Chapter 10:
Characteristics of
Gases
The Kinetic-molecular theory-particles
of matter are always in motion
The kinetic-molecular theory has 5
assumptions:
1. Gases consist of large numbers of
tiny particles that are far apart relative to
their size (gases are tiny particles that
are far apart)
2. Collisions between gas particles and
between particles and container walls are
elastic collisions (the particles bounce off
each other and the walls with no loss of
energy)
3. Gas particles are in continuous, rapid,
random motion. They therefore possess
kinetic energy, which is energy of motion
(gases move in random directions)
4. There are no forces of attraction or
repulsion between gas particles (gas particles
not attracted to each other and bounce apart
when they hit)
5. The average kinetic energy of gas
particles depends on the temperature of the
gas (energy of gases depends on their
speed, speed will increase with increased
temperature)
Kinetic energy: KE = ½ mv2
Ideal gas=imaginary gas that fits all 5
assumptions
Ideal gases do not really exist; real
gases will behave almost like ideal
gases at low pressure and high
temperature
Physical Properties of
gases
1. Expansion-gases can completely fill
a container
2. Fluidity-gases can slide past one
another easily, similar to how liquids
flow. Both are called fluids.
3. Low density-particles are far apart so
they are less dense than solid and liquid
4. Compressibility-gases can be
squeezed together in smaller volumes
under pressure
5. Diffusion-gases spread out and mix
with other gases because of their
random motion
Effusion-process by which gases
move through a tiny opening.
Real Gases
1.
2.
Real gases-does not behave according to
the 5 assumptions of the kinetic-molecular
theory
2 factors van der Waals proposed to explain
why real gases are different than ideal
gases:
Gases occupy space
Gases exert attractive force on each other
Conditions that real gases will act
similar to ideal gases:
1. High temperature
2. Low pressure
Gases that act similar to ideal gases:
Nonpolar
Diatomic
Noble gases
Ex: H2 N2 He
Gases that act different from ideal gases:
Polar molecules
10-2 Pressure
Pressure-the force per unit area on a
surface, expressed in Newtons (N)
P=F
A
Consider the ballerina: Which has more
pressure- flat footed, 2 feet tippy toes, 1
foot tippy toed
Each kilogram exerts 9.8N of force due
to gravity
If the ballerina has a mass of 51 kg,
then she exerts a force of 500 N (51 X
9.8)
Atmospheric Pressure
The atmosphere is a blanket of gases
surrounding Earth. These gases exert a
pressure downward, atmospheric
pressure.
The atmospheric pressure exerts a
pressure on everything so why doesn’t it
crush us?
Crushing can demo
Measuring Atmospheric
Pressure
Torricelli discovered the first barometer
Barometer-measures atmospheric
pressure
Units of Pressure
mm Hg (millimeters of mercury)
Torr
atm (atmospheres)
Pascal (Pa)
Kilopascals (kPa)
Atmospheric pressure =
760 mm Hg = 1 atm = 1.01325 X 105 Pa =
101.325 kPa = 760 torr
Standard temperature
and pressure
STP = 1 atm and 0ºC
Converting Units of
Pressure
Sample Problem 10-1 pg. 312
The average atmospheric pressure in
Denver, Colorado, is 0.830 atm.
Express this pressure in mm Hg and
kPa.
1. Convert a pressure of 1.75 atm to kPa
to mm Hg.
2. Convert a pressure of 570 torr to
atmospheres and to kPa.
10-3 Gas Laws
Simple mathematical relationships
between the volume, temperature,
pressure, and amount in a gas.
Boyle’s Law: PressureVolume Relationship
Boyle discovered that doubling the
pressure on a gas will reduce the
volume by one-half.
If you reduce the volume of a container
of gas, there are still the same number
of gas particles that are colliding,
increasing the pressure
Boyle’s Law-the volume of a fixed mass
of gas varies inversely with the pressure
at constant temperature
PV = k
Use Boyle’s Law to compare changing
conditions for a gas
P1V1 = P2V2
Sample problem 10-2
A sample of oxygen gas has a volume
of 150 mL when its pressure is 0.947
atm. What will the volume of the gas be
at a pressure of 0.987 atm if the
temperature remains constant?
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?
Charles’s Law: VolumeTemperature
Relationship
Jacques Charles discovered that as
temperature of a gas increases, volume
also increases
Charles’s law states that the volume of
a fixed mass of gas as constant
temperature varies directly with the
Kelvin temperature.
Converting Temperature
Kelvin (K) = 273.15 + ºC
Charles’s Law:
V=k
T
V1 = V2
T1 T2
Sample problem
A sample of neon gas occupies a
volume of 752 mL at 25ºC. What
volume will the gas occupy at 50ºC if
the pressure remains constant?
Charles’s Law practice
A gas at 65ºC occupies 4.22L. At what
Celsius temperature will the volume be
3.87L, assuming the same pressure?
Gay-Lussac’s LawPressure/Temperature
Joseph Gay-Lussac discovered
Gay-Lussac’s Law-the pressure of a
fixed mass of gas at constant volume
varies directly with the Kelvin
temperature
P=k
P1 = P2
T
T1 T2
Sample Problem
The gas in an aerosol can is at a
pressure of 3.00 atm at 25ºC.
Directions on the can warn the user not
to keep the can in a place where the
temperature exceeds 52ºC. What
would the gas pressure in the can be at
52ºC?
Gay-Lussac’s Practice
A sample of helium gas has a pressure
of 1.20 atm at 22ºC. At what Celsius
temperature will the helium reach a
pressure of 2.00 atm?
Combined Gas Law
Expresses the relationship between
pressure, volume, and temperature of a
fixed amount of gas.
PV = k
T
P1V1 = P2V2
T1
T2
Sample Problem
A helium-filled balloon has a volume of
50 L at 25ºC and 1.08 atm. What
volume will it have at 0.855 atm and
10ºC?
Combined Gas Law
Practice
A 700 mL gas sample at STP is
compressed to a volume of 200 mL, and
the temperature is increased to 30ºC.
What is the new pressure of the gas in
Pa?
Dalton’s Law of Partial
Pressures
The pressure of each gas in a mixture is
called the partial pressure of that gas.
Dalton’s Law of Partial Pressures states
that the total pressure of a mixture of
gases is equal to the sum of the partial
pressures of the component gases.
Dalton’s Partial Pressure
Formula
PT = P1 + P2 + P3 +…
Patm = Pgas + PH2O
Dalton’s Law Sample
A sample of nitrogen gas is collected
over water at a temperature of 23ºC.
What is the pressure of the nitrogen gas
if atmospheric pressure is 785 mm Hg?
Use Table A-8