Air Bags (Lesson Plan)
(Exploring the Science of Automotive Air Bags)
Suggested Grade Level
8-9
Standard Statements (Pennsylvania)
3.1.10A Discriminate among the concepts of systems, subsystems, feedback, and control in
solving technological problems.
3.1.10E Describe patterns of change in nature, physical and man made systems.
3.2.10B Apply the elements of scientific inquiry to solve problems.
3.2.10C Identify and apply the technological design process to solve problems.
3.4.10A Explain concepts about the structure and properties of matter.
3.4.10B Analyze energy sources and transfers of heat.
3.8.10A Analyze the relationship between societal demands and scientific and technological
enterprises.
Content Objectives
Students will know that
1. Acetic acid reacts with sodium bicarbonate to produce a gas.
2. Acetic acid and sodium bicarbonate are the chemical names for vinegar and baking soda,
respectively.
3. The two main purposes of an air bag are to slow a person’s forward movement into the steering
wheel (or dashboard) and to provide a cushion between the person and the steering wheel (or
dashboard).
4. The three parts of an air bag are the bag itself, the sensors, and the inflation system (or gas
generator).
5. The main chemical responsible for the inflation of an air bag is sodium azide, which rapidly
decomposes into nitrogen gas and sodium metal when it is ignited.
6. Other reactions occurring within an air bag ensure that highly reactive, dangerous byproducts
are changed into stable and safe compounds.
7. The inflation of an air bag and the other chemical reactions occur in a very short amount of
time (1/25th of a second).
8. The reaction between acetic acid and sodium bicarbonate is similar to the decomposition of
sodium azide because both reactions produce a gas.
Process Objectives
Students will be able to
1. Provide evidence that a chemical reaction occurs between acetic acid and sodium bicarbonate.
2. Point out how the reaction between acetic acid and sodium bicarbonate is similar to the
decomposition of sodium azide.
3. Create a mock air bag by determining a quantity of sodium bicarbonate that will react with 25
mL of acetic acid and inflate a zip-seal plastic bag.
4. Test their mock air bag during an egg-drop crash test and determine the maximum height that
their protected egg can be dropped without the egg cracking.
Assessment Strategies
1. Evaluation of completed student handout.
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Lesson Plan
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2. Group presentations of completed mock air bag designs.
3. Evaluation of student essays from Part 8.
Materials
Per class
 A deflated balloon (to be blown up by you or a student)
 Computer equipped with PowerPoint software
 Projector and projector screen
 PowerPoint presentation that accompanies this activity
 Plastic sheet or other appropriate material to create an egg-drop crash zone
Per group of students
 zip-seal baggies (~3)
 sodium bicarbonate (baking soda), ~5g
 acetic acid (vinegar), ~100 mL
 graduated cylinder
 balance
 raw eggs (can be substituted with boiled eggs for less mess)
 facial tissue
 paper cup
 tape
 meter stick
Per student
 Copy of the history of air bag comic strip (pdf file)
Procedures
Part 1
(1, 45-min class period)
1. Assess what the students already know about automotive air bags by having them answer the
questions that appear in their Student Handout (What does an automotive air bag do?; How
does an automotive air bag work; Where are air bags found?)
2. Have the students share what they wrote down.
3. Show the first seven slides of the PowerPoint presentation on air bags to help the students
understand what an air bag looks like and where it is located.
4. Have the students sketch their own picture of an air bag in the box provided in their handout.
5. Have your students watch as you (or a student) blow up a balloon. Discuss what makes the
balloon inflate. Help the students understand that the balloon inflated because carbon dioxide
gas was exhaled into it. Emphasize that gases cause inflatable materials (like balloons and
bags) to inflate.
6. Allow the students to think about how they would get an air bag to inflate by completing the
prompt box in their handout.
Part 2
(30 min)
1. Have the students observe the production of gas from the chemical reaction between acetic acid
and sodium bicarbonate by following the directions provided in their handout.
2. Ask the students to provide evidence that a chemical reaction occurred between these two
substances.
3. Guide the students in recognizing acetic acid and sodium bicarbonate as the common
household substances, vinegar and baking soda, respectively.
