The Property of Gases –
Kinetic Molecular Theory
And Pressure
Kinetic Molecular Theory of Gases
• The word kinetic refers to motion.
• Kinetic energy is the energy an object has because of its
motion.
• Kinetic Molecular Theory makes assumptions about:
▫ Size
▫ Motion
▫ Energy of gas particles
Kinetic Molecular Theory Part 1
1. According to the KMT all matter consists of tiny particles
that are in constant, random motion
• Move in a straight line until they collide with other particles or with
the walls of the container.
Kinetic Molecular Theory Part 2
2. Gas particles are much smaller than the distances
between them. Most of a gas consists of empty space.

Gas consists of small particles that are separated from one
another by empty space

Most of the volume of a gas consists of empty space

Because they are so far apart, there are no attractive or
repulsive forces between the gas molecules

The motion of one particle is independent of the motion of
other particles
Kinetic Molecular Theory Part 3
3. No kinetic energy is lost when gas particles collide with
each other or with the walls of the container (elastic
collision)

Undergoes elastic collision – no kinetic energy is lost when
particles collide.

The total amount of kinetic energy remains constant.
Kinetic Molecular Theory Part 4
4. All gases have the same average kinetic energy at a
given temperature
 Temperature is a measure of average kinetic energy of particle
in a sample of matter.
 Kinetic energy and temperature are directly related
 The higher the temperature, the greater the kinetic energy
 The Kelvin temperature of a substance is directly proportional to
the average kinetic energy of the particles of the substance.
273 + _____oC = _______Kelvin
 There is no temperature lower than 0 Kelvin (Absolute Zero).
 Kinetic Energy = ½ mv2; where m = mass and v = velocity
Absolute Zero
• The greater the atomic and molecular motion, the greater the
temperature is of a substance.
• If all atomic and molecular motion would stop, the temperature
would be at absolute zero (0 Kelvin or -273 oC)
Diffusion and Effusion
• Diffusion – describes the movement of one material
through another
▫ Particles diffuse from an area of high concentration to low
concentration
• Effusion – gas escapes through a tiny opening.
• The heavier the molecule, the slower it will effuse or diffuse
Diffusion and Effusion
Diffusion
Effusion
Pressure
• Pressure is the force per unit area
• Gas pressure is the force exerted by a gas per unit
surface area of an object.
 Gas pressure is the result of billions of collisions of billions of
gas molecules with an object
• Atmospheric pressure (air pressure) results from the
collisions of air molecules with objects.
 The air pressure at higher altitudes is slightly lower than at sea
level because the density of the Earth’s atmosphere decreases
as elevation increases.
• Vacuum - Empty space with no particles and no pressure
Measuring Pressure
• Barometer – an
instrument used to
measure atmospheric
pressure
Measuring Pressure
• Manometer – an
instrument used to
measure gas pressure in
a closed container
Units of Pressure and STP
• Average atmospheric pressure is 1 atm
• 1atm = 760 torr = 760 mmHg = 101.3 kPa
• STP (Standard Temperature and Pressure)
▫ 1 atm and 0oC or 1 atm and 273 K
S T P
e e
n r
f
e
c
t
1
0
Dalton’s Partial Pressure
• Dalton’s law of partial pressures states that the total
pressure of a mixture of gases is equal to the sum of the
pressures of all the gases in the mixture.
• Ptotal = P1 + P2 +P3 + . . . Pn
Conversion Factors for Pressure
1 atm = 760 torr = 760 mmHg = 101.3 kPa
Example 1
Convert 2.5 atm into torr, mmHg, kPa
2.5 atm
760 torr
1 atm
2.5 atm
760 mmHg
1 atm
2.5 atm
101.3 kPa
1 atm
= 1900 torr
= 1900 mmHg
= 250 kPa
Example 2
Convert 215 kPa into torr, mmHg, atm
215 atm
760 torr
101.3 kPa
215 atm
760 mmHg
101.3 kPa
215 atm
1 atm
101.3 kPa
= 1610 torr
= 1610 mmHg
= 2.12 atm
Dalton’s Law of Partial Pressures
Ptotal = P1 + P2 + P3 … + Pn
Example 1
Ptotal = PO2 + PCO2 + PN2
0.97 atm = PO2 + 0.70 atm + 0.12 atm
PO2 = 0.15 atm
Example 2
Ptotal = PO2 + PCO2 + PCO
235 kPa
1 atm
101.3 kPa
= 2.32 atm
455 torr
1 atm
760 torr
= 0.599 atm
Ptotal = 0.563 atm + 2.32 atm + 0.599 atm
Ptotal = 3.48 atm
**You first
have to put
everything in
the same units!
COCl2
C2H2AsCl3
Cl3CNO2
C4H8Cl2S