Topic
12
Table of Contents
Topic
12
Topic 12: Kinetic Theory
Basic Concepts
Additional Concepts
Kinetic Theory: Basic Concepts
Topic
12
Gases
• In the late 1800s, two scientists,
Ludwig Boltzmann and James
Maxwell, independently proposed
a model to explain the properties
of gases in terms of particles in
motion. This model is now known
as the kinetic-molecular theory.
• The model makes the following
assumptions about the size, motion,
and energy of gas particles.
Kinetic Theory: Basic Concepts
Topic
12
Particle size
• The particles in a gas are separated from
one another by empty space.
• The volume of the empty space is much
greater than the volume of the gas particles
themselves.
• Because gas particles are far apart, there are
no significant attractive or repulsive forces
among them.
Kinetic Theory: Basic Concepts
Topic
12
Particle motion
• Gas particles are in constant, random
motion. Until they bump into something
(another particle or the side of a container),
particles move in a straight line.
• When gas particles do collide with
something, the collision is said to be elastic.
• An elastic collision is one in which no
kinetic energy is lost.
Kinetic Theory: Basic Concepts
Topic
12
Particle energy
• Mass and velocity determine the kinetic
energy of a particle, as represented in the
equation below.
KE = kinetic energy
m= mass of the particle
v = velocity of the particle
Kinetic Theory: Basic Concepts
Topic
12
Particle energy
• The velocity of a particle includes both its
speed and its direction.
• Each particle in a sample containing only
one gas will have the same mass but not the
same velocity.
• Thus, all the particles in a sample of gas do
not have the same kinetic energy.
Kinetic Theory: Basic Concepts
Topic
12
Particle energy
• Temperature is a measure of the average
kinetic energy of the particles in a sample
of matter.
• At a given temperature, all gases have the
same average kinetic energy.
Kinetic Theory: Basic Concepts
Topic
12
Explaining the behavior of gases
• The kinetic-molecular theory explains the
following behavior of gases.
• Low Density Density is a measure of mass
per unit volume. The difference between the
high density of a solid and the low density of
a gas is due mainly to the large amount of
space between the particles in the gas. There
are fewer particles in a gas than in a solid of
the same volume.
Kinetic Theory: Basic