AIR BAGS R US
John Woolcock (IUP Dept of Chemistry)
Revised by Phil Palko (2012)
Based on experiments developed by Jim Klent and Angelica Stacy (ChemConnections)
Scenario
In this experiment, you have been asked by your boss, I. M. Gaseous, of the AIR BAGS R US
Co. to investigate the design of an small-scale airbag system they want to produce as a child
safety device to put in baby carriages. Because of sodium azide’s toxicity, he suggests you use
the reaction of NaHCO3 (sodium bicarbonate or baking soda) with an aqueous solution of HCl
(stomach acid) to produce CO2 gas to test your engineering design. The reaction of hydrochloric
acid and sodium bicarbonate is: HCl + NaHCO3 ---> NaCl + CO2 + H2O. So, for every mole of
HCl and NaHCO3, one mole of CO2 gas is produced. With regard to the other products, NaCl
dissolves in the water to create a salt solution which occupies only a small portion of the volume
inside the bag.
Background Information About Automobile Airbags
Chemists can use concepts like stoichiometry, molar mass, and balanced chemical equations to
predict just about anything about a chemical reaction. You may think that this kind of stuff never
leaves the lecture hall, but virtually everything you can buy depends on a chemist determining
how to mix things up. In industry, where huge amounts of chemicals are used, careful
calculations can save millions of dollars in chemicals and disposal costs and so they make good
economic and environmental sense. The invention of the automobile air bag, for instance
required consideration of not only the chemistry, but economic and environmental factors.
Chemists are trained to take this "big picture" approach to problem solving. In this investigation
you’ll design a model air bag. You’ll use the theoretical tools you have learned about gases and
stoichiometry to decide how to optimize the reaction that inflates the bag.
The automobile air bag is a remarkable device. It is deceptively simple in concept, but the design
requirements of the system are actually very demanding. When a crash occurs, an air bag must
inflate rapidly (within about 40 milliseconds), cushioning the occupants against impact. The gas
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produced must be nontoxic, odorless, and cool enough to avoid burning the occupants. You’d
also like the compounds used to be stable, and nontoxic; so they don’t expand unexpectedly and
are easy and safe to dispose of and don’t create hazards in a land fill or junk yard.
Sensors detect impact and electrically initiate the reaction to activate modern air bags.
Sodium azide (NaN3) is a stable solid and a small pellet can easily be stored in air bag
compartments. A huge volume of nitrogen (N2) gas is produced rapidly and is nontoxic and
relatively cool. Sodium azide itself, however, is pretty nasty. Toxic chemicals like sodium azide
are a major concern in landfills where undeployed air bags are discarded.
The sodium metal produced reacts violently with water. Fortunately, sodium metal can be
transformed into the relatively inert compound NaFeO2 by adding Fe2O3 (iron oxide or rust) to
the pellet. The pellets also contain additives that enhance the rate at which the gas is produced
while minimizing the heat. Some formulations are trade secrets, but all require a careful
chemical, environmental and economic analysis. These issues, as well as the quest for better and
cheaper systems, keeps open the search for other compounds and formulations for the gasproducing reaction used in the system.
The Ideal Gas Law and Mass-Mole Relationships
In the reaction shown above the CO2 gas produced obeys the Ideal Gas Law. In, fact at everyday
temperatures and pressures, all gases obey the Ideal Gas Law. This gas law combines together all
the major properties of a gas: pressure, volume, temperature and amount (moles) of gas. The
algebraic expression for this law is:
PV = nRT
where P is the pressure of the gas in atmospheres (atm), V is the volume in liters (L), n is the
moles of gas, R is the Ideal Gas Constant and has a value of 0.0821 atm-L/mole-K and T is the
temperature of the gas in degrees Kelvin (K). With this algebraic equation and the measurement
of any three properties of a gas, the fourth property can be calculated. For example, if P, V and T
are measured, then the moles of gas can be found by substituting these measured values into the
Ideal Gas Law equation, along with the value for R and solving for the moles of gas.
In most laboratories the properties of P, V and T are often measured in units different from those
used in the Ideal Gas Law. In the lab, pressure is typically measured in mm Hg or torr, volume is
typically measured in milliliters (mL) and temperature is typically measured in degrees Celsius
(°C). Properties of a gas measured with the these units must be converted to those in the Ideal
Gas Constant before being used in the Ideal Gas Law. To do this you can use the following
conversion factors: 760 torr (or mm Hg) = 1 atm, 1 L = 1000 mL and K = 273 + °C.
Once the number of moles of gas needed to fill the bag has been calculated, the exact mass of
NaHCO3 solid and the volume of 6 M HCl solution needed to make just that amount of gas can
be calculated. Let's suppose that we need one mole of CO2 gas to fill the air bag. The balanced
reaction equation above shows us that 1 mole of CO2 gas can be produced from 1 mole of
NaHCO3 and 1 mole of HCl. To find the mass of solid NaHCO3 that is required, you must
multiply the moles of NaHCO3 by its molar mass in grams/mole. This is obtained by adding up
the molar masses of the elements in the formula using the relative atomic weights on the periodic
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table. The volume of HCl required is calculated differently since it is a solution instead of a
solid. The quantity 6 M indicates a solution that has a concentration of 6 moles/Liter. Therefore
to find the volume you must divide the moles needed, in this case one, by the molarity.
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Procedure
Your main objective is to design a model automobile airbag which expands to the largest
possible volume without breaking the seal on the bag. To simulate the airbag we will use a zip
lock bag.
Part I: Formation of Groups
Work in groups of two or three.
