Particle Theory of Matter
• Are able to move independently
• Random motion
• Move is straight lines until course is altered by
collisions with other particles
• Motion is fast
• Liquids flow because they are influences by gravity
• Gas particles move so fast that gravity does not affect
them as much
• Compressible
• Motion in all directions until all available empty space
is filled (expand)
Kinetic Molecular Theory
• Theory developed to explain gas behavior.
• Theory of moving molecules.
• Assumptions:
– Gases consist of a large number of molecules in
constant random motion.
– Volume of individual molecules negligible
compared to volume of container.
– Intermolecular forces (forces between gas
molecules) negligible.
– Energy can be transferred between molecules,
but total kinetic energy is constant at constant
temperature.
– Average kinetic energy of molecules is
proportional to temperature.
• Kinetic molecular theory gives us an understanding of
pressure and temperature on the molecular level.
• Gas molecules have an average kinetic energy.
• Each molecule has a different energy.
• When particles are heated:
• Particles move faster and have more collisions
• Higher kinetic energy helps overcome
attractive forces
• Particles need more space, making the object
expand
• Pressure of a gas results
from the number of
collisions per unit time on
the walls of container
• Magnitude of pressure
given by how often and
how hard the molecules
strike.
• There is a spread of individual energies of gas
molecules in any sample of gas.
• As the temperature increases, the average kinetic
energy of the gas molecules increases.
Pressure
• Pressure is the force acting on an object per unit
area:
• Gravity exerts a force on the earth’s atmosphere
• Atmospheric pressure is the force per unit area
exerted against a surface by the weight of the air
above that surface.
• A column of air 1 m2 in cross section exerts a
force of 105 N.
• The pressure of a 1 m2 column of air is 100 kPa.
force
pressure 
area
Barometer
A
Torricellian
Barometer
• At sea level, the
average atmospheric
pressure supports a
column of mercury
760 mm high in a
simple mercury
barometer at 0oC
• This average pressure
is called one
atmosphere.
Atmosphere Pressure and the
Barometer
• The height of the
mercury column is
a measure of the
atmosphere’s
pressure.
• The height
changes as
atmospheric
pressure changes.
Pressure and the Manometer
• A manometer consists of a bulb
of gas attached to a U-tube
containing Hg
• The pressures of gases not open
to the atmosphere are
measured in manometers.
• The height of the mercury is in
millimeters (mmHg)
– If Pgas < Patm then Pgas + Ph = Patm.
– If Pgas > Patm then Pgas = Patm + Ph.
Units of Pressure
• atmospheres (symbol = atm)
• millimeters of mercury (symbol = mm Hg). This
unit is also called a torr.
• Pascals (symbol = Pa). A Pa is N/m2. More
commonly used is kiloPascals (symbol = kPa). This
is the SI unit for pressure.
Conversion Between Units
1 atm = 760 mm Hg = 101.325 kPa = 101325 Pa
• Example #1 - Convert 0.875 atm to mmHg
1 atm =
760 mmHg
0.875 atm
?
? = 665 mmHg
• Example #2 - Convert 745.0 mmHg to atm.
1 atm =
760 mmHg
?
745.0 mmHg
? = 0.9802 atm
• Example #3 - Convert 0.955 atm to kPa
1 atm
=
101.325 kPa
0.955 atm
?
? = 96.8 kPa
• Example #4 - Convert 98.35 kPa to atm
1 atm =
101.325 kPa
?
98.35 kPa
? = 1.030 atm
• Example #5 - Convert 740.0 mmHg to kPa
760 mmHg =
101.325 kPa
740 mmHg
?
? = 98.6 (99) kPa
• Example #6 - Convert 99.25 kPa to mmHg
760 mmHg =
101.325 kPa
?
99.25 kPa
? = 744.4 mmHg
Convert the following pressures:
1. 658.2 mm Hg to kPa
2. 1.85 atm to torr
3. 337.3 kPa to atm
4. 6.6 x 10-2 torr to atm
5. 147.2 kPa to torr
Convert the following pressures:
1. 658.2 mm Hg to kPa
87.75 kPa
2. 1.85 atm to torr
1.41 x 103 torr
3. 337.3 kPa to atm
3.329 atm
4. 6.6 x 10-2 torr to atm
8.7 x 10-5 atm
5. 147.2 kPa to torr
1104 torr
The mercury in a manometer is 46 mm higher on the
open end than on the gas bulb end. If atmospheric
pressure is 102.2 kPa, what is the pressure of the gas
in the bulb?
On a certain day the barometer in a laboratory
indicates that the atmospheric pressure is 764.7 torr.
A sample of gas is placed in a vessel attached to an
open-end mercury manometer. A meter stick is used
to measure the height of the mercury above the
bottom of the manometer. The level of mercury in the
open end arm of the manometer has a measured
height of 136.4 mm. In the arm that is in contact with
the gas has a height of 103.8 mm. What is the
pressure of the gas (a) in atm and (b) in kPa?