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Lesson Plan
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Part 3
(40 min)
1. Have the students read “Air Bags: What’s Their Purpose?” in the Additional Resources section
of their handout and then have them summarize the two main purposes of air bags in the space
provided in their handout.
2. Help the students interpret what they read by showing them the slide in the PowerPoint that
illustrates how force is distributed with and without an air bag (slide #8). Emphasize that an air
bag helps to distribute the force of impact during a crash over a larger area.
3. Have the students read “Timing is Everything” in the Additional Resources section, and then
help them answer the questions in their handout on this reading.
4. To make the time scale of air bag deployment more concrete for the students, have them view
the 10-second online video clip entitled “Real-time deployment of air bag” from the General
Motors website (refer to student handout for URL).
5. Have students read about new design technologies for automotive air bags (“Advanced Air Bag
Design” article in the Additional Resources section of the handout).
Part 4
(Hmwk Assignment)
1. Distribute the history of air bag comic strip to each student. Have the students read it and then
answer the questions in their handout about it for a homework assignment.
2. Review the questions with the students to help make aspects of the nature of science embedded
in the comic more explicit.
Part 5
(20 min)
1. Have the students read “What’s Going On In There” in the Additional Resources section.
Assist them in completing the comprehension questions associated with this reading.
2. Help the students to realize that even though the chemicals they used in Part 2 are not the same
as the ones used in automotive air bags, both chemical reactions produce a gas. (The questions
in their handout are intended to help them with this concept.)
3. Tell the students that the reason why they are using acetic acid and sodium bicarbonate in this
activity is because sodium azide is highly toxic and explosive, making it too dangerous for
classroom use. Acetic acid and sodium bicarbonate, on the other hand, are common household
substances, safe enough to be used in a classroom.
Part 6
(20 min)
1. Show the remaining slides of the PowerPoint to introduce students to the three main parts of an
air bag. While doing so, select students to read the following parts from “The Three Parts of an
Automotive Air Bag” (Additional Resources section) aloud to the class: The bag, The sensors,
and The inflation system / gas generator.
2. Have the students construct an illustration of an air bag and its three major components using
the space provided in their handout.
Part 7
(1, 45-min class period)
1. Assign the reading entitled “Engineering Hurdles to Automotive Air Bag Development” and
the corresponding questions in their handout as a homework assignment (items #19 and #20).
2. Select students to read the four engineering hurdles in their handout aloud to the class.
3. Tell the students that in this part of the activity, they will figure out how to create a mock air
bag using the gas produced in the reaction between acetic acid and sodium bicarbonate.
Emphasize the fact that they will be responsible for addressing each of the four engineering
hurdles in this endeavor.
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4. Divide the students up into groups of three or four. Have the students follow the procedure in
their handout for determining an appropriate amount of sodium bicarbonate to react with 25 mL
of acetic acid (Engineering Hurdles #1, 2, & 3). Before allowing the students to work, point
out the data table in their handout and emphasize the categories in this data table for which they
will need to collect data.
5. Prepare a crash zone (or multiple crash zone stations) by laying out sheets of plastic on the
floor.
6. Have the students follow the directions in their handout for determining how well their mock
air bags perform (Engineering Hurdle #4).
7. Allow each group of students to present how they met each engineering hurdle to the class.
Part 8
(Hmwk Assignment)
1. As a post-assessment activity, have each student write an essay on how their group met each of
the four engineering hurdles. If needed, explain to them what is meant by a “trade-off,” and
instruct them to include which trade-offs they encountered while overcoming the four
engineering hurdles.
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Lesson Plan
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Air Bags (Teacher Notes)
(Exploring the Science of Automotive Air Bags)
General Lesson Notes

