Physical Characteristics
of Gases
Chapter 10
Kinetic Molecular Theory
KMT is based on the idea that particles of matter are always in
motion
 Explains the behavior of gas particles and their physical
properties
 Provides a model of what is called an ideal gas
Kinetic Molecular Theory
Five assumptions of the KMT
1. Gases consist of large numbers of tiny particles that are
relatively far apart in comparison to their size
4’ x 8’
Kinetic Molecular Theory
Five assumptions of the KMT
2. Collisions involving gas particles are elastic (no net loss
of kinetic energy)
Kinetic Molecular Theory
Five assumptions of the KMT
3. Gas particles are in continuous, rapid, random motion
4. There are no forces of attraction or repulsion between
gas particles
Kinetic Molecular Theory
Five assumptions of the KMT
5. The average KE of the particles depends on their Kelvin
temperature
KMT and the Nature of Gases
Expansion
 Gases have an indefinite shape and indefinite volume
 Completely take the shape of their container
KMT and the Nature of Gases
Fluidity
 Particles are not attracted to each other so they simply
move past each and flow
Lake Nyos
KMT and the Nature of Gases
Fluidity
 Particles are not attracted to each other so they simply
move past each and flow
KMT and the Nature of Gases
Low Density
 The density of a substance in the gas state is about 1/1000
the density of the same substance in the liquid or solid
state
KMT and the Nature of Gases
Compressibility
 The volume of a gas can be decreased greatly
Diffusion and Effusion
 Diffusion is mixing of two substances caused by their
random motion
 Effusion is the process by which a gas passes through a
tiny opening
 Both are proportional to the velocity of the particles and
inversely proportion to their mass
Deviations from Ideal Behavior
A real gas is a gas that does not adhere to all of the
assumptions of the KMT
1873 – Johannes van der Waals points out that real gas
particles occupy space and exert attractive forces on each
other
Real gases that behave like ideal gases are
 Small (in mass)
 Non-polar
Real gases can exhibit ideal behavior under the conditions
of
 High temperature (well above their boiling points)
 Low pressure
Pressure
To fully describe a gas you need to state four measureable
quantities
1. Volume
2. Temperature
3. # of particles
4. Pressure
Pressure is defined as the force per unit area.
𝑓𝑜𝑟𝑐𝑒
𝑃=
𝑎𝑟𝑒𝑎
SI unit for pressure is the Newton
Pressure
Pressure
Pressure
Barometer – device used to
measure atmospheric
pressure
Normal atmospheric
pressure can support a
column of Hg 760 mm high
760mm Hg =
760 torr =
1 atmosphere =
101.3 kilopascals
Pressure
Pressure
Barometer – device used to
measure atmospheric
pressure
Normal atmospheric
pressure can support a
column of Hg 760 mm high
760mm Hg =
760 torr =
1 atmosphere =
101.3 kilopascals
Pressure
 Convert the following
pressures.
 151 kPa to atm
 456 torr to mmHg
 1.24 atm to mmHg
Pressure
Weather barometer
The Gas Laws
 1662 – Robert Boyle
discovered that gas
pressure and volume are
related mathematically
 Found that when he
double the pressure on a
gas sample at constant
temperature decreased its
volume by one-half
PV=k
P1V1=P2V2
The Gas Laws
The Gas Laws
Divers know that the pressure exerted
by water increases about 100 kPa with
every 10.2 m of depth. This means
that at 10.2 m below the surface, the
pressure is 201 kPa; at 20.4 m, the
pressure is 301 kPa; and so forth.
Given that the volume of a balloon is
3.5 L at STP and that the temperature
of the water remains the same, what is
the volume of the balloon 51 m below
the surface of the water?
The Gas Laws
A gas has a pressure of 1.26 atm and occupies a volume of 7.40
L. If the gas is compressed to a volume of 2.93 L, what will its
pressure be, assuming constant temperature?
The Gas Laws
 The quantitative relationship
between temperature and
volume was discovered by
Jacques Charles in 1787
 Kelvin temperature and
volume are directly related
𝑉
=𝑘
𝑇
𝑉1 𝑉2
=
𝑇1 𝑇2
The Gas Laws
A sample of nitrogen gas is contained in a piston with a freely
moving cylinder. At 0.0C, the volume of the gas is 375 mL.
To what temperature must the gas be heated to occupy a
volume of 500. mL?
The Gas Laws
The quantitative relationship
between temperature and
pressure was discovered by
Joseph Gay-Lussac in 1802
Pressure of a fixed gas
(constant volume) varies
directly with the Kelvin
temperature
𝑃
=𝑘
𝑇
𝑃1 𝑃2
=
𝑇1 𝑇2
The Gas Laws
Before a trip from NY to Boston, the pressure in a car tire is
1.8 atm at 20°C. At the end of the trip, the pressure gauge
reads 1.9 atm. What is the new Celsius temperature of the air
inside the tire?
The Gas Laws
The gas in an aerosol can is at a pressure of 3.00 atm at
25°C. Directions on the can warn the user not to keep the
can in a place where the temperature exceeds 52°C. What
would the pressure in the can be at 52°C?
The Gas Laws
Boyle’s Law, Charles’s Law, and Gay-Lussac’s Law can all be
combined into a single expression.
𝑃1𝑉1
𝑃2𝑉2
=
𝑇1
𝑇2
The Gas Laws
A 700.0 mL gas sample at STP is compressed to a volume of
200.0 mL, and the temperature is increased to 30.0°C. What
is the new pressure of the gas in kPa?
The Gas Laws
A helium filled balloon has a volume of 50.0 L at 25°C and
1.08 atm. What will the volume be at 0.855 atm and 10.°C?
The Gas Laws
 John Dalton found that in the absence of a chemical
reaction, the pressure of a gas mixture is the sum of the
individual pressures of the gas alone.
 The whole is equal to the sum of its parts
PT = P1 + P2 + P3 + …
Pair = PN2 + PO2 + PH2 + PHe + PNe + …
The Gas Laws
PT = P1 + P2
Ptotal = PCO2 + PH20
NaHCO3
CO2 and H2O
The Gas Laws
The Gas Laws
760 mm Hg = PCO2 + 17.5 mm Hg
PCO2 = 742.5 mm Hg
NaHCO3
CO2 and H2O
The Gas Laws
Oxygen gas from the decomposition of potassium chlorate
was collected by water displacement. The barometric
pressure and the temperature during the experiment were
731.0 torr and 20.0°C, respectively. What was the partial
pressure of the oxygen collected?
The Gas Laws