Chapter 3: Airbags

Airbags

This chapter will introduce the chemistry needed to understand how airbags workSection 3.1: States of matter & Phase

DiagramsSection 3.2: Properties & Changes of matterSection 3.3: DensitySection 3.4: Counting MoleculesSection 3.5: Gas Behavior & Gas Laws

Intro—Airbags

http://youtu.be/dZfLOnXoVOQ

How do airbags work in your car?

Nylon bag inside your steering wheelSolid 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

Problems with this reaction?

NaN3 is very toxic

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. An exothermic reaction.

The FIX!

Addition of potassium nitrate!It reacts with the sodium metal to form

potassium oxide & sodium oxide and nitrogen gas

Following up with addition of silicon dioxide which reacts with the oxides to form silcates (glass).

Why do we use it?

It produces the gas very quickly,(at about 35 ms) – 4x faster than a blink of an eye

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

Section 3.1

Solid

Closely packedstrong attractive forcesVibrate in place; low kinetic energy of particlesCan’t switch placesDefinite shapeDefinite volumeIncompressible

Liquid

Particles more spread out than solidWeaker attractions Particles are free to move past each other:

flow Indefinite shape – takes shape of containerDefinite volumeSlightly compressible

Gas

Particles are very spread out; weakest attractive forcesRapid, random motion; High kinetic

energy of particles Indefinite shape—take shape of container Indefinite volume—they will fill containerHighly compressible

Changes in State: EndothermicKinetic energy must be put INTO the substance

in order to increase the motions of the molecules thus breaking the intermolecular forces holding the particles together.

Melting: change of state from a solid to a liquidVaporization(Boiling or Evaporation): change of

state from a liquid to a gasSublimation: change of state directly from a solid

to a gas

Physical Changes: Names of the Phase Changes

Solid

LiquidGas

Changes in State: ExothermicKinetic energy must be taken OUT of the

substance in order for the molecules to slow down so the intermolecular forces can begin draw & hold the particles closer together.

Freezing: change of state from a liquid to a solidCondensation: change of state from a gas to a

liquidDeposition: change of state from a gas to a solid

Solid

Liquid

GasSublimation

Melting

Boiling or Evaporating

Condensing

Freezing

Deposition

Incr

easi

ng m

olec

ular

mot

ion

(tem

pera

ture

)

Changes in State

Endothermic

Exothermic

Temperature of state changes

Freezing point (fp) is the temperature at which a liquid turns into a solid

Melting Point (mp) is the temperature at which a solid turns into a liquid

freezing point is the same as melting point

Example: water has a melting point or freezing point of 0°C

Temperature of state changes

Boiling point (bp) is the temperature at which a liquid turns into a gas

Condensation Point (cp) is the temperature at which a gas turns into a liquid

boiling point is the same as condensation point

Example: water has a boiling point or dew point of 100°C

Physical Property

All substances have their own freezing and boiling points which make this property a great way to identify a unknown substance.

Atmospheric Pressure vs Vapor PressurePressure Atmospheric Pressure Vapor Pressure

Force per unit area created as gas molecules collide with objects

Force per unit area exerted against a surface by the weight of the air molecules above the surface

Force per unit area of the gas molecules above a liquid colliding

Usually measured in N/m2 but in chemistry we use atm or millimeters of mercury (mm Hg)

The more air molecules above a surface, the more molecules to exert a force and thus higher air pressure

The lower the attractive forces, the higher the vapor pressure

At sea level, atmospheric pressure equals 1 atm

Substance with high vapor pressure are called volatile

Temperature Controls Vapor Pressure

Only 2 factors control the Vapor Pressure of a Liquid: Nothing ELSE!!!!