Warnings. Even though acetic acid (vinegar) is a household substance, students should avoid
getting it near or in their eyes. Students should also thoroughly wash their hands after handling
the raw eggs.

PowerPoint Presentation. The PowerPoint that accompanies this activity is mainly illustrative
in nature. Its intent is to serve as a set of visual aids as students go through specific points of the
activity.
Part 1 Notes

Tie between balloons & air bags. (This explanation pertains to Question #3 in the Student
Handout) In Part 1, you or a student will blow up a balloon by exhaling into it. This is
intended to get the students thinking about how inflatable materials (balloons, bags, etc.)
become inflated. A balloon inflates when a person exhales CO2 gas into it. An air bag inflates
when it becomes filled with N2 gas generated from a chemical reaction. The commonality
between the two is that in both cases, a gas causes something to inflate. Why do gases cause
inflatable materials to inflate? The kinetic theory of gases can be useful in explaining this
concept to your students (see next note), if it is desired. Otherwise, students should minimally
understand that a gas causes a balloon to inflate in the same way that a gas causes an air bag to
inflate. The only difference is that in a balloon, we exhale a gas into it, whereas in an air bag,
the gas is supplied from a chemical reaction.

Explanation of inflation using the kinetic theory of gases. The kinetic theory of gases
assumes that each gas molecule is a tiny point particle that moves randomly through space,
colliding with other gas molecules and with the walls of its container. As gas molecules collide
with the walls of their container, they exert a tiny force on the wall. Since there are many
molecules hitting the walls of the container over and over again, an aggregate force is exerted
over the entire container surface, thus causing pressure. Pressure causes the bag or balloon to
expand, and is defined as the force exerted by a gas per unit area (A) on the walls of the
container, (Pressure = Force / Surface Area).1 A diagram like the following may be helpful in
explaining these ideas to your students (if you wish to do so). It could easily be reproduced on a
chalkboard or whiteboard or copied into the PowerPoint that accompanies this activity.
1
http://www.chemistry.wustl.edu/~edudev/LabTutorials/Airbags/airbags.html (accessed 5/16/05; 10/12/05)
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Teacher Notes
1
Source (point particle gas image on far right):
http://www.chemistry.wustl.edu/~edudev/LabTutorials/Airbags/airbags.html (accessed 10/12/05).
Part 2 Notes

Chemistry of acetic acid and sodium bicarbonate. (This explanation pertains to Questions
#6 and #7 in the Student Handout) If an air bag inflates because a gas is produced in a
chemical reaction, what chemical reactions produce gases? Well, lots! The reaction between
acetic acid (vinegar) and sodium bicarbonate (baking soda) is just one example, and it is useful
for educational purposes because the chemicals are relatively safe and inexpensive. In this
reaction, acetic acid combines with sodium bicarbonate to form carbonic acid (H2CO3), which
then rapidly decomposes into carbon dioxide and water. We can write this equation as follows:
HC2H3O2
acetic acid
+
NaHCO3
→
sodium bicarbonate
H2CO3
carbonic acid
NaC2H3O2
+
H2CO3
sodium acetate
→→
H2O
water
+
carbonic acid
CO2
carbon dioxide
----------------------------------------------------------------------------------------Overall reaction:
HC2H3O2 + NaHCO3 → NaC2H3O2 + H2O + CO2
The gas produced in these reactions is carbon dioxide. The sodium acetate ends up in solution
(as aqueous, dissolved in water). The production of a gas (and thus the inflation of the zip-seal
baggie) is the visible indicator that a chemical reaction occurred. The decomposition of sodium
azide is another chemical reaction that produces a gas and is the one utilized in automotive air
bag technology. However, sodium azide is highly toxic and is explosive at high temperatures,
making it inappropriate for use in a classroom setting.
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Teacher Notes
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Part 3 Notes

Timing of air bag deployment. The deployment of an air bag happens extremely quickly,
faster than the blink of an eye. The 10-second online video clip (URL provided in the Student
Handout) is intended to help students visualize this. The three chemical reactions occurring
within an air bag all happen in 1/25th of a second!
Part 4 Notes

The history of air bag comic strip. The historical comic strip that accompanies this activity is
taken from the American Society of Mechanical Engineers (www.asme.org). To access the
comic, follow this URL: http://anniversary.asme.org/comicbook.shtml. Click on
“Determination – Safety Air Bag.” The artist is Ron Spellman. It is free to download.

Air bag history and the Nature of Science. (This explanation pertains to Question #11 in the
Student Handout) The questions in the student handout pertaining to the comic strip are
intended to emphasize aspects of the nature and history of science. The fact that Allan Breed
used what he had learned from making fuses, timers, and sensors for U.S. Military weapons to
come up with an innovative way to protect passengers during crashes was highly creative
(question a and c). His initial ball-in-tube design using magnetism and an electrical circuit was
also very creative (question a). Because scientific research is a human activity, there is always
some measure of subjectivity involved. It is clear that Breed’s prior knowledge of how to
construct fuses and sensors for Military weapons influenced his breakthrough on how to
construct a safety air bag for automobiles (question c). Had a different engineer with a different
background set out to solve this problem, it is unlikely he or she would have come up with
exactly the same design or solution to the problem (question d). Allen Breed struggled to get
automotive companies to support his idea, until a political move by congress mandated it
(question e). This is illustrative of there being political and social influences on science. It is
because science is really just apart of the greater human social system that some scientific
research projects are favored and some are discouraged at different times throughout history.
Part 5 Notes

The chemistry of automotive safety air bags. Please refer to the reading entitled What’s
Going On In There? for an explanation of the chemistry occurring inside automotive safety air
bags.

