Gas Laws

1. "Imagine the air particles inside the balloon. How
do you think they are moving? Are they still or
constantly moving?"

2. 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

3. Kinetic Molecular Theory (KMT)
1. …are so small that they are assumed to have zero volume
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 theabsolute temp. of gas (i.e., Kelvin temp.)
AS TEMP. , KE
 explains why gases behave as they do
 deals w/“ideal” gas particles…

4. 8
Elastic vs. Inelastic Collisions
8
v1
elastic collision
inelastic collision
POW v2
v3 v4

5. 8
Elastic Collision
8
v1
before
v2
after

6. What is an Ideal Gas?
Gases are complicated. They're full of billions and billions of energetic
gas molecules that can collide and possibly interact with each other.
Since it's hard to exactly describe a real gas, people created the
concept of an Ideal gas as an approximation that helps us model and
predict the behavior of real gases. The term ideal gas refers to a
hypothetical gas composed of molecules which follow a few rules:
Ideal gas molecules do not attract or repel each
other. The only interaction between ideal gas
molecules would be an elastic collision upon
impact with each other or an elastic collision with
the walls of the container.

7. What is an Ideal Gas?
Gases are complicated. They're full of billions and billions of energetic
gas molecules that can collide and possibly interact with each other.
Since it's hard to exactly describe a real gas, people created the
concept of an Ideal gas as an approximation that helps us model and
predict the behavior of real gases. The term ideal gas refers to a
hypothetical gas composed of molecules which follow a few rules:
Ideal gas molecules themselves take up no
volume. The gas takes up volume since the
molecules expand into a large region of space, but
the Ideal gas molecules are approximated as point
particles that have no volume in and of
themselves.

8. In conclusion…
The ideal gas law states that the product
of the pressure and the volume of one
gram molecule of an ideal gas is equal to
the product of the absolute temperature of
the gas and the universal gas constant.

9. How can we describe empirically
an ideal gas?
How are the four variables related
to each other?

10. Volume to Pressure
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
The ideal gas law tells us that there's
an inverse relationship between the
pressure and volume of a gas,
considering a constant temperature
and amount of gas. This means:
•If you increase the pressure on a
gas (squeeze it in), its volume will
decrease.
•If you decrease the pressure on a
gas (let it expand), its volume will

17. Volume to Temperature
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
According to the ideal gas law, there's a
direct relationship between the volume
and temperature of a gas, assuming
constant pressure and amount of gas.
This means:
•If you increase the temperature of a
gas (heat it up), its volume will also
increase.
•If you decrease the temperature of a
gas (cool it down), its volume will

22. 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

23. 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

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

25. Pressure - Temperature - Volume
Relationship
P T V
Gay-Lussac’s P T
a
Charles V T
a
Boyle’s P
1
V
a
___

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

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

28. 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?
Balloon Riddle
A
B
C

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

30. Kinetic Molecular Theory
Rules:
1. Gases consist of small particles that
are far apart, relative to their size.
Gas particles are teeeny tiiiny. The particles that you’ve
seen on the screen are huge. If gas particles were the size
of a marble, the container that’s been shown would be the
size of a football stadium

31. Kinetic Molecular Theory
2. Gas particles are in constant, random
motion. The moving particles are
constantly colliding with each other and
with the walls of the container.
Like bouncy balls that never loses their energy!
Or three year olds at a birthday party.

32. Kinetic Molecular Theory
3. Collisions between other gas particles
and the container are elastic collisions.

33. Kinetic Molecular Theory
3. Collisions between other gas particles and
the container are elastic collisions.
The energy isn’t wasted, it just changes
direction.
Gas Particles are elastic.
Did it bounce?
Not elastic!
Did it bounce?
Elastic!

34. Kinetic Molecular Theory
4. There are no forces of attraction or
repulsion between gas particles.
Water likes to bond to each other. Remember
surface tension? Water likes to bead up and
bond.
Gas Particles
Don’t Care

35. Kinetic Molecular Theory
MOST IMPORTANT RULE OF ALL!
5. The Average Kinetic Energy depends on
the temperature!
The hotter the material, the faster the molecules are moving!