1.Temperature

2. Attractive forces of the liquid

Vaporization: Differences between Evaporation and Boiling

Evaporation occurs spontaneously at all temperaturesat the surface of the liquid

Boiling occurs when extra kinetic energy(heat) is added.at only 1 temperature dependent on pressureIt takes place within the body of the liquid

Real Definition of Boiling Point

external atmospheric pressure = vapor pressure of the liquid

Real Definition of Boiling Point

Since atmospheric pressure changes at various altitudes, “normal” boiling point is used to describe the temp at which a LG at “1 atm or 760 mmHg” of pressure

69.0 29,000 Mount Everest

Important Ideas

The higher the

altitude, the lower

the atmospheric

pressure!• At higher altitudes, the

boiling point is lower

• It takes longer to cook foods at higher altitudes (lower atmospheric pressures)

Boiling Water By Changing Pressure

http://www.kentchemistry.com/moviesfiles/Units/GasLaws/vacuum.htm

Heating & Cooling Curves

shows how solids, liquids & gases change state when temperature is changedPlateaus : the changes of state (freezing, melting, boiling & condensation) Freezing Point & Melting Point are at the same temperature or plateauBoiling Point & Condensation Point are at the same temperature or plateau

Slopes= pure states (solid, liquid, gas)

At the plateaus, kinetic energy remains constant (temp. remains constant) while potential energy changes

At the slopes, kinetic energy changes (temp. changes) while potential energy remains constant

*** DANGER!!***

Notice that a gas

can get higher

than boiling point!

Phase Changes

0

100

KE is constant/PE in changing

KE is changing/PE in constant

BP

FP

Heating Curve

Examples: 1. What is the boiling point of the substance?

2. What letter represents the solid state only?

3. What letter represents the melting process?

100°C

A

B

Cooling Curve

While the substance is cooling during the liquid phase, the average kinetic energy of the molecules of the substance:

a) decreasesb) increasesc) remains the same

Cooling Curve

Examples: 1. What is the freezing point of this substance?

2. How long does it take for the gas to completely liquefy?

70°C

9 -2= 7 min

Animation of Heating Curve

http://www.kentchemistry.com/links/Matter/HeatingCurve.htm

Phase Diagramsshows how solids, liquids & gases change state as both temperature and pressure are changedCrossing a line between states determines the change state (boiling, melting, etc)A point directly on a line will identify the pressure and temperature (boiling point, melting point, etc.) of the change

Important Points on Phase Diagram

Triple Point is the temperature and pressure in which all 3 of the states coexist

Critical Point is the temperature & pressure at which a gas can no longer liquefy

Phase Diagrams of Water & Carbon Dioxide

Phase Diagram of Water

Temp. of line B at 1 atm (freezing point): 273 K

Temp. of line C at 1 atm (boiling point): 373KD is the triple point

Phase Diagrams of Water

E is the critical point

What change of state happens when you cross line B at a constant pressure of 10 atm & increasing temp? melting

What change of state occurs when you cross line A at constant pressure of .001 atm? sublimation

What change of state happens when you cross line C at 400 K to 300K at approx. 5 atm?condensation

Section 3.2What are the properties & changes occurring within the airbag?

Physical versus Chemical Properties

Chemical PropertyPhysical Property

Characteristic that can be determined

or measured without changing the substance’s

identity

Characteristic that can only be

determined or measured as the

substance changes into different substances

Examples of Physical Properties

•COLOR ODOR•TEXTURE BOILING POINT•DENSITY SOLUBILITY•VOLUME MASS•HARDNESS•MALLEABILITY /BRITTLENESS

Physical Properties

Examples of Chemical Properties

•BURNING/COMBUSTING•RUSTING•ROTTING•FLAMMABILITY•REACTIVITY•NEUTRALIZATION•DECOMPOSING

Intensive and Extensive Properties

Extensive PropertyIntensive Property

Size of the sample DOES NOT matter

— a big piece & a small piece are the

same with respect to the property

Size of the sample DOES matter

—a big piece & a small piece would be

different with respect to the property

IntensiveMelting point/boiling pointDensityColor/SmellConductivityHardness

ExtensiveMassVolumeEnergyLengthShape

SELF CHECK

Flammability

Boiling point

Solubility

Malleability

Reactivity with oxygen

Chemical

Physical, Intensive

Physical, Intensive

Physical, Intensive

Chemical

Flammability

Boiling point

Solubility

Malleability

Reactivity with oxygen

Physical VS Chemical Changes

Physical Change: the chemical structure of the substances is not changed.