Chemicals. (This explanation pertains to Questions #15 and #16 in the Student Handout) It
is important for students to realize that the chemicals they are using in this activity are not the
same ones used in a real air bag. The reaction that occurs between vinegar and baking soda is
analogous to the air bag reaction because it produces a gas. Students should be helped to
understand that sodium azide is not being used in this activity because it is highly toxic and
explosive, whereas the reaction between acetic acid and sodium bicarbonate is much safer.
Part 6 Notes

The parts of an automotive air bag. (This explanation pertains to Question#18 in the
Student Handout) Please refer to the reading entitled The Three Parts of an Automotive Air
Bag for an explanation of the sensors, the inflation system, and the bag itself.
Part 7 Notes
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Teacher Notes
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
Why must engineers determine the exact quantity of chemicals to use in the air bag? (This
explanation pertains to Question #20 in the Student Handout) The answer to this is
essentially two-fold. One, the exact quantity of reactants needs to be calculated so that just the
right amount of gas gets produced. If the air bag under-inflates, the air bag will not provide
enough protection for the driver. If the air bag over-inflates, the bag could burst or be too hard
for cushioning (thus causing more injury). Two, chemicals cost money! Air bag manufacturers
only want to use what is needed, else the cost of making the air bag increases unnecessarily. To
stress the importance of not adding an excess of sodium bicarbonate, one of the four
Engineering Hurdles is to consider cost of the sodium bicarbonate. This is not unlike what
engineers must do.

Designing the mock air bag. In Part 7, students will attempt to find the amount of sodium
bicarbonate (baking soda) required to react with 25 mL of acetic acid (vinegar) so that their
zipper-seal bag is inflated but does not have a significant quantity of either reactant left over.
Four “Engineering Hurdles” are presented to the students as they embark on this task. An
understanding of how to do reaction stoichiometry is not required here. The intent of the
exercise is for students to use trial and error in somewhat of a systematic fashion so that they
gain an understanding that gases can be generated in a controlled manner from chemical
reactions by varying the amount of reactants. Just as they can control the amount of carbon
dioxide gas that gets produced in their mock air bag, engineers and air bag manufacturers can
control the amount of nitrogen gas that gets produced to ensure that the air bag fully inflates.

Amount of baking soda. Depending on the atmospheric pressure, the amount of baking soda
required to fully inflate the bag containing 25 mL of acetic acid should be between 1.5 and 3.0
grams.

Crash testing with eggs. At this point of the activity, students are told that they may inflate
multiple air bags to protect their egg, but that they should still consider the engineering hurdle
that addresses cost. If students wish to inflate 5 or 6 zipper-seal bags, remind them of
engineering hurdle #2. You may want to have students drop their protected eggs (in the paper
cup, attached to their mock air bag) into a shoe box or some other plastic container, to help
contain potential messes as they are dropped.
Additional References

http://www.falstad.com/mathphysics.html. Educational java applets in math, physics, and
engineering. The one entitled “Gas Molecules Simulation Applet” provides an excellent visual
for the kinetic theory of gases. (Accessed 10/13/05).

