Chapter 11
Gases
Section 2
The Kinetic-Molecular Theory of
Matter
Kinetic-Molecular Theory of Gases:
 Particles of matter are always in motion.
Can help us understand the behavior of gas
molecules and the physical properties of
gases.
 Use kinetic theory to explain the properties
and behavior of gases.
5 Assumptions of Kinetic Theory for
Ideal Gases
 1. motion of gas particles is constant and
random
 2. collisions are elastic, no energy is lost,
energy may be transferred
 3. gases are “point masses” – extremely
small and very far apart
 4. gas particles do not exert attractive forces
on each other
 5. gases have higher kinetic energy (move
faster) at higher temperatures, have lower
kinetic energy (move slower) at low
temperatures
Properties of Gases
 1. have mass
 2. expansion – gases have no definite
shape or volume, they expand to fill the
container they are in
 3. Fluidity – ability to flow (glide past one
another, just like liquids)
 4. low density
 5. are compressible
 6. diffuse through other gases (gases spread out
and mix with one another)
Diffusion – spontaneous mixing of the particles
of two substances caused by their random motion
→Rate of diffusion of one gas through another
depends on three properties
1. speed of gas particles
2. diameter (size) of particles
3. attractive forces between particle
 7. Effuse through small openings
Effusion – process by which gas particles
pass through a tiny opening
 8. Exert pressure
Deviations of Real Gases from Ideal
Behavior
 Ideal Gas – an imaginary gas that perfectly fits all
the assumptions of the kinetic theory
 Ideal behavior happens when gas particles have
little attraction for each other
 Examples of Ideal Gas
Noble gases
Nonpolar diatomic molecules like H2 and N2
Other small, nonpolar gases
 Real Gas – a gas that does not behave
completely according to the assumptions of
the kinetic-molecular theory
 Real Gases have mass and attractive forces
between the particles.
 Examples of Real Gases
Polar molecules like NH3 and H2O
Gases at LOW temperatures
Gases at HIGH pressures
Homework
 Read pages 359-362
 Answer questions page 399 #1-5