Transcript Ch. 3 ppt modified.ppt

Chapter 3: Airbags
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
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
Airbags
Use different
States
of
Matter
With different
Work because of changes
Changes
To produce
Which is a
Gas
Properties
Properties explained by
One of which is
Density
Kinetic
Molecular
Theory
Explanation for
Gas Laws
Intro—Airbags
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
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
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
Section 3.1—States of Matter
Solid
 Closely packed together particles
 Vibrate in place
 Can’t switch places
 Definite shape
 Definite volume
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
Gas
 Particles very spread out
 Rapid, random motion
 Highly compressible
 No definite volume—they will fill container
 No definite shape—take shape of container
Increasing molecular motion (temperature)
Changes in State
Sublimation
Boiling or
Evaporating
Liquid
Melting
Gas
Condensing
Freezing
Solid
Deposition
Temperature of state changes
Freezing point = melting point
Boiling point = condensation point
What’s between the particles?
?
Nothing! There is absolutely nothing between the particles!
Section 3.2—Properties of
Matter
What properties are useful or not useful in an airbag?
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)
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
Let’s Practice
Flammability
Example:
Are the
following
properties are
physical or
chemical?
Boiling point
Solubility
Malleability
Reactivity with oxygen
Let’s Practice
Mass
Example:
Are the
following
properties are
intensive or
extensive?
Volume
Color
Flammability
Texture
Section 3.3—Density
Do you want high or low density in your airbag?
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
Density
Mass
Density
In grams (g)
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!
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 2—Solving for Mass
Example:
What is the
mass of a 2.34
mL sample with
a density of
2.78 g/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?
Graphing Density
y2  y1
Slope 
x2  x1
Density
Mass (g)
If we make the y-axis
mass and the x-axis
volume then…
mass
Slope 
volume
Volume (mL)
Then the slope equals
Density!
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
Let’s Practice 1
Example:
If a 22.7 g
sample has a
volume of 47.8
mL, will it float
in water?
Let’s Practice 2
Example:
What volume is
a sample that is
27.5 g and has
a density of 3.5
g/mL?
Section 3.4—Changes in
Matter
What type of changes can produce a gas for an airbag?
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.
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.
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)
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)
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!
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.
Confusing changes
People often use the following terms incorrectly.
Term
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
Heating a sample to
evaporate the water
Physical
Drying
Physical
Section 3.5—Gas Behavior
How does the behavior of gases affect airbags?
What is pressure?
Pressure – Force of gas particles
running into a surface
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
As # of molecules increases, pressure increases
Pressure
increases
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
As volume increases, pressure decreases
Pressure
decreases
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
Pressure and Temperature
If temperature is related to molecular motion…
and pressure is molecular collisions with the
container…
As temperature
increases,
molecular
motion
increases
Collisions
between
molecules and
the wall
increase
As temperature increases, pressure increases
Pressure
increases
Pressure Inside and Outside a
Container
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.
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)
Lower
pressure
Lower
Higher
pressure
The external pressure is high
altitude (low pressure).
The internal pressure is higher than the external pressure.
The bag will expand in order to reduce the internal pressure.
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.
Lower
pressure
Higher
Can
Explodes!
pressure
As temperature increases, pressure
increases.
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.
Kinetic Molecular Theory
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
Assumptions of the KMT
1
All gases are made of atoms or molecules
2
Gas particles are in constant, rapid, random motion
3
The temperature of a gas is proportional to the average
kinetic energy of the particles
4
Gas particles are not attracted nor repelled from one
another
5
All gas particle collisions are perfectly elastic (no kinetic
energy is lost to other forms)
6
The volume of gas particles is so small compared to the
space between the particles, that the volume of the
particle itself is insignificant
Real Gases
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
Effusion & Diffusion
Effusion
Effusion –gas escapes from a tiny hole
in the container
Effusion is why
balloons deflate
over time!
Diffusion
Diffusion –gas moves across a space
Diffusion is the reason we can smell perfume across the room
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.
Rate of Diffusion & Particle Mass
Watch as larger particles take longer to get to your nose
Section 3.6—Counting
Molecules
So the number of molecules affects pressure of an
airbag…how do we “count” molecules?
What is a mole?
Definition
Mole – SI unit for counting
The only acceptable abbreviation for “mole” is
“mol”…not “m”!!
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
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
355 mL 6.021023 molecules H2O 1.19  1025
= _________ molecules H2O
mL
18
This means a 12 ounce bottle of water (355 mL) would
have 1.19  1025 molecules of water.
1.19  1025 molecules
1 dozen
12 molecules
9.89  1023 dozen
= _________
That would be 9.89  1023 “dozen” water molecules.
These huge numbers are impractical!
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!
Example: Molecules & Moles
Example:
How many
molecules
of water
are in 1.25
moles?
Let’s Practice #1
Example:
How many
moles are
equal to
2.8 × 1022
molecules
Molar Mass
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
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
Molar mass for molecules
The molar mass for a molecule = the
sum of the molar masses of all the
atoms
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
Example: Molar Mass
Example:
Find the
molar
mass for
CaBr2
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
Let’s Practice #2
Example:
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 78.01 g
Using Molar Mass in
Conversions
Example: Moles to Grams
Example:
How many
grams are
in 1.25
moles of
water?
Example: Grams to Molecules
Example:
How many
molecules
are in
25.5 g
NaCl?
Let’s Practice #3
Example:
How many
moles are
in 25.5 g
NaCl?
Let’s Practice #4
Example:
How many
grams is a
sample of
2.75 × 1024
molecules of
SrCl2?
Section 3.7—Gas Laws
How can we calculate Pressure, Volume and Temperature of
our airbag?
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 = 101300 Pa = 101.3 kPa = 760 mm Hg = 14.7 psi
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