Possible extensions. The concept of limiting reactants, stoichiometry, and Newton’s laws could
easily be incorporated into this lesson for more advanced students or upper grade levels.
Consult “The Chemistry behind the Air Bag” (Journal of Chemical Education, J. Madlung,
1996, 73, 347) for ways to incorporate stoichiometry or “Chemistry Behind Airbags: What
about the Gas Used to Fill the Airbag” (from
http://www.chemistry.wustl.edu/~edudev/LabTutorials/Airbags/airbags.html) for ways to
incorporate physics calculations and concepts.
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Teacher Notes
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Name: ________________________
Air Bags
(Exploring the Science of Automotive Air Bags)
Overview
In this activity, you will learn how automotive air bags work. You will get to make a mock air bag
using substances which are safe enough to use in the classroom, and then you will test this air bag
during an egg-drop crash test! Have fun!
Part 1 (What do I know about automotive air bags?)
1) Without reading any of the material in the Additional Resources section, answer the following
questions to the best of your knowledge:
a) What does an automotive air bag do?
b) How does an automotive air bag work?
c) Where are air bags found?
What I know about air bags
a)
b)
c)
2) After your teacher has shown you several pictures of air bags, sketch your own diagram of an
air bag in the box provided.
My sketch of an air bag
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Student Handout
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3) Watch as your teacher or one of your classmates blows up a balloon. What causes the balloon
to inflate?
________________________________________________________________________________
________________________________________________________________________________
4) Obviously, an automotive air bag does not inflate by a person exhaling into it like they would
to blow up a balloon! If you were a scientist or engineer, how would you get an air bag in a car to
inflate? List your ideas in the space provided.
To get an air bag to inflate, I would…
Part 2 (How are gases generated?)
5) You will now observe the interaction between two chemicals that can be found in a household
kitchen. Follow the procedure provided to observe the interaction between acetic acid and sodium
bicarbonate:
a) Add 25 mL of acetic acid to a zip-seal plastic bag.
b) Place a piece of tissue on a balance and tare the balance. Measure out 0.5g of sodium
bicarbonate. Carefully wrap up the sodium bicarbonate in the tissue, being careful not to
loose any of it in the process.
c) Drop the tissue bundle containing the sodium bicarbonate in the bag. Quickly flatten the
bag to remove any air and then seal it.
d) Help the two substances come in contact by gently pressing on the tissue bundle while it is
immersed in the acetic acid. This will help the acetic acid soak through the tissue.
e) Observe what happens, and write your observations in the box provided. Record what you
see and hear.
f) After several seconds, make sure that the reaction is complete by swishing the liquid in the
bag around and pressing it against the tissue (do not open the bag to do this; simply press on
the plastic).
6) What evidence is there that a chemical reaction occurred between acetic acid and sodium
bicarbonate? Record your evidence in the box provided.
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Student Handout
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Evidence that a chemical reaction occurred
7) What are the common household names for acetic acid and sodium bicarbonate?
Acetic acid
=
_____________________________________________
Sodium bicarbonate =
_____________________________________________
Part 3 (What is the purpose of air bags?)
8) Read Air Bags: What’s Their Purpose? in Additional Resources. What are the two main
purposes of air bags?
________________________________________________________________________________
________________________________________________________________________________
9) Read the paragraph in Additional Resources entitled Timing is Everything, and then answer
the following questions:
a) Why would it be bad for a person to collide with an air bag the instant it is inflated?
_____________________________________________________________________________
b) What is present in the air bag to allow it to deflate some before a person hits it?
_____________________________________________________________________________
10) View the online video clip entitled “Real-time deployment of air bag” from the General Motors
website. Use the following URL:
http://www.gm.com/company/gmability/safety/protect_occupants/air_bags/questions/video_highlights.html
11) Read about advanced air bag technologies currently under development in the Advanced Air
Bag Design article in Additional Resources. List some of these technologies in the space provided.
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
Part 4 (Who invented air bags?)
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Student Handout
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12) Read the comic strip that your teacher provides on the history of air bags. Answer the
following questions:
a) How does this story show that the scientific process involves creativity?
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
b) Using words or a sketch, explain Allen Breed’s original air bag idea.
Allen Breed’s 1st Air Bag
c) How does this story show that scientists’ and engineers’ prior knowledge and experiences
influence how they solve a particular problem?
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