You’ll have the following to work with:
 plastic zip-loc bags
 6 M HCl
 NaHCO3
 timer
 1 ruler or vernier caliper
You may need other items to complete the design of the air bag. First look at the equipment and
supplies available. You should ask your instructor if there are other items you need, but you
should be ready to explain why you need them.
Next, your group needs to decide how to measure the pressure, temperature and the volume of
gas that fills the air bag. So you may need things such as a barometer and a large graduated
cylinder. Once you have agreed on how this can be accomplished, make these initial
measurements and record the values in your lab notebook.
Part II: Design of the Air Bag
The specifics of the design is up to you, but we suggest the following:
 You should calculate the exact amounts of solid NaHCO3 and 6 M HCl solution that are
needed to produce just enough gas to create a cushion of the largest possible volume
without breaking the seal on the bag. Record the amounts you use in your lab notebook.
 The trigger and deployment system should require minimal external parts or assistance.
 You should have the acid and the NaHCO3 inside the model air bag in a stable configuration
that will survive handling without deploying until triggered.
 You will also want to measure the thickness of the bag and how fast it inflates.
You should describe or sketch in your notebook the design of your airbag, including the
mechanism that you are using to trigger the reaction and deploy the bag. After each test of the air
bag, empty all the chemical waste into the sink. The bags may be rinsed in the sink and wiped
out with a paper towel to dry them for the next trial.
Part III: Optimization of Air Bag Design
Once you have a successful air bag design and you can operate it, use your model air bag to
determine if changing the amount of one or both of the reactants changes the speed of inflation
of the bag or creates a better or poorer cushion. To do this you may want to consider doubling or
cutting in half the amount of either or both of the reactants, deploying the air bag and measuring
the width of the bag and how fast the gas fills it.
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For Your Presentation:
Use the following checklist to write your report for this experiment. Your “report” will
be presented in a poster session.
Experiment Objective
 Identify the members of your group and the title of your presentation.
 Write your own description of the objective of the experiment in no more than one or two
sentences.
Summary of Procedure
 All aspects of the procedure have been noted and described clearly. This means you should
describe or diagram the methods your group used to measure P, V and T and how you
designed your the air bag. You should also describe how you decided to check for the
optimum amounts of reactant used in the bag. You should also have a detailed sketch of
the air bag.
Summary of Data
 All measured data must be tabulated, including non-numerical data or observations. Each
part of the experiment should have its own data table.
 The tables should have clear headings describing what was measured, with units of
measurement for all columns and rows.
 The value in the tables should be written with the correct precision (significant figures)
based on estimation between the calibration marks of each measuring device.
Calculations and Data Analysis
 A set of example calculations for determination of the mass of NaHCO3 and mL of HCl
needed to inflate the bag must be included.
 All results from Part II and III should tabulated with row and column headings.
 Correct significant figures should be used for all calculated values.
Calculations, Data Analysis and Conclusions
 Correct conversion of P, V and T measurements to atm, liters and K.
 Correct calculation of the moles of CO2 in the bag using the Ideal Gas Law
 Correct calculation of the mass of NaHCO3 needed to inflate the bag using the
stoichiometry of the reaction.
 Correct calculation of the mL of HCl needed to inflate the bag using the stoichiometry of
the reaction.
 Conclusions about the effectiveness of your design of a model air bag. Can it be handled
without initiating the reaction prematurely? Does it inflate quickly? Does it create an
effective cushion?
 Conclusions about the optimization of the air bag. Does changing the amount of either of
the reactants change the speed at which it inflates? Does changing the amount of either of
the reactants change the height of the cushion? For each of these questions, explain why
you made each conclusion.
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Application Lab-Design of a Model Air Bag
Pre-Lab
Procedure Design
1. Your group’s main objective is to design an airbag which expands to the largest possible
volume without breaking the seal on the bag. To simulate the airbag we will use a zip lock bag.
Use the design specifications in Part II of the experiment to come up with a preliminary design.
2. Your group will need to decide how to measure the pressure, temperature and the volume of
the gas that fills the air bag. Describe what procedures you will use make each of these
measurements on the gas in your airbag.
3. Once you have a successful air bag design you will need to optimize its operation. Do you
think changing the amount of one or both of the reactants will change the speed of inflation of
the bag or create a better or poorer cushion? Explain your answers.
Applying Gas Stoichiometry
4. Suppose a model air bag contained 500. mL of gas at a temperature of 0°C and 760. torr.
Show how to convert these measurements to units of liters(L), degrees kelvin (K) and
atmospheres (atm). (Answers: 273 K, 1.00 atm, 0.500L)
5. Use the Ideal gas Law to show how to calculate the number of moles of the CO2 gas present
in the airbag (Answer: 0.0223 moles)
6. The balanced reaction equations for the reaction in the airbag is:
HCl + NaHCO3 ---> NaCl + CO2 + H2O
(a) Use the moles of gas in the bag and the ratio of moles NaHCO3/mole CO2 and show how
to calculate the moles of NaHCO3 needed to fill the bag with gas. (answer: 0.0223 moles)
(b) Use the moles of gas in the bag and the ratio of moles HCl/mole CO2 to show how to
calculate the moles of HCl needed to fill the bag with gas (answer: 0.0223 moles)
7. (c) Use the moles of NaHCO3 and the molar mass of NaHCO3 to show how to calculate the
grams of NaHCO3 needed to fill the bag with gas. (answer: 1.88 g)
(d) Use the moles of HCl and the molarity (or moles/liter) of the HCl (6.0 M) to show how to
calculate the mL of HCl needed to fill the bag with gas. (answer: 3.7 mL)
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