C  273  K
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!
Gas Laws
KMT and Gas Laws
The Gas Laws are the experimental
observations of the gas behavior that
the Kinetic Molecular Theory explains.
“Before” and “After” in Gas Laws
This section has 4 gas laws which have
“before” and “after” conditions.
For example:
P1 P2

n1 n2
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
Avogadro’s Law
Avogadro’s Law relates # of particles (moles) and
volume.
Where Temperature and Pressure are held constant
V1 V2

n1 n2
Example:
V = Volume
n = # of moles of gas
The two volume units must match!
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?
Avogadro’s Law
Avogadro’s Law relates # of particles (moles) and
volume.
Where Temperature and Pressure are held constant
V1 V2

n1 n2
Example:
V = Volume
n = # of moles of gas
The two volume units must match!
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
2.5 L
V2

0.15mole 0.55mole
0.55mole  2.5 L
 V2
0.15mole
V2 = 9.2 L
Boyles’ Law
Boyles’ Law relates pressure and volume
Where temperature and # of molecules are
held constant
P1V1  P2V2
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 0.980 atm?
Boyles’ Law
Boyles’ Law relates pressure and volume
Where temperature and # of molecules are
held constant
P1V1  P2V2
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 0.980 atm?
P1 = 1.05 atm
V1 = 2.5 L
P2 = 0.980 atm
V2 = ? L
1.05atm 2.5L  0.980atmV2
1.05atm  2.5L
 V2
0.980atm
V2 = 2.7 L
Charles’ Law
Charles’ Law relates temperature and pressure
V1 V2

T1 T2
Example:
V1 = 10.5 L
T1 = 25C
Where pressure and # of molecules are held
constant
V = Volume
T = Temperature
The two volume units must match and
temperature must be in Kelvin!
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!
25C + 273 = 298 K
V2 = ? L
T2 = 50C
50C + 273 = 323 K
Charles’ Law
Charles’ Law relates temperature and pressure
Where pressure and # of molecules are held
constant
V1 V2

T1 T2
Example:
V = Volume
T = Temperature
The two volume units must match and
temperature must be in Kelvin!
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 = 298 K
V2 = ? L
T2 = 50C = 323 K
10.5 L
V2

298 K 323K
323K 10.5 L
 V2
298K
V2 = 11.4 L
Combined Gas Law
P1V1 P2V2

n1T1 n2T2
Example:
P = Pressure
V = Volume
n = # of moles
T = Temperature
Each “pair” of units must
match and
temperature must be in
Kelvin!
What is the final pressure if a 0.125 mole sample of gas at
1.7 atm, 1.5 L and 298 K is changed to STP and particles
are added to 0.225 mole?
Why you really only need 1 of these
The combined gas law can be used for all “before”
and “after” gas law problems!
P1V1 P2V2

n1T1 n2T2
For example, if volume is held constant, then
V1  V2
and the combined gas law becomes: P1V1
P2V1

n1T1 n2T2
When two variables on opposites sides are the same, they cancel
out and the rest of the equation can be used.
P1
P2

n1T1 n2T2
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
P1V1 P2V2

n1T1 n2T2
Avogadro’s Law
Hold moles and
temperature constant
P1V1 P2V2

n1T1 n2T2
Boyles’ Law
Hold pressure and
moles constant
P1V1 P2V2

n1T1 n2T2
Charles’ Law
The Ideal Gas Law
The Ideal Gas Law does not compare situations—it
describes a gas in one situation.
PV  nRT
P = Pressure
V = Volume
n = moles
R = Gas Law Constant
T = Temperature
There are two possibilities for “R”:
L * atm
0.0821
mole * K
L * kPa
8.31
mole * K
Choose the one with
units that match your
pressure units!
Volume must be in Liters when using “R” to allow the unit to cancel!
The Ideal Gas Law Example
The Ideal Gas Law does not compare situations—it
describes a gas in one situation.
PV  nRT
Example:
P = Pressure
V = Volume (in L)
n = moles
R = Gas Law Constant
T = Temperature
A sample with 0.55 moles of gas is at 105.7 kPa and 27°C.
What volume does it occupy?
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?
What did you learn about
airbags?
Airbags
Use different
States
of
Matter
With different
Work because of changes
Changes
To produce
Which is a
Gas
Properties
Properties explained by
One of which is
Density
Kinetic
Molecular
Theory
Explanation for
Gas Laws