d) Do you think a different scientist / engineer would have come up with exactly the same air
bag idea as Allen Breed’s? Why / Why not?
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
e) How does this story demonstrate that scientific research can be influenced by political
factors?
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
Part 5 (The Chemistry of Automotive Air Bags)
13) Read What’s Going On In There? in Additional Resources, and then answer the following
questions:
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Student Handout
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a) What are the three chemical reactants found in an air bag’s inflation system? List these on
the lines provided.
_________________________________
_________________________________
_________________________________
b) Put a star next to the chemical in list a) that rapidly decomposes to produce a gas.
c) What gas fills the air bag?
_________________________________
d) What is the purpose of the potassium nitrate and silicon dioxide?
_____________________________________________________________________________
e) How many different reactions are going on inside an air bag when it is deployed? _______
f) How long does it take for all of these reactions to occur? ___________________________
14) In Part 2 of this activity, you observed a chemical reaction between acetic acid and sodium
bicarbonate. Circle the product in the word equation written below that was the gas produced.
Acetic Acid + Sodium bicarbonate→ Sodium Acetate + Water + Carbon dioxide
15) Compare the products of the acetic acid / sodium bicarbonate reaction (written below) with the
products of the decomposition of sodium azide that occurs in an air bag (also written below). What
is similar? (Hint: focus on the states of matter in the products.)
________________________________________________________________________________
________________________________________________________________________________
HC2H3O2(aq) + NaHCO3(s) → NaC2H3O2(aq) + H2O(l) + CO2(g)
2NaN3(s) → 2Na(s) + 3N2(g)
Acetic acid / sodium bicarbonate
Decomposition of sodium azide
*(aq) means dissolved in water
16) Why are we using acetic acid and sodium bicarbonate in this activity instead of sodium azide?
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
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Student Handout
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Part 6 (What parts make up an automotive air bag?)
17) Read The Parts of an Automotive Air Bag in the Additional Resources section.
18) After your teacher discusses the three main parts of an air bag, draw and label a diagram that
illustrates these parts. Use the box provided.
The Parts of an Automotive Air Bag
Part 7 (Constructing Your Mock Air Bag)
19) Scientists faced some engineering challenges in developing automotive air bags. Read
Engineering Hurdles to Automotive Air Bag Development to find out what these were. List
these hurdles in the space provided.
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
20) The engineers who design air bags must determine the exact quantity of chemicals needed to
inflate the air bag to the proper volume. List several reasons why you think this may be so.
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
21) Now its your turn to design an air bag! However, there are several engineering hurdles you will
need to do your best to overcome. Carefully read the list of four engineering hurdles before
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Student Handout
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proceeding. At the end of this activity, you will be asked to address how your group tackled each
hurdle. Do your best and good luck!
Engineering Hurdle #1:
The amount of acetic acid you will use in your mock air bag must be
25 mL. However, the amount of sodium bicarbonate is unknown.
Determine an amount of sodium bicarbonate so that all of the acetic
acid in the bag is used up in the chemical reaction.
Engineering Hurdle #2:
Chemicals cost money. Sodium bicarbonate costs approximately 3¢
per gram. Determine an amount of sodium bicarbonate so that the
chemicals for your mock air bag will be as cheap as possible.
Engineering Hurdle #3:
The zip-seal bag (your mock air bag) must inflate, but not pop open.
Engineering Hurdle #4:
Your mock air bag must be inflated enough so that it can protect a
falling egg.
22) Follow the procedure provided to get started on Engineering Hurdles #1, #2, & #3. (Note:
This procedure is very similar to what you did in Part 1.)
a) Add 25 mL of acetic acid to a zip-seal plastic bag.
b) Place a piece of tissue on a balance and tare the balance. Measure out an amount of sodium
bicarbonate and note it in the Data Table provided. Carefully wrap up the sodium
bicarbonate in the tissue, being careful not to loose any of it in the process.
c) Drop the tissue bundle containing the sodium bicarbonate in the bag. Flatten the bag to
remove any air and then seal it.
d) Help the two substances come in contact by gently pressing on the tissue bundle while it is
immersed in the acetic acid. This will help the acetic acid soak through the tissue.
e) After several seconds, make sure that the reaction is complete by swishing the liquid in the
bag around and pressing it against the tissue (do not open the bag to do this; simply press on
the plastic). Continue to do this until there is no more sign of the chemicals reacting.
f) Test how inflated the bag is by pinching it. Write a description of your pinch test in the data
table provided.
g) Now open the bag and add a small amount of sodium bicarbonate. If there is any sign of
