Kinetic Molecular theory of Gases

Contributed by:

Jonathan James Fri, Jan 14, 2022 09:44 PM UTC

The highlights are:
1. Collision of gas particles
2. Kinetic Theory
3. Elastic vs inelastic collisions
4. Molecular velocities
5. Real gases
6. Properties and characteristics of gases
7. Kinetic theory and gas laws

1. Collisions of Gas Particles

2. Collisions of Gas Particles

3. Kinetic Theory

4. Kinetic Molecular Theory
Postulates of the Kinetic Molecular Theory of Gases
1. Gases consist of tiny particles (atoms or molecules)
2. These particles are so small, compared with the distances between
them, that the volume (size) of the individual particles can be assumed
to be negligible (zero).
3. The particles are in constant random motion, colliding with the walls of
the container. These collisions with the walls cause the pressure exerted
by the gas.
4. The particles are assumed not to attract or to repel each other.
5. The average kinetic energy of the gas particles is directly proportional
to the Kelvin temperature of the gas

5. Kinetic Molecular Theory (KMT)
 explains why gases behave as they do
 deals w/“ideal” gas particles…
1. …are so small that they are assumed to have zero
2.…are in constant, straight-line motion
3.…experience elastic collisions in which no energy is lost
4.…have no attractive or repulsive forces toward each other
5.…have an average kinetic energy (KE) that is proportional
to the absolute temp. of gas (i.e., Kelvin temp.)
AS TEMP. , KE

6. Elastic vs. Inelastic Collisions
8
3

7. Elastic vs. Inelastic Collisions
v1 POW v2
8
elastic collision
v3 v4
8
inelastic collision

8. Elastic Collision
v1
8
v2
8

9. Model Gas Behavior
• All collisions must be elastic
• Take one step per beat of the
metronome  Mark area of container
• Container with tape on ground.
– Class stands outside tape box  Add only a few
• Higher temperature molecules of inert gas
– Faster beats of metronome  Increase temperature
• Decreased volume  Decrease volume
– Divide box in half  Add more gas
• More Moles  Effect of diffusion
– More students are inside box  Effect of effusion
(opening size)

10. Kinetic Molecular Theory
• Particles in an ideal gas…
– have no volume.
– have elastic collisions.
– are in constant, random, straight-line motion.
– don’t attract or repel each other.
– have an avg. KE directly related to Kelvin temperature.
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

11. Molecular Velocities
molecules sorted by speed
many different molecular speeds
Fractions of particles
the Maxwell speed distribution
speed

12. Real Gases
• Particles in a REAL gas…
– have their own volume
– attract each other
• Gas behavior is most ideal…
– at low pressures
– at high temperatures
– in nonpolar atoms/molecules
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

13. Characteristics of Gases
Gases expand to fill any container.
– random motion, no attraction
Gases are fluids (like liquids).
– no attraction
Gases have very low densities.
– no volume = lots of empty space
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

14. Characteristics of Gases
• Gases can be compressed.
– no volume = lots of empty space
• Gases undergo diffusion & effusion.
– random motion
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

15. Properties of Gases
Gas properties can be modeled using math.
Model depends on:
V = volume of the gas (liters, L)
T = temperature (Kelvin, K)
P = pressure (atmospheres, atm)
n = amount (moles, mol)

16. Pressure - Temperature - Volume
Relationship
P V
P T
T V
PP VV
1
___
Boyle’s PV
Charles VT
Gay-Lussac’s PT

17. Pressure - Temperature - Volume
Relationship
P T
n V
1
___
Boyle’s P
V
Charles VT
Gay-Lussac’s PT

18. Pressure and Balloons
When balloon is being filled:
PA > P B
A When balloon is filled and tied:
PA = PB
When balloon deflates:
PA < PB
A = pressure exerted BY balloon
B = pressure exerted ON balloon

19. Balloon Riddle
When the balloons are untied, A
will the large balloon (A) inflate
the small balloon (B); will they
end up the same size or will the
small balloon inflate the large B
C

20. Kinetic Theory and the Gas Laws
10
10 10 10
(a) (b) (c)
original temperature increased temperature increased temperature
original pressure increased pressure original pressure
original volume original volume increased volume
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 323 (newer book)

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