Kinetic Molecular
Theory of Gases
These ideas have withstood the
test of time and much
experimentation.
Six Postulates
of the
Kinetic Molecular
Theory of Gases
A scientific postulate is a
statement assumed to be true
unless proven otherwise.
Six Postulates
1. A gas consists of very small
particles, each of which has
a mass.
Example:
An inflated basketball weighs more
than a deflated basketball.
Six Postulates
2. The distances separating gas
particles are relatively large.
The volume of the gas particles is
assumed to be zero because it is
negligible compared with the total
volume in which the gas is contained.
This is a fair assumption.
Six Postulates
3. Gas particles are in constant,
rapid, random motion.
Gases immediately fill a
container and quickly diffuse
from one area to another.
This is a good description of how
gas molecules behave
Six Postulates
4. Collisions of gas particles with
each other or with the walls of
the container are perfectly
elastic.
Unlike “bouncing balls,” no
energy of motion is lost.
Six Postulates
5. The average kinetic energy of
gas particles depends only on
the temperature of the gas.
The kinetic energy of gas molecules is
proportional to their temperature in
Kelvins -- a good description.
KE = mv2 / 2
High T  Higher KE
Low T  Lower KE
Six Postulates
6.
Gas particles exert no force on one
another. Attractive forces between gas
particles is assumed to be zero.
Gas particles do not slow down and condense
into a liquid because they exert only very weak
attractive forces upon each other.
Gas molecules don’t interact with one another.
Depending on the gas, this can be good or bad
assumption. H2O vapor has much stronger
intermolecular interactions than He.
Real Gas -- Ideal Gas
Very high temperatures and very low
pressure help real gases simulate ideal
gases.
@ High Temperatures: Gas molecules
move so quickly that there is not time
to interact.
@ Low Pressure: Gas molecules don’t
encounter each other very often.
Avogadro’s Principle
When measured at constant
temperature and pressure,
equal volumes of gases
contain equal numbers of
moles.
Avogadro’s Principle
• The volume of a gas is directly
proportional its number of
moles (n), regardless of the
identity of the gas
V n
V  n
• Ideal gas molecules don’t
have volume, molecule size
doesn’t matter
• Ideal gases don’t interact with
each other, no intermolecular
forces change the volume of
the gas
V  n
Therefore…
Ideal gas molecules behave
according to Avogadro’s
Principle.
Standard Temperature
& Pressure
STP
• 1 atmosphere pressure (atm)
• 273 Kelvin (K) or 0 0C
Standard Molar Volume
• The volume occupied by one
mole of gas at STP
• 22.4 L