Physical Characteristics
of GasesChapter 10
Chemistry Chapter 10 2
Kinetic-molecular theory• Particles of matter are always in
motion
Chemistry Chapter 10 3
Ideal gas• An imaginary gas that perfectly fits
all the assumptions of the kinetic-molecular theory.
• We can often treat real gases as ideal gases and still get good results.
Chemistry Chapter 10 4
Assumptions of KMT of gases
1. Large numbers of tiny particles that are far apart compared to their size
• Low density• Easily compressed
2. Elastic collisions• No kinetic energy is lost when gas particles
collide with each other or their container• It can be transferred between particles, but
the total kinetic energy remains the same
Chemistry Chapter 10 5
3. Gas particles are in continuous, rapid, random motion
4. There are no attractive or repulsive forces between gas particles
• When they hit, they don’t stick together
5. The average kinetic energy of gas particles depends on the temperature of the gas
• Direct relationship
Chemistry Chapter 10 6
Expansion• Gases have indefinite shape and
volume.• They completely fill any container
they are in.• They also take the shape of that
container.• Because:
– they move rapidly in all directions and don’t stick together
Chemistry Chapter 10 7
Fluidity• Gas particles slide past each other.• They can flow
– Fluid: something that can flow (can be gas or liquid)
Chemistry Chapter 10 8
Diffusion• Spontaneous mixing of the particles
of two substances caused by their random motion.
• Gas particles spread out to fill their new container
Chemistry Chapter 10 9
Effusion• When gas particles pass through a
small opening• Particles leak out of the container
Chemistry Chapter 10 10
Real gases• Do not completely follow kinetic-
molecular theory• Especially deviant at high pressures
and low temperatures• Noble gases are closest to ideal• Very polar gases are farthest from
ideal
Chemistry Chapter 10 11
Discuss• Describe the conditions under which
a real gas is most likely to behave ideally.
• Explain the following properties of gases using the kinetic-molecular theory: expansion, fluidity, low density, compressibility, and diffusion.
Chemistry Chapter 10 12
Describing gases• Needed:
– Volume– Temperature– Number of molecules– Pressure
• They are mathematically related.
Chemistry Chapter 10 13
Pressure• balloon• The force per unit area on a surface.
area
forcepressure
Chemistry Chapter 10 14
Force• A push or a pull• Measured in newtons (N).• At the Earth’s surface, 1 kg of mass
exerts 9.8 N of force due to gravity.
Chemistry Chapter 10 15
Chemistry Chapter 10 16
Pressure of gases• Gases exert pressure on any surface
with which they collide.– Depends on volume, temperature, and
number of molecules
Chemistry Chapter 10 17
Atmospheric pressure• Air around Earth exerts a pressure on
it’s surface and everything on it.– Like the weight of all the molecules
pressing down.
Chemistry Chapter 10 18
Barometer• Used to measure
atmospheric pressure.• Height of liquid (usually
mercury) in tube can be used to express atmospheric pressure.
• At sea level, the average is 760 mm Hg.
h
Chemistry Chapter 10 19
Manometer• Used to measure
the pressure of gases.
• The height difference between the two arms is the pressure.
Chemistry Chapter 10 20
Pressure Units
Chemistry Chapter 10 21
STP• Standard temperature and pressure.• 0 °C and 1 atm• Used to compare volumes of gases.
Chemistry Chapter 10 22
Example• A weather report gives a current
atmospheric pressure of 745.8 mm Hg. Convert this to – Atmospheres
• 0.9813 atm– Torr
• 745.8 torr– Kilopascals
• 99.43 kPa
Chemistry Chapter 10 23
Discuss• Define pressure• What is STP?• Convert 151.98 kPa to atmospheres
– 1.4999 atm
Chemistry Chapter 10 24
Boyle’s Law• Fixed: mass and temperature• The volume varies inversely with
pressure– Less volume, means the particles hit the
walls more often.– This increases the pressure
Chemistry Chapter 10 25
Boyle’s Law• Mathematically:
• Each sample of gas has its own k.