reaction after doing this, this indicates that not enough sodium bicarbonate was added to
completely react with all of the acetic acid in the bag. If there is no sign of reaction, then
there was enough sodium bicarbonate so that all of the acetic acid reacted -- but is there too
much sodium bicarbonate? (Remember, you want to make this air bag as cheaply as
possible.)
h) In the data table provided, indicate whether or not the amount of sodium bicarbonate you
started out with in step b) was enough to react with all of the acetic acid in the bag.
i) Repeat steps a) through g), using different starting amounts of sodium bicarbonate, but
always using the same amount of acetic acid (25 mL). Record the amounts of sodium
bicarbonate you use in each case, the results of your pinch test for each amount, and whether
or not there was unreacted acetic acid for this amount (as determined in step g).
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Student Handout
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To test your air bag against Engineering Hurdle #4, follow the directions provided for crash
testing in item #23.
Data Table for determining amount of sodium bicarbonate to use in your air bag.
Amount of
sodium
bicarbonate
used (grams)
Description of pinch test
Unreacted
acetic acid for
this amount?
(Yes or No)
Do I need to
add more or
less sodium
bicarbonate?
23) Directions for crash testing (Engineering Hurdle #4)
a) Inflate a zip-seal bag, using 25 mL of acetic acid and the amount of sodium bicarbonate you
decided on in Part 6. You may inflate multiple bags to better protect your egg if you wish,
but remember Engineering Hurdle #2!
b) Obtain a paper cup and tape from you teacher. Devise a way to attach the paper cup to your
air bag(s) so that it acts as a holder for the egg.
c) Obtain a meter stick from your teacher. Starting with small distances from the ground and
then increasing to larger distances, drop your protected egg to determine the maximum
height at which the egg is protected by your air bag(s).
d) Create a table that will allow you to keep track of the heights tested and the results (whether
the egg survived the crash).
e) Any cracks or breaks in the egg means the egg was not protected sufficiently.
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24) Record the maximum height at which your protected egg and air bag(s) can be dropped without
cracking:
___________________meters
25) Record the total amount of sodium bicarbonate you ended up using to design your mock air bag
(if you chose to use more than one bag, be sure to calculate a TOTAL amount):
______________________g
26) Calculate the cost of sodium bicarbonate for your air bag (sodium bicarbonate costs about 3 ¢
per gram):
$_______________________
27) Compare your air bag design, maximum height, amount of sodium bicarbonate used, and the
cost of your mock air bag with those of your classmates.
Part 8 (Analyzing the trade-offs)
28) Write an essay that addresses how your group tackled EACH ONE of the four Engineering
Hurdles. Include the following items in your essay:
a) Any problems you ran into as you sought to overcome these hurdles in creating your mock
air bag.
b) Some of the trade-offs you encountered. For example, how did you address cost
(Engineering Hurdle #2) while at the same time ensure good performance (Engineering
Hurdle #4)?
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Additional Resources
Automotive Air Bags: What’s Their Purpose?
The purpose of an air bag is to slow the passenger’s forward movement into the steering wheel (or
dash board) during a collision and also to provide a cushion between the passenger and the steering
wheel. During a collision, the driver’s head and torso are thrown forward rapidly. The goal of an
air bag is to help the passenger come to a stop while doing as little damage to them as possible. If
there is no restraining force (i.e., from a seat belt and air bag), the driver’s body hits the steering
wheel instantaneously, causing serious head and spine injury. One of the ways an air bag helps
reduce injury is by spreading the force of impact with the dashboard or steering wheel over a larger
area, as illustrated in Figure 1. When the force is spread over a larger area of the body, the injuries
are less severe.
Figure 1. Force distribution during collision comparing air bag to no air bag.
Source: Gas Laws Save Lives: The Chemistry Behind Air Bags. Washington University in St. Louis, Chemistry
Department. http://www.chemistry.wustl.edu/~edudev/LabTutorials/Airbags/airbags.html (accessed May 16, 2005).
Timing is everything!
The air bag must inflate within milliseconds and then begin deflating before the driver’s head and
body actually make contact with it. This is because the explosive decomposition of sodium azide
occurs so rapidly and produces so much nitrogen gas that the bag is initially as hard as stone. 2 If the
driver were to make contact with the bag at this point, the bag would add to injury instead of
preventing it. Therefore, vents or small holes in the air bag allow the gas generated to escape,
creating a soft cushion instead of a brick wall for the passenger!
2
Madlung, A. (1996). The chemistry behind the air bag. Journal of Chemical Education, 73(4), 347-348.
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Advanced Air Bag Design
(An excerpt taken from wikipedia.org)3
Many advanced air bag technologies are being developed to tailor air bag deployment to the
severity of the crash, the size and posture of the vehicle occupant, belt usage and how close that
person is to the air bag module. Many of these systems will use multi-stage inflators that deploy less
