THE KINETIC THEORY OF GASES

1
Is the air we breath a solid, a liquid or a gas?
What is a gas?
2
Pressure?
What is Air
?
78.08 Nitrogen Gas 20.95 Oxygen Gas 0.93
Argon 0.03 Carbon Dioxide
3
Air Molecules are constantly colliding against
everything.
These collisions create the atmospheric air
pressure. (approx 1 atm)
What is pressure?
4
PRESSURE Force per unit area
Force/Area F/A
The weight (Force) of the knife does not change,
but the amount of surface area in contact with
your skin does.
Smaller Area Greater Pressure
Greater Area Lower Pressure
5
Gas Pressure (P) - the force exerted by a gas
per unit surface area. Measured in pascals (Pa)

• Pressure Force / Area
• Force due to collisions / Area
• PSI Pounds per square inch
• Pounds/in2

Gas Laws Simulations http//phet.colorado.edu/sim
ulations/sims.php?simGas_Properties
6
Can we FEEL Air Pressure?
7
Can we FEEL Air Pressure?
1 m2 10,000 kg
1 cm2 1 kg
One Atmosphere 1atm 101,325 Pa
101.3 kPa 760 mmHg
1.01325 bar 14.7 psi
8
Air pressure varies with altitude
? 1 atm
9
Is the air pressure inside the bottle different
than the air pressure outside?
Air Molecules colliding with the interior walls
of the container creating an internal pressure
External Pressure
Air Molecules (the atmosphere) colliding with the
exterior of the container creating an external
pressure
Internal Pressure
10
-Pressure is EQUAL in all directions -Force of
collisions are equal in ALL directions
Can the Internal Pressure be different than the
External Pressure?
Internal Pressure VS. External Pressure
11
Internal Pressure VS. External Pressure
-Pressure is EQUAL in all directions
12
Differences in Pressure
Vacuum chamber removed some air
By removing the air within the vacuum chamber,
the external air pressure was decreasing The
internal air pressure (within the bottle) was
becoming greater than the external air pressure
allowing the bottle to expand
Before vacuum
Internal external
13
Kinetic Molecular Theory (KMT)
Structure of Matter- http//preparatorychemistry.c
om/Bishop_KMT_frames.htm
Speed of gas particles - http//www.chm.davidson.
edu/vce/KineticMolecularTheory/BasicConcepts.html
Average Speed of gas particles http//www.chm.davi
dson.edu/vce/KineticMolecularTheory/Maxwell.html
14
Kinetic Energy (KE) The energy of motion
KE ½ Mass (Velocity)2 ½ MV2
http//www.physicsclassroom.com/mmedia/energy/pe.c
fm
15
Kinetic Molecular Theory (KMT)

• All Matter is composed of tiny particles
• These particles are in constant motion
• The amount of motion is proportional to the
  temperature.
• (Increased temperature increased motion)

Attraction (IMF) VS. Motion (State)
The States of Matter (Solids, Liquids, and gases)
differ in the freedom of motion of their
particles and the extent to which the particles
interact (Attraction)
Structure of Matter- http//preparatorychemistry.c
om/Bishop_KMT_frames.htm
16
The Nature of Gases

• Three basic assumptions of the kinetic theory
  as it applies to gases

• 1. Gas is composed of particles-
• usually molecules or atoms
• Small, hard spheres
• Insignificant volume relatively far apart from
  each other
• No attraction or repulsion between particles

17
The Nature of Gases

• Three basic assumptions of the kinetic theory
  as it applies to gases

2. Particles in a gas move rapidly in constant
random motion -Move in straight paths, changing
direction only when colliding with one another or
other objects
Average speed of O2 in air at 20 oC is an amazing
1660 km/h! (1.6km1mile)
18
The Nature of Gases

• Three basic assumptions of the kinetic theory
  as it applies to gases

3. Collisions are perfectly elastic-
Meaning kinetic energy is transferred without
loss from one particle to another- the total
kinetic energy remains constant (KE1/2MV2)
Gas Laws Simulations http//phet.colorado.edu/sim
ulations/sims.php?simGas_Properties
19
1 atm 101.3 kPa 760 mmHg
1 atm (One Atmosphere) 101.3 kPa
(kilopascals) 760 mmHg (millimeters of
Mercury)
Standard Temperature and Pressure (STP) -STP 1
Atm (of pressure) at 273 K (0 C)
NOTE You need to be able to convert between the
different pressures and different temperatures.
20
Conversion Factors
1 atm 101.3 kPa 760 mmHg
101.3 kPa
760 mmHg



1 atm
1 atm
760 mmHg
1



101.3 kPa
21
A gas is at a pressure of 1.5 atm. Convert this
pressure to kilopascals
1 atm 101.3 kPa 760 mmHg
Start with what you have
Multiply by a conversion factor to cancel units
Perform the calculations to find your answer
1.5 atm
151.95 kPa

22

1. What pressure, in kilopascals and in atmospheres,
   does a gas exert at 385 mmHg?
2. Express the pressure 125.2 kPa in millimeters of
   mercury and atm.

385 mmHg 51.3 kPa 0.507 atm
125.2 kPa 939.3 mmHg 1.24 atm