P
kV kPV
2211 VPVP
Chemistry Chapter 10 26
Example• A helium-filled balloon contains
125 mL of gas at a pressure of 0.974 atm. What volume will the gas occupy at standard pressure, assuming constant temperature?
• 122 mL
Chemistry Chapter 10 27
You try• A weather balloon with a volume of
1.375 L is released from Earth’s surface at sea level. What volume will the balloon occupy at an altitude of 20.0 km, where the air pressure is 10.0 kPa, assuming constant temperature?
• 13.9 L
Chemistry Chapter 10 28
Charles’s Law• Fixed: mass and pressure• Volume varies directly with
temperature.– As temperature goes up, the particles
have more energy, so they hit the walls more often and with more force
– This pushes the walls outward.
Chemistry Chapter 10 29
Charles’s Law• Mathematically
2
2
1
1
T
V
T
V
kT
V
Chemistry Chapter 10 30
Kelvin Scale• Charles’s law works more elegantly
on the Kelvin Scale than the Celsius Scale.– If you double the temperature, the
volume doubles.• Not true with Celsius
• We must use Kelvin for Charles’s Law.
Chemistry Chapter 10 31
Kelvin Scale• Absolute zero: lowest possible
temperature– All particle motion stops– 0 K, -273.15 °C
• Often rounded to 273
CK 15.273
Chemistry Chapter 10 32
Example• A balloon filled with oxygen gas
occupies a volume of 5.5 L at 25 °C. What volume will the gas occupy at 100. °C, assuming constant pressure?
• 6.9 L
Chemistry Chapter 10 33
You try• A sample of nitrogen gas is contained
in a piston with a freely moving cylinder. At 0.0 °C, the volume of the gas is 375 mL. To what temperature must the gas be heated to occupy a volume of 500. mL, assuming constant pressure?
• 91 °C
Chemistry Chapter 10 34
Gay-Lussac’s Law• Fixed: mass and volume• Pressure varies directly with
temperature (in Kelvin)– As temperature goes up, energy of
particles goes up.– They go faster and hit the walls harder.– If the walls can’t move, the pressure
goes up.
Chemistry Chapter 10 35
Gay-Lussac’s Law• Mathematically:
kT
P
2
2
1
1
T
P
T
P
Chemistry Chapter 10 36
You try• The temperature within an
automobile tire at the beginning of a long trip is 25 °C. At the conclusion of the trip, the tire has a pressure of 1.80 atm. What is the final Celsius temperature within the tire if its original pressure was 1.75 atm? Assume constant volume.
• 34 °C
Chemistry Chapter 10 37
Combined gas law• Expresses the relationship between
pressure, volume, and temperature of a fixed amount of gas.
kT
PV
2
22
1
11
T
VP
T
VP
Chemistry Chapter 10 38
You try• The volume of a gas at 27.0 °C and
0.200 atm is 80.0 mL. What volume will the same gas sample occupy at standard conditions?
• 14.6 mL
Chemistry Chapter 10 39
Dalton’s Law• The total pressure in a container is
the sum of the partial pressures of all the gases in the container.
...321 PPPPT
Chemistry Chapter 10 40
Application• We can collect gases by displacing
water.• When we do this,
• Read Patm from the barometer. Look up Pwater in table A-8 in the appendix.
watergasatm PPP
Chemistry Chapter 10 41
Example• A student has stored 100.0 mL of
neon gas over water on a day when the temperature is 27.0 °C. If the barometer in the room reads 743.3 mm Hg, what is the pressure of the neon gas in its container?
• 716.6 mm Hg
Chemistry Chapter 10 42
You try• A sample of nitrogen gas is collected
over water at a temperature of 23.0 °C. What is the pressure of the nitrogen gas if atmospheric pressure is 785 mm Hg?
• 764 mm Hg
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