forcefully in stages in moderate crashes than in very severe crashes. Occupant sensing devices let
the air bag diagnostic unit know if someone is occupying a seat in front of an air bag, whether the
person is an adult or a child, whether a seat belt or child restraint is being used and whether the
person is forward in the seat and close to the air bag module. Based on this information and crash
severity information, the air bag is deployed at either a high force level, a less forceful level or not
at all.
Many new vehicles are also equipped with side air bags. While there are several types of side air
bags, all are designed to reduce the risk of injury in moderate to severe side impact crashes. These
air bags are generally located in the outboard edge of the seat back, in the door or in the roof rail
above the door.
Seat and door-mounted air bags all provide upper body protection. Some also extend upwards to
provide head protection. Two types of side air bags, known as inflatable tubular structures and
inflatable curtains, are specifically designed to reduce the risk of head injury and/or help keep the
head and upper body inside the vehicle. A few vehicles are now being equipped with a different
type of inflatable curtain designed to help reduce injury and ejection from the vehicle in rollover
crashes.
What’s Going On In There?
(The Chemistry of Automotive Air Bags)
The three chemical reactants contained in the inflation system of an air bag are sodium azide
(NaN3), potassium nitrate (KNO3), and silicon dioxide (SiO2). Sodium azide is a highly toxic
chemical that reacts explosively at high temperatures. It rapidly decomposes into sodium metal
(Na) and nitrogen gas (N2) at ~300oC. That’s around 570oF! Because the decomposition of sodium
azide produces a large volume of gas very rapidly, this chemical reaction is ideal for an air bag.
The high-temperature necessary for sodium azide to decompose is obtained when the mixture of
chemicals is ignited by an electrical impulse from the sensors. The chemical reaction proceeds
almost instantaneously, and nitrogen gas fills the air bag at 200 mph!
While the nitrogen gas generated is harmless, the sodium metal produced is extremely reactive and
potentially explosive. Therefore, the purpose of the other two chemicals (potassium nitrate and
silicon dioxide) is to convert the sodium metal into a much safer and more stable substance. First,
the sodium metal reacts with potassium nitrate to give potassium oxide (K2O), sodium oxide
(Na2O), and more nitrogen gas. Potassium and sodium oxide are highly reactive compounds, so
they must be reacted with something else to make them safe. Silicon dioxide combines with the
potassium and sodium oxides to give alkaline silicates, which are essentially forms of glass and are
3
http://en.wikipedia.org/wiki/Air_bag
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harmless. Keep in mind that all of this occurs in 1/25th of a second! The three chemical reactions
that take place in an air bag are as follows:
2NaN3(s) → 2Na(s) + 3N2(g)
Equation 1
10Na(s) + 2KNO3(s) → K2O(s) + 5Na2O(s) + N2(g)
Equation 2
K2O(s) + Na2O(s) + SiO2(s) → alkaline silicates (“glass”) Equation 3
The Three Parts of an Automotive Air Bag
An air bag has three main parts: the bag itself, sensors, and the inflation system (or gas generator).
The bag. The bag is made of a thin, lightweight nylon fabric that is folded up and stored compactly
in the steering wheel, dashboard, seat back, or door. When fully inflated, the steering wheel air bag
is approximately the diameter of a large beach ball4 (a volume between 65 and 70 liters). Air bags
in seat backs and doors are more recent developments.
The sensors. When special sensors detect a sudden decrease in acceleration brought about during a
collision, they tell the inflation system to inflate the bag. In order for inflation to occur the collision
must at least be equal to the decrease in acceleration and force your car would experience from
running into a brick wall at 10-15 miles per hour. The maximum deceleration generated in even the
severest braking is only a small fraction of that necessary to activate the sensors.5
The inflation system / gas generator. The inflation system (or gas generator) consists of an igniter
and chemicals. The explosion that occurs when the chemicals are ignited produces harmless
nitrogen gas which fills the air bag in 1/25th of a second! This system is very similar to what is
often used to propel rocket engines.
Engineering Hurdles to Automotive Air Bag Development
(An excerpt taken from www.howstuffworks.com)6
According to Scientific American:
The idea of using a rapidly inflating cushion to prevent crash injuries had a long history
before the U.S. Department of Transportation called for the equipment to be adapted for
automobiles in the 1980s. The first patent on an inflatable crash-landing device for airplanes
was filed during World War II.
Early efforts to adapt the air bag for use in cars bumped up against prohibitive prices and technical
hurdles involving the storage and release of compressed gas. Researchers wondered:
“Air Bag,” http://en.wikipedia.org/wiki/Air_bag, accessed September 23, 2005.
“Air Bag,” http://en.wikipedia.org/wiki/Air_bag, accessed September 23, 2005.
6
http://autohowstuffworks.com/airbag2.htm
4
5
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


If there was enough room in a car for a gas canister
Whether the gas would remain contained at high pressure for the life of the car
How the bag could be made to expand quickly and reliably at a variety of operating
temperatures and without emitting an ear-splitting bang
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