Limiting Reactant
Ck12 Science
Say Thanks to the Authors
Click http://www.ck12.org/saythanks
(No sign in required)
To access a customizable version of this book, as well as other
interactive content, visit www.ck12.org
CK-12 Foundation is a non-profit organization with a mission to
reduce the cost of textbook materials for the K-12 market both in
the U.S. and worldwide. Using an open-source, collaborative, and
web-based compilation model, CK-12 pioneers and promotes the
creation and distribution of high-quality, adaptive online textbooks
that can be mixed, modified and printed (i.e., the FlexBook®
textbooks).
Copyright © 2015 CK-12 Foundation, www.ck12.org
The names “CK-12” and “CK12” and associated logos and the
terms “FlexBook®” and “FlexBook Platform®” (collectively
“CK-12 Marks”) are trademarks and service marks of CK-12
Foundation and are protected by federal, state, and international
laws.
Any form of reproduction of this book in any format or medium,
in whole or in sections must include the referral attribution link
http://www.ck12.org/saythanks (placed in a visible location) in
addition to the following terms.
Except as otherwise noted, all CK-12 Content (including CK-12
Curriculum Material) is made available to Users in accordance
with the Creative Commons Attribution-Non-Commercial 3.0
Unported (CC BY-NC 3.0) License (http://creativecommons.org/
licenses/by-nc/3.0/), as amended and updated by Creative Commons from time to time (the “CC License”), which is incorporated
herein by this reference.
Complete terms can be found at http://www.ck12.org/about/
terms-of-use.
Printed: August 11, 2015
AUTHOR
Ck12 Science
www.ck12.org
C HAPTER
Chapter 1. Limiting Reactant
1
Limiting Reactant
• Define limiting reactant.
• Describe how to determine which component in a reaction is the limiting reactant.
Don’t you hate running out of cooking ingredients?
Cooking is a great example of everyday chemistry. In order to correctly follow a recipe, a cook needs to make sure
that he has plenty of all the necessary ingredients in order to make his dish. Let us suppose that you are deciding to
make some pancakes for a large group of people. The recipe on the box indicates that the following ingredients are
needed for each batch of pancakes:
1 cup of pancake mix
3
4
cup milk
1 egg
1 tablespoon vegetable oil
Now you check the pantry and the refrigerator and see that you have the following ingredients available:
2 boxes of pancake mix (8 cups)
Half gallon of milk (4 cups)
2 eggs
Full bottle of vegetable oil (about 3 cups)
The question that you must ask is: How many batches of pancakes can I make? The answer is two. Even though
you have enough pancake mix, milk, and oil to make many more batches of pancakes, you are limited by the fact
that you only have two eggs. As soon as you have made two batches of pancakes, you will be out of eggs and your
“reaction” will be complete.
1
www.ck12.org
Limiting Reactant
For a chemist, the balanced chemical equation is the recipe that must be followed. As you have seen earlier, the
Haber process is a reaction in which nitrogen gas is combined with hydrogen gas to form ammonia. The balanced
equation is shown below.
N2 (g) + 3H2 (g) → 2NH3 (g)
We know that the coefficients of the balanced equation tell us the mole ratio that is required for this reaction to occur.
One mole of N2 will react with three moles of H2 to form two moles of NH3 .
Now let us suppose that a chemist were to react three moles of N2 with six moles of H2 (see Figure 1.1).
FIGURE 1.1
Reaction in presence of limiting reagent.
So what happened in this reaction? The chemist started with 3 moles of N2 . You may think of this as being 3 times
as much as the “recipe” (the balanced equation) requires since the coefficient for the N2 is a 1. However, the 6 moles
of H2 that the chemist started with is only two times as much as the “recipe” requires, since the coefficient for the
H2 is a 3 and 3 × 2 = 6. So the hydrogen gas will be completely used up while there will be 1 mole of nitrogen gas
left over after the reaction is complete. Finally, the reaction will produce 4 moles of NH3 because that is also two
times as much as shown in the balanced equation. The overall reaction that occurred in words:
2 mol N2 + 6 mol H2 → 4 mol NH3
All the amounts are doubled from the original balanced equation.
The limiting reactant (or limiting reagent) is the reactant that determines the amount of product that can be formed
in a chemical reaction. The reaction proceeds until the limiting reactant is completely used up. In our example above,
the H2 is the limiting reactant. The excess reactant (or excess reagent) is the reactant that is initially present in
a greater amount than will eventually be reacted. In other words, there is always excess reactant left over after the
reaction is complete. In the above example, the N2 is the excess reactant.
Summary
• The amount of limiting reactant determines how much product will be formed in a chemical reaction.
2
www.ck12.org
Chapter 1. Limiting Reactant
Practice
Questions
Watch the video at the link below and answer the following questions:
http://www.sophia.org/limiting-reactant-definition/limiting-reactant-definition--2-tutorial
1. What reaction is occurring?
2. How is the reaction measured?
3. What do the balloons tell us?
Review
Questions
1. In the Haber reaction illustrated above, how do we know that hydrogen is the limiting reactant?
2. What if hydrogen were left over?
3. Which material would be limiting if no hydrogen or nitrogen were left over?
• excess reactant (or excess reagent): The reactant that is initially present in a greater amount than will
eventually be reacted.
• limiting reactant (or limiting reagent): The reactant that determines the amount of product that can be
formed in a chemical reaction.
References
1. Courtesy of Renee Comet and the National Cancer Institute. http://commons.wikimedia.org/wiki/File:Panca
kes_%281%29.jpg .
2. CK-12 Foundation - Christopher Auyeung. .
3