Gas Laws, Explosions, and Air Bags
1 Gas Laws, Explosions, and Air Bags
- Gas Laws
- Hot Air balloons
- Air Bags
2 Ideal Gas Law
PV = nRT
P = Pressure (in atmospheres)
V = Volume (in liters)
n = Amount of gas present (in moles)
R = Gas Law Constant (0.082 [ L atm/mol K] )
T = Absolute temperature (in degrees Kelvin)
A 'mole' is a counting unit used for atoms, molecules, ...
Analogous to a six-pack (6), a dozen (12), or a ream (500)
A mole is NOT:
- Weight (how much does a dozen weigh?)
- Dependent on type of stuff (a dozen is 12)
4 Why Use Moles ?
Water has the structure
and the formula H 2
- Chemical reactions based on atoms, which are too small to measure.
|atoms||dozens of atoms||moles of atoms|
5 Moles and Weight
We don't buy jellybeans, flour, gasoline, or countless other small
items by 'each'.
We buy these by weight or volume.
Chemists use compounds by mass, then convert to moles to get ratios.
2.0 g of hydrogen reacts with 16.0 g of oxygen to make
18.0 g of water
2.0 moles hydrogen atoms react 1.0 mole of oxygen atoms
to make 1.0 mole of water.
2 H + 1 O ® 1
H 2 O
6 Water to Steam
18.0 grams of water = 1 mole
- Volume » 18 mL (3-4 Tablespoons)
- Volume = [ nRT/P] = [ (1mole)(0.082)(298 K)/(1 atm)]
= 22.4 Liters (about 5.5 gallons)
- (Assumes room temperature and 1 atmosphere pressure)
7 Volume and Temperature
PV = nRT
Pressure and amount of gas constant
V µ T
If temperature increases, volume increases.
8 Hot Air balloons
Assume a balloon has a volume of is 300,000 L and the temperature
is room temperature.
If the average temperature is increased 50 C 0 , then the
volume of gas should increase to about 350,000L.
Since the volume of the balloon is roughly fixed, about 50,000 L of
air is 'lost'.
50,000 L of air weighs » 50 kg
9 Pressure and Volume
PV = nRT
Amount of gas and pressure held constant
PV = constant
If pressure increases, volume decreases.
- Muscles pull lungs further open to increase volume.
- Pressure inside lungs decreases.
- Outside air enters to equalize pressure
- Muscle squeeze lungs to decrease volume
- Pressure inside lungs increases.
- Air leaves lungs to equalize pressure
11 Pressure and Temperature
PV = nRT
Volume and amount of gas constant
P µ T
If temperature increases, pressure increases.
12 Tires, Pressure Cookers, and Coke
Heating a closed container increases the pressure of the gas inside
- Friction of tires with road surface increases temperature of air in
tires and increases the air pressure.
- In pressure cookers, higher pressure makes it more difficult for water
to boil. Result is that water boils at a higher temperature.
- Pop left in car on hot summer day can 'explode'.
An explosion is due to rapid expansion of gas.
Expansion due to:
- Increased temperature (reaction gives off heat)
- Creation of a gas (gas is product of chemical reaction)
TNT is 2,4,6-trinitrotoluene
2 C 7 H 5 N 3 O 6 + 10.5
O 2 ® 14 CO 2 + 5
H 2 O + 3 N 2
10.5 moles gas ® 22 moles
15 Air Bags
16 Airbag chemistry
Sodium azide (NaN 3 ) reacts to create sodium metal and N 2
Other chemicals added to 'neutralize' sodium
2 NaN 3 ® 2 Na + 3
10 Na + 2 KNO 3 ® K 2 O + 5
Na 2 O + N 2
K 2 O + Na 2 O + SiO 2 ® Na 2 K 2 SiO 4
17 Sodium Azide
Very toxic compound. (More toxic than cyanide)
While some of this might escape during deployment, a more serious
concern is what this chemical is doing in junkyards.
Recycling or controlled deployment would solve problem.
18 Deployment Details
- Air bag volume is » 70 L
- Requires » 130 grams of sodium azide.
- Full deployment in approximately 0.03 seconds
- Velocity of airbag typically close to 200 mph during inflation
19 Safety Claims
National Safety Council
- 175 deaths from airbags
- 3.3 million deployments
- 6377 lives saved
- ? injuries caused/prevented
Area with 10 inches (25 cm) of air bag opening is 'danger zone'.
- Arms, fingers, faces, ... in this area during deployment likely to
be seriously injured.
Individuals in most danger:
- shorter adult drivers (too close to steering wheel)
- any unrestrained individual (no seat beat)
File translated from
On 25 Apr 2002, 07:59.