen
21. Oxygen and hydrogen gas combine to produce water.
Write a balanced equation below then complete the data table.
balanced equation: O2 + 2H2  2H2O
Have
(Given amounts)
12 moles oxygen
15 moles hydrogen
Show work
Example Starting with Mass
Consider the following reaction:
2 Al + 6HBr  2AlBr3 + H2
When 5.45 g of Al and 5.45 g of HBr are reacted, what is the theoretical yield of AlBr3?
(Remember…theoretical yield the predicted mass of the product if the reaction occurs perfectly).
Step 1: Do a mass to mass Stoichiometry calculation from each reactant to the
product.
5.45 g Al
1 mol Al
27 g Al
2 mol AlBr3
2 mol Al
266.7 g AlBr3 = 53.8 g AlBr3
1 mol AlBr3
5.45 g HBr 1 mol HBr 2 mol AlBr3
80.9 g HBr 6 mol HBr
266.7 g AlBr3 = 5.99 g AlBr3
1 mol AlBr3
Step 2: Determining the Limiting Reactant
The limiting reactant limits the amount of product produced…it has run out! You can figure out
the limiting reactant by choosing the reactant that produced less product.
Answer: HBr is the limiting reactant because it produces less product that Al.
Step 3: Determining the Theoretical Yield
The mass produced by the limiting reactant is the theoretical yield.
Answer: 5.99 g AlBr3 is the theoretical yield because that is the maximum quantity able to be
produced by the limiting reactant.
Exercises
22. Consider the synthesis of water as shown in Model 5. A container is filled with 10.0 g
of H2 and 5.0 g of O2.
a. Write the balanced equation
b. What is the theoretical yield of water? Show your work. Once this is determined,
what is the limiting reactant?
c. Which reactant is the limiting reactant? Which reactant is the excess reactant?
23. Given the balanced chemical equation:
2NO(g) + O2(g)  2NO2(g)
a. Calculate the mass of nitrogen dioxide that can be made from 30.0 grams of NO and
30.0 grams of O2 (Hint: do two mass-to mass stoichiometry)
b. Which produced the least amount of nitrogen dioxide? Which reactant is the
limiting reactant? Which is the excess reactant?
Information: Calculating Percent Yield
Equation:
Percent Yield= ___Actual Yield
Theoretical Yield
x100
Percent Yield is the actual yield (what you measured in the lab) divided by the theoretical
yield (the mass you should have obtained in the lab based on Stoichiometry) multiplied by 100
to get the percent. This is used a way to measure how good your experiment was. The closer to
100% yield you are, the better your experiment was.
Answer the following question using the data from question 23.
24. What is the percent yield of NO2 if your actual yield was 42 g?
Teacher Notes:
For a class of 26 and groups of 3 (with 2 groups of 4)
Bag
M
G
Cs
1
2
3*
4
5
6
7*
8
*group of four
4
4
5
4
4
4
4
3
6
6
8
8
8
8
10
8
4
4
5
3
3
3
5
4
Limiting
Reactant
G
G
G
Cs
Cs
Cs
M
M
Cs(chocolate squares) You need 128 pieces for 4 classes. I used 32 pieces for each class. I bought
2 bags (18.50 oz) of the Hershey miniatures (not nuggets) but I can only find them in assorted
flavors so warn you student about nut allergies. You may need a few extra pieces. I liked how
they are individually wrapped.
M (marshmallows) You will need 124 marshmallows for 4 classes. I used ¾ of a 10 oz bags (big
ones) for each class (so 3 bags in total with about ½ bag left over).
G(graham crackers) You will need 248 half(quarter) pieces for 4 classes. I used 62 half(quarter)
pieces for each class. Honey maid sells them in the half pieces, “Honey Maid Fresh Stacks” so
they are in stacks of 8 quarters (half pieces) wrapped. So I used about 1 and 1/3 box for each
class (so 5 boxes in total).