H2O(l) H2O(g)

Physical VS Chemical Changes

Chemical Change: the chemical structures of the substances are changed.

RECALL: Another name for a chemical change is called a CHEMICAL REACTION.

H2O(l) H2(g) + O2(g)

Physical & Chemical Changes Physical changes

do not produce new

substances

breaking, dissolving, distilling, cutting,Changes in state (boiling, condensing, melting & freezing)

Chemical changes do produce new substances

rusting, burning, metabolizing food, oxidation or reduction, reacting with oxygen, etc.

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…

But if the water is evaporated (a physical change), the salt is left in the container.

many people think this is a chemical change.

Since the original change was un-done with a physical change, then the original change (the dissolving) was a physical change as well.

Definition

Changes a solid into a liquid

Reacting with oxygen to produce CO2 and H2O

Confusing changes

People often use the following terms incorrectly.

Term Type of Change

Melting Physical

Burning Chemical

Adding one substance to another to form a homogeneous mixture

Dissolving Physical

Heating a sample to evaporate the water

Drying Physical

RECALL the Possible Signs of Chemical Changes

① Gas production (bubbling)

② Energy change (getting hot or cold)

③ Color change

④ Light given off

⑤ Formation of a precipitate (an insoluble substance formed from two soluble substances)

They’re “Possible” signs

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)

Sometimes these “signs” accompany physical changes as well!

Section 3.3

Do you want high or low density in your airbag?

Density

How heavy is it for its size?

the ratio of mass to volume of a sample

Lead = high density…small size is very heavyAir = low density…large sample has very little mass

Density

D =m

V

MassIn grams (g)

VolumeIn liters (L) or mL

DensityIn g/L or g/mL

Don’t try to cancel out the units…density has “2 units” – a mass unit over a volume unit!

Floating

Objects float when they are less dense than the substance they are in! Is vegatable oil more or less dense than water?

Fewer particles in the same space = less dense

More particles in the same space = More dense

Density Values: the larger the value, the more dense

Density Varies with TemperatureWHY?

Most substances will expand when heated, increasing the volume & decreasing the density.

Water is an exception: As water is cooled, it expands, increasing the volume & decreasing the density.

Thus, ICE is less dense than WATER!

Calculating Volume using Water Displacement

The volume is the difference between the final volume and the initial volume of water.

What is the volume of the dinosaur? ______________

Example 1—Solving for Density

Example:What is the density of a

sample with a mass of 2.50 g and a volume

of 1.7 mL?

Example 1—Solving for Density

V

mD

mL

gD

7.1

50.2

Example:What is the density of a

sample with a mass of 2.50 g and a volume

of 1.7 mL?

D = ?

m = 2.50 g

V = 1.7 mL

mLgD 5.1

Example 2—Solving for Mass

Example:What is the

mass of a 2.34 mL sample with

a density of 2.78 g/mL?

Example 2—Solving for Mass

V

mD

mL

mmLg

34.278.2

Example:What is the

mass of a 2.34 mL sample with

a density of 2.78 g/mL?

m = ?

V = 2.34 mL

D = 2.78 g/mL

gm 51.6

× 2.34 mL2.34 mL ×

Example 3—Solving for Volume

Example:A sample is

45.4 g and has a density of 0.87 g/mL. What is the

volume?

Example 3—Solving for Volume

V

mD

V

gmLg 4.45

87.0

Example:A sample is

45.4 g and has a density of 0.87 g/mL. What is the

volume?

m = 45.4 g

D = 0.87 g/mL

V = ?

mLV 52

× VV ×

gmLgV 4.4587.0

0.87 g/mL 0.87 g/mL

SELF CHECK

Example:Is it

aluminum? The metal

has a mass of 612 g and a volume of

345 cm3.

The accepted density of aluminum is 2.70 g/cm3

Graphing Density

volume

massSlope

12

12

xx

yySlope

If we make the y-axis mass and the x-axis volume then…

Volume (mL)

Mas

s (g

)

Density

Then the slope equals Density!