Kinetic Molecular Theory

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Kinetic Molecular Theory
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Collisions of Gas Particles
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Collisions of Gas Particles
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Kinetic Theory
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Kinetic Molecular Theory
Postulates of the Kinetic Molecular Theory of
Gases

• Gases consist of tiny particles (atoms or
  molecules)
• 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

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Kinetic Molecular Theory
Evidence
Postulates
1. Gases are tiny molecules in mostly empty space. The compressibility of gases.
2. There are no attractive forces between molecules. Gases do not clump.
3. The molecules move in constant, rapid, random, straight-line motion. Gases mix rapidly.
4. The molecules collide classically with container walls and one another. Gases exert pressure that does not diminish over time.
5. The average kinetic energy of the molecules is proportional to the Kelvin temperature of the sample. Charles Law
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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 volume

• are in constant, straight-line motion
• experience elastic collisions in which no energy
  is lost
• have no attractive or repulsive forces toward
  each other
• have an average kinetic energy (KE) that is
  proportional
• to the absolute temp. of gas (i.e., Kelvin temp.)

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Newtons First Law of Motion (Law of Inertia)
Object at rest tends to stay at rest, and object
in motion tends to stay in motion at constant
velocity unless object is acted upon by an
unbalanced, external force.
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Elastic vs. Inelastic Collisions
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3
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Elastic vs. Inelastic Collisions
POW
v1
v2
elastic collision
v3
v4
inelastic collision
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Elastic Collision
v1
before
v2
after
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Model Gas Behavior

• All collisions must be elastic
• Take one step per beat of the metronome
• Container
• Class stands outside tape box
• Higher temperature
• Faster beats of metronome
• Decreased volume
• Divide box in half
• More Moles
• More students are inside box

• Mark area of container with tape on ground.
• Add only a few molecules of inert gas
• Increase temperature
• Decrease volume
• Add more gas
• Effect of diffusion
• Effect of effusion (opening size)

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Kinetic Molecular Theory

• Particles in an ideal gas
• have no volume.
• have elastic collisions.
• are in constant, random, straight-line motion.
• dont attract or repel each other.
• have an avg. KE directly related to Kelvin
  temperature.

Courtesy Christy Johannesson www.nisd.net/communic
ationsarts/pages/chem
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Molecular Velocities
Fractions of particles
the Maxwell speed distribution
speed
http//antoine.frostburg.edu/chem/senese/101/gases
/slides/sld016.htm
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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/communic
ationsarts/pages/chem
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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/communic
ationsarts/pages/chem
17
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/communic
ationsarts/pages/chem
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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)

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Pressure - Temperature - Volume Relationship
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Pressure - Temperature - Volume Relationship
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Pressure and Balloons
B
When balloon is being filled PA gt PB
A
When balloon is filled and tied PA PB
When balloon deflates PA lt PB
A pressure exerted BY balloon
B pressure exerted ON balloon
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Balloon Riddle
When the balloons are untied, 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 balloon? Why?
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Behavior of Gases
Behavior of Gases
Behavior of Gases
Keys
http//www.unit5.org/chemistry/GasLaws.html
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Kinetic Theory and the Gas Laws
(a)
(b)
(c)
increased temperature increased pressure original
volume
original temperature original pressure original
volume
increased temperature original pressure increased
volume
Dorin, Demmin, Gabel, Chemistry The Study of
Matter , 3rd Edition, 1990, page 323 (newer book)
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Kinetic Theory and the Gas Laws
(a)
(c)
increased temperature increased pressure original
volume
original temperature original pressure original
volume
increased temperature original pressure increased
volume
Dorin, Demmin, Gabel, Chemistry The Study of
Matter , 3rd Edition, 1990, page 323 (newer book)