1. Chapter 3: Airbags

2. Introductory Activity
What makes an effective airbag?
List criteria necessary to consider an airbag effective.
List characteristics that would be good in an airbag
List characteristics that you’d want to avoid in an airbag

3. Airbags
This chapter will introduce the chemistry needed to understand how airbags work
Section 3.1: States of matter
Section 3.2: Properties of matter
Section 3.3: Density
Section 3.4: Changes in matter
Section 3.5: Gas Behavior
Section 3.6: Counting Molecules
Section 3.7: Gas Laws

4. Kinetic Molecular Theory
Airbags
Use different
Work because of changes
Changes
States of Matter
To produce
Which is a
Gas
With different
Properties
Properties explained by
One of which is
Kinetic Molecular Theory
Density
Gas Laws
Explanation for

5. Intro—Airbags

6. How do airbags work in your car?
Nylon bag inside your steering wheel
Solid sodium azide (NaN3) with is ignited with electricity when a crash sets off the trigger
2 NaN3 (s)  2 Na (s) + 3 N2 (g)
The nitrogen gas fills the airbag

7. Problems with this reaction?
It produces sodium metal, which reacts with water to form hydrogen gas & enough heat to ignite that hydrogen gas
Reaction produces heat, so gas is very hot in airbag
NaN3 is very toxic

8. Why do we use it?
It produces the gas very quickly, but not so quick that it’s more of a hazard
Reactants are small to store before needed
Amount of dangerous chemicals is minimal
Heat from reaction is absorbed, in part, by the physical components of the airbag system

9. Section 3.1—States of Matter

10. Solid Closely packed together particles Vibrate in place
Can’t switch places
Definite shape
Definite volume

11. Liquid Particles more spread out than solid
Particles are free to move past each other
Slightly compressible
Definite volume
No definite shape – take shape of container

12. Gas Particles very spread out Rapid, random motion Highly compressible
No definite volume—they will fill container
No definite shape—take shape of container

13. Changes in State Gas Increasing molecular motion (temperature) Liquid
Sublimation
Boiling or
Evaporating
Liquid
Melting
Deposition
Condensing
Solid
Freezing

14. Temperature of state changes
Freezing point = melting point
Boiling point = condensation point

15. What’s between the particles?
Nothing! There is absolutely nothing between the particles!

16. Section 3.2—Properties of Matter
What properties are useful or not useful in an airbag?

17. Physical versus Chemical Properties
Physical Property
Chemical Property
Can be observed or tested without changing the atoms or molecules
In the process of observing or testing, the atoms or molecules are changed into different substance(s)

18. Intensive and Extensive Properties
Intensive Property
Extensive Property
Size of the sample doesn’t matter—you’d say a big piece and a small piece were the same with respect to this property
Size of the sample does matter—a big piece and a small piece would be different with respect to this property

19. Are the following properties are physical or chemical?
Let’s Practice
Flammability
Boiling point
Solubility
Malleability
Reactivity with oxygen
Example:
Are the following properties are physical or chemical?

20. Are the following properties are intensive or extensive?
Let’s Practice
Mass
Volume
Color
Flammability
Texture
Example:
Are the following properties are intensive or extensive?

21. Section 3.3—Density
Do you want high or low density in your airbag?

22. Definitions Density- the ratio of mass to volume of a sample
How heavy is it for its size?
Lead = high density…small size is very heavy
Air = low density…large sample has very little mass

23. Density Mass Density m D = V Volume
In grams (g)
Density
In g/L or g/mL
D = m V
Volume
In liters (L) or mL
Don’t try to cancel out the units…density has “2 units” – a mass unit over a volume unit!

24. Example 1—Solving for Density
What is the density of a sample with a mass of 2.50 g and a volume of 1.7 mL?

25. Example 2—Solving for Mass
What is the mass of a 2.34 mL sample with a density of 2.78 g/mL?

26. Example 3—Solving for Volume
A sample is 45.4 g and has a density of 0.87 g/mL. What is the volume?

27. Graphing Density Density
Volume (mL)
Mass (g)
Density
If we make the y-axis mass and the x-axis volume then…
Then the slope equals Density!

28. Floating
Objects float when they are less dense than the substance they are in!
Fewer particles in the same space = less dense
More particles in the same space = More dense

29. If a 22.7 g sample has a volume of 47.8 mL, will it float in water?
Let’s Practice 1
Example:
If a 22.7 g sample has a volume of 47.8 mL, will it float in water?

30. What volume is a sample that is 27.5 g and has a density of 3.5 g/mL?
Let’s Practice 2
Example:
What volume is a sample that is 27.5 g and has a density of 3.5 g/mL?

31. Section 3.4—Changes in Matter
What type of changes can produce a gas for an airbag?

32. Definition
Physical Change - n. Change in which the chemical structure of the substances is not changed.
Chemical Change - n. Change in which the chemical structures of the substances are changed.

33. Physical & Chemical Changes
Physical changes do not produce new substances
breaking, dissolving, distilling, cutting, etc.
Changes in state are physical changes (boiling, condensing, melting and freezing)
Chemical changes do produce new substances
rusting, burning, metabolizing food, oxidation or reduction, reacting with oxygen, etc.

34. Possible Signs of Chemical Changes
Gas production (bubbling)
Energy change (getting hot or cold)
Color change
Light given off
Formation of a precipitate (making an insoluble substance from two soluble substances)

35. They’re “Possible” signs
Sometimes these “signs” accompany physical changes as well!
Gas production (bubbling). Bubbles are formed during boiling (a physical change)
Energy change (getting hot or cold). Energy changes accompany changes in state (physical changes)
Color change. Color change can occur due to dissolving a substance (a physical change)
However, some of these signs also accompany physical changes, so you must take into account many observations to determine if the change was in fact chemical.

36. How do you know for sure?
Measure and observe chemical and physical properties before the change in question.
Measure and observe the properties after the change.
If the properties are the same, then it was a physical change!

37. Physical & Chemical Changes
Also…if a change can be un-done by a physical change, then the original change was physical as well.
If salt is dissolved in water, it seems to disappear…
many people think this is a chemical change.
But if the water is evaporated (a physical change), the salt is left in the container.
Since the original change was un-done with a physical change, then the original change (the dissolving) was a physical change as well.

38. Confusing changes People often use the following terms incorrectly.
Definition
Type of Change
Melting
Changes a solid into a liquid
Physical
Burning
Reacting with oxygen to produce CO2 and H2O
Chemical
Dissolving
Adding one substance to another to form a homogeneous mixture
Physical
Drying
Heating a sample to evaporate the water
Physical

39. Section 3.5—Gas Behavior
How does the behavior of gases affect airbags?

40. What is pressure?
Pressure – Force of gas particles running into a surface

41. Pressure and Number of Molecules
If pressure is molecular collisions with the container…
As number of molecules increases, there are more molecules to collide with the wall
Collisions between molecules and the wall increase
Pressure increases
As # of molecules increases, pressure increases

42. Pressure and Volume
If pressure is molecular collisions with the container…
As volume increases, molecules can travel farther before hitting the wall
Collisions between molecules and the wall decrease
Pressure decreases
As volume increases, pressure decreases

43. What is “Temperature”?
Temperature – proportional to the average kinetic energy of the molecules
Energy due to motion
(Related to how fast the molecules are moving)
As temperature increases
Molecular motion increases

44. Pressure and Temperature
If temperature is related to molecular motion…
and pressure is molecular collisions with the container…
Pressure increases
As temperature increases, molecular motion increases
Collisions between molecules and the wall increase
As temperature increases, pressure increases

45. Pressure Inside and Outside a Container

46. What is Atmospheric Pressure?
Atmospheric Pressure – Pressure due to the layers of air in the atmosphere.
Climb in altitude
Less layers of air
Lower atmospheric pressure
As altitude increases, atmospheric pressure decreases.

47. Pressure In Versus Out
A container will expand or contract until the pressure inside = atmospheric pressure outside
Example:
A bag of chips is bagged at sea level. What happens if the bag is then brought up to the top of a mountain.
The internal pressure is from low altitude (high presser)
The external pressure is high altitude (low pressure).
Lower
pressure
Higher
pressure
Lower
pressure
The internal pressure is higher than the external pressure.
The bag will expand in order to reduce the internal pressure.

48. When Expansion Isn’t Possible
Rigid containers cannot expand
Example:
An aerosol can is left in a car trunk in the summer. What happens?
The temperature inside the can begins to rise.
As temperature increases, pressure increases.
Lower
pressure
Can
Explodes!
Higher
pressure
The internal pressure is higher than the external pressure.
The can is rigid—it cannot expand, it explodes!
Soft containers or “movable pistons” can expand and contract.
Rigid containers cannot.

49. Kinetic Molecular Theory
What is the Kinetic Molecular Theory?

50. Definition
Theory – An attempt to explain why or how behavior or properties are as they are. Based on empirical evidence
Kinetic Molecular Theory (KMT) – An attempt to explain gas behavior based upon the motion of molecules

51. Assumptions of the KMT 1
All gases are made of atoms or molecules 2
Gas particles are in constant, rapid, random motion
The temperature of a gas is proportional to the average kinetic energy of the particles 3
Gas particles are not attracted nor repelled from one another 4
All gas particle collisions are perfectly elastic (no kinetic energy is lost to other forms) 5
The volume of gas particles is so small compared to the space between the particles, that the volume of the particle itself is insignificant 6

52. Real Gases
What are real gases?

53. What is a “real gas”?
Real Gas – 2 of the assumptions of the Kinetic Molecular Theory are not valid
Gas particles are not attracted nor repelled from one another
Gas particles do have attractions and repulsions towards one another
The volume of gas particles is so small compared to the space between the particles, that the volume of the particle itself is insignificant
Gas particles do take up space—thereby reducing the space available for other particles to be

54. Effusion & Diffusion

55. Effusion Effusion –gas escapes from a tiny hole in the container
Effusion is why balloons deflate over time!

56. Diffusion Diffusion –gas moves across a space
Diffusion is the reason we can smell perfume across the room

57. Effusion, Diffusion & Particle Mass
How are particle size (mass) and these concepts related?
As particle size (mass) increases, the particles move slower
it takes them more time to find the hole or to go across the room
Rate of effusion and diffusion is lower
As mass of the particles increases, rate of effusion and diffusion is lowered.

58. Rate of Diffusion & Particle Mass
Watch as larger particles take longer to get to your nose

59. Section 3.6—Counting Molecules
So the number of molecules affects pressure of an airbag…how do we “count” molecules?

60. What is a mole?

61. Definition Mole – SI unit for counting
The only acceptable abbreviation for “mole” is “mol”…not “m”!!

62. What is a counting unit?
You’re already familiar with one counting unit…a “dozen”
A dozen = 12
“Dozen” 12
A dozen doughnuts
12 doughnuts
A dozen books
12 books
A dozen cars
12 cars
A dozen people
12 people

63. What can’t we count atoms in “dozens”?
Atoms and molecules are extremely small
There are 6.02  1023 water molecules in 18mL of water mL
355
molecules H2O
6.021023 18
= _________ molecules H2O
1.19  1025
This means a 12 ounce bottle of water (355 mL) would have 1.19  1025 molecules of water.
molecules
1.19  1025
dozen 1 12
= _________ dozen
9.89  1023
That would be 9.89  1023 “dozen” water molecules.
These huge numbers are impractical!

64. What does a “mole” count in?
A mole = 6.02  1023 (called Avogadro’s number)
6.02  1023 = 602,000,000,000,000,000,000,000
“mole”
6.02  1023
1 mole of doughnuts
6.02  1023 doughnuts
1 mole of atoms
6.02  1023 atoms
1 mole of molecules
6.02  1023 molecules
This means a 12 ounce bottle of water would have
19.7 “moles” of water…a much easier-to-work-with number!

65. Example: Molecules & Moles
How many molecules of water are in 1.25 moles?

66. How many moles are equal to 2.8 × 1022 molecules
Let’s Practice #1
Example:
How many moles are equal to 2.8 × 1022 molecules

67. Molar Mass
What is molar mass?

68. Definition Molar Mass – The mass for one mole of an atom or molecule.
Other terms commonly used for the same meaning:
Molecular Weight
Molecular Mass
Formula Weight
Formula Mass

69. Mass for 1 mole of atoms The average atomic mass = grams for 1 mole
Average atomic mass is found on the periodic table
Element
Mass
1 mole of carbon atoms
12.01 g
1 mole of oxygen atoms
16.00 g
1 mole of hydrogen atoms
1.01 g
Unit for molar mass: g/mole or g/mol

70. Molar mass for molecules
The molar mass for a molecule = the sum of the molar masses of all the atoms

71. Calculating a Molecule’s Mass
To find the molar mass of a molecule: 1
Count the number of each type of atom 2
Find the molar mass of each atom on the periodic table 3
Multiple the # of atoms  molar mass for each atom 4
Find the sum of all the masses

72. Find the molar mass for CaBr2
Example: Molar Mass
Example:
Find the molar mass for CaBr2

73. Example: Molar Mass & Parenthesis
Be sure to distribute the subscript outside the parenthesis to each element inside the parenthesis.
Example:
Find the molar mass for Sr(NO3)2

74. Find the molar mass for Al(OH)3
Let’s Practice #2
Example:
Find the molar mass for Al(OH)3

75. Find the molar mass for Al(OH)3
Let’s Practice #2
Be sure to distribute the subscript outside the parenthesis to each element inside the parenthesis.
Example:
Find the molar mass for Al(OH)3 Al 1 
26.98 g/mole =
26.98 g/mole O 3 
16.00 g/mole =
48.00 g/mole H 3 
1.01 g/mole = +
3.03 g/mole
78.01 g/mole
1 mole of Al(OH)3 molecules would have a mass of g

76. Using Molar Mass in Conversions
What is molar mass?

77. Example: Moles to Grams
How many grams are in 1.25 moles of water?

78. Example: Grams to Molecules
How many molecules are in
25.5 g NaCl?

79. How many moles are in 25.5 g NaCl?
Let’s Practice #3
Example:
How many moles are in 25.5 g NaCl?

80. How many grams is a sample of 2.75 × 1024 molecules of SrCl2?
Let’s Practice #4
Example:
How many grams is a sample of 2.75 × 1024 molecules of SrCl2?

81. Section 3.7—Gas Laws
How can we calculate Pressure, Volume and Temperature of our airbag?

82. Pressure Units Several units are used when describing pressure Unit
Symbol
atmospheres
atm
Pascals, kiloPascals
Pa, kPa
millimeters of mercury
mm Hg
pounds per square inch
psi
1 atm = Pa = kPa = 760 mm Hg = 14.7 psi

83. Temperatures cannot fall below an absolute zero
Definition
Kelvin (K)– temperature scale with an absolute zero
Temperatures cannot fall below an absolute zero
A temperature scale with absolute zero is needed in Gas Law calculations because you can’t have negative pressures or volumes

84. Definition
Standard Temperature and Pressure (STP) – 1 atm (or the equivalent in another unit) and 0°C (273 K)
Problems often use “STP” to indicate quantities…don’t forget this “hidden” information when making your list!

85. Gas Laws
What are the gas laws?

86. KMT and Gas Laws
The Gas Laws are the experimental observations of the gas behavior that the Kinetic Molecular Theory explains.

87. “Before” and “After” in Gas Laws
This section has 4 gas laws which have “before” and “after” conditions.
For example:
Where P1 and n1 are pressure and # of moles “before”
and P2 and n2 are pressure and # of moles “after”
Both sides of the equation are talking about the same sample of gas—with the “1” variables before a change, and the “2” variables after the change

88. Avogadro’s Law
Avogadro’s Law relates # of particles (moles) and volume.
Where Temperature and Pressure are held constant
V = Volume
n = # of moles of gas
The two volume units must match!
Example:
A sample with 0.15 moles of gas has a volume of 2.5 L. What is the volume if the sample is increased to 0.55 moles?

89. Avogadro’s Law
Avogadro’s Law relates # of particles (moles) and volume.
Where Temperature and Pressure are held constant
V = Volume
n = # of moles of gas
The two volume units must match!
Example:
A sample with 0.15 moles of gas has a volume of 2.5 L. What is the volume if the sample is increased to 0.55 moles?
n1 = 0.15 moles
V1 = 2.5 L
n2 = 0.55 moles
V2 = ? L
V2 = 9.2 L

90. Boyles’ Law Boyles’ Law relates pressure and volume
Where temperature and # of molecules are held constant
P = pressure
V = volume
The two pressure units must match and
the two volume units must match!
Example:
A gas sample is 1.05 atm when 2.5 L. What volume is it if the pressure is changed to atm?

91. Boyles’ Law Boyles’ Law relates pressure and volume
Where temperature and # of molecules are held constant
P = pressure
V = volume
The two pressure units must match and
the two volume units must match!
Example:
A gas sample is 1.05 atm when 2.5 L. What volume is it if the pressure is changed to atm?
P1 = 1.05 atm
V1 = 2.5 L
P2 = atm
V2 = ? L
V2 = 2.7 L

92. Charles’ Law Charles’ Law relates temperature and pressure
Where pressure and # of molecules are held constant
V = Volume
T = Temperature
The two volume units must match and
temperature must be in Kelvin!
Example:
What is the final volume if a 10.5 L sample of gas is changed from 25C to 50C?
Temperature needs to be in Kelvin!
V1 = 10.5 L
T1 = 25C
V2 = ? L
T2 = 50C
25C = 298 K
50C = 323 K

93. Charles’ Law Charles’ Law relates temperature and pressure
Where pressure and # of molecules are held constant
V = Volume
T = Temperature
The two volume units must match and
temperature must be in Kelvin!
Example:
What is the final volume if a 10.5 L sample of gas is changed from 25C to 50C?
V1 = 10.5 L
T1 = 25C
V2 = ? L
T2 = 50C
= 298 K
= 323 K
V2 = 11.4 L

94. Combined Gas Law P = Pressure Each “pair” of units must match and
V = Volume
n = # of moles
T = Temperature
Each “pair” of units must match and
temperature must be in Kelvin!
Example:
What is the final pressure if a mole sample of gas at 1.7 atm, 1.5 L and 298 K is changed to STP and particles are added to mole?

95. Why you really only need 1 of these
The combined gas law can be used for all “before” and “after” gas law problems!
For example, if volume is held constant, then
and the combined gas law becomes:
When two variables on opposites sides are the same, they cancel out and the rest of the equation can be used.

96. Transforming the Combined Law
Watch as variables are held constant and the combined gas law “becomes” the other 3 laws
Hold pressure and temperature constant
Avogadro’s Law
Hold moles and temperature constant
Boyles’ Law
Hold pressure and moles constant
Charles’ Law

97. The Ideal Gas Law
The Ideal Gas Law does not compare situations—it describes a gas in one situation.
P = Pressure
V = Volume
n = moles
R = Gas Law Constant
T = Temperature
There are two possibilities for “R”:
Choose the one with units that match your pressure units!
Volume must be in Liters when using “R” to allow the unit to cancel!

98. The Ideal Gas Law Example
The Ideal Gas Law does not compare situations—it describes a gas in one situation.
P = Pressure
V = Volume (in L)
n = moles
R = Gas Law Constant
T = Temperature
Example:
A sample with 0.55 moles of gas is at kPa and 27°C. What volume does it occupy?

99. Let’s Practice Example:
What is the final volume if a 15.5 L sample of gas at 755 mm Hg and 298 K is changed to STP?

100. What did you learn about airbags?

101. Kinetic Molecular Theory
Airbags
Use different
Work because of changes
Changes
States of Matter
To produce
Which is a
Gas
With different
Properties
Properties explained by
One of which is
Kinetic Molecular Theory
Density
Gas Laws
Explanation for