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4 Postulates of Kinetic Theory
1) Spherical molecules in constant, random straight-line motion
2) “Elastic” collisions3) Point masses4) No interactions
Ideal Gas
Gas that obeys all 4 assumptions of the kinetic theory all of the time. It doesn’t exist. It’s a model.
Real Gas
Most real gases obey the kinetic theory most of the time.
Which assumptions of the kinetic theory hold up?
1. Spherical molecules in random, straight-line motion
2. “Elastic” collisions
Which assumptions of the kinetic theory break
down?
1. Point masses2. No interactions
When do the assumptions of the kinetic theory
break down?
When the gas molecules are close to each other.
When are the gas molecules close to each
other?
At high pressure & low temperature.
When are the gas molecules far apart from
each other?
At low pressure & high temperature.
How do the gas molecules act when they are far apart from each
other?
Good!
What are the properties of gases?
1. Have mass2. Take the shape & volume of their
container3. Compressible4. Diffuse (spread spontaneously)5. Exert Pressure
Pressure
Force/Area
Pressure results from?
Collisions of the gas molecules with the walls of the
container.
Pressure depends on?(microscopically)
# of impacts per unit time and force of each impact
Pressure depends on?(macroscopically)
# of gas molecules per unit volumeAnd
temperature
Units of Pressure?
1 atm = 760 mm Hg =
760 torr = 101.3 kPa
STP
P = 1 atmT = 0C = 273 K
Molar Volume at STP
The volume of 1 mole of any gas at STP = 22.4 Liters
Temperature
A measure of the avg. kinetic energy of the particles of a
substance.
4 variables needed to completely describe a
gas-phase system?
1.Temperature2.Pressure3.Volume4.# of moles
Boyle’s Law
For a fixed mass and temperature, the pressure-
volume product is a constant.
Boyle’s Law
Volume of a gas varies inversely with the pressure.
Boyle’s Law
PV = k where k = a constant
Boyle’s Law
P1V1 = P2V2
Graph of Boyle’s Law
Hyperbola – it’s an inverse relationship!
Graph of Boyle’s Law, Pressure vs. Volume
Double the pressure
Volume goes to ½ the original volume
Triple the pressure
Volume goes to 1/3 the original volume
Halve the pressure
Volume goes to 2 X the original volume
Quadruple the pressure
Volume goes to 1/4 of the original volume
What does the graph of a direct relationship look
like?
Which temperature scale has a direct relationship to molecular velocity?
Kelvin: 0 K means 0 speed.
Absolute Temperature Scale
Kelvin: 0 K means 0 speed.
Graph of Volume vs. Kelvin Temperature
It’s a direct relationship.
Math expression of relationship between
volume & Kelvin temperature
V1/T1 = V2/T2
Charles’ Law
Verbal expression of Charles’ Law
Volume of a gas varies directly with the Kelvin temperature.
What happens to the volume when the Kelvin temperature is doubled?
The volume doubles!
What happens to the volume when the Kelvin temperature is tripled?
The volume triples!
What happens to the volume when the Kelvin temperature is halved?
The volume is halved!
What happens to the Kelvin temperature when the
volume is halved?
It’s halved!
Graph of Pressure vs. Kelvin Temperature
It’s a direct relationship.
Math expression for relationship between
pressure & Kelvin temperature.
P1/T1 = P2/T2
Gay-Lussac’s Law
Verbal expression of relationship between
pressure & Kelvin temperature.
The pressure of a gas varies directly with the Kelvin temperature.
What happens to the pressure when the Kelvin temperature is doubled?
The pressure is doubled.
What happens to the pressure when the Kelvin temperature is halved?
The pressure is halved.
What happens to the pressure when the Kelvin temperature is tripled?
The pressure is tripled.
What happens to the Kelvin temperature when the pressure is doubled?
The Kelvin temperature is doubled.
Combined Gas Law
P1V1 = P2V2 T1 T2
Same pressure & volume units on both sides. Temperature must be in Kelvins.
What is Avogadro’s Principle?
Equal volumes of gases at the same temperature & pressure have equal numbers of molecules.
These 2 boxes have the same:
a) Mass c) # of molecules
b) Density d) # of atoms
He N2
What is Dalton’s Law of Partial Pressures?
Ptot = P1 + P2 + P3 + P4 + …
At the same temperature, which of the following gases diffuses most
rapidly?
He Ne Ar Kr Xe
Lightest is fastest!
At the same temperature, which of the following gases diffuses most
slowly?
He Ne Ar Kr Xe
Heaviest is slowest!
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