# Gas Laws, Explosions, and Air Bags

### 1  Gas Laws, Explosions, and Air Bags

Outline

• Gas Laws
• Applications

• Hot Air balloons
• Explosions
• 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)

### 3  Mole

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

H-O-H

and the formula H 2 O.

• Chemical reactions based on atoms, which are too small to measure.

 atoms dozens of atoms moles of atoms # H 2 2 2 # O 1 1 1

### 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.

Ex.

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

• Liquid

• Volume » 18 mL (3-4 Tablespoons)
• Gas

• 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

• Given
PV = nRT

• If
Pressure and amount of gas constant

• Then
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

• Given
PV = nRT

• If
Amount of gas and pressure held constant

• Then
PV = constant

If pressure increases, volume decreases.

### 10  Breathing

Inhale

• Muscles pull lungs further open to increase volume.
• Pressure inside lungs decreases.
• Outside air enters to equalize pressure
Exhale

• Muscle squeeze lungs to decrease volume
• Pressure inside lungs increases.
• Air leaves lungs to equalize pressure

### 11  Pressure and Temperature

• Given
PV = nRT

• If
Volume and amount of gas constant

• Then
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 the container.

• 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'.

### 13  Explosions

An explosion is due to rapid expansion of gas.

Expansion due to:

• Increased temperature (reaction gives off heat)
and/or

• Creation of a gas (gas is product of chemical reaction)

### 14  TNT

TNT is 2,4,6-trinitrotoluene

Reaction is:

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 gas

### 16  Airbag chemistry

Sodium azide (NaN 3 ) reacts to create sodium metal and N 2 gas.

Other chemicals added to 'neutralize' sodium

2 NaN 3     ®    2 Na  +  3 N 2

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 (glass)

### 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.

Currently unregulated.

### 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

From 1990-2001

• 175 deaths from airbags
• 3.3 million deployments
• 6377 lives saved
• ? injuries caused/prevented

### 20  Risks

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:

• children
• shorter adult drivers (too close to steering wheel)
• any unrestrained individual (no seat beat)

File translated from TEX by TTH, version 3.02.
On 25 Apr 2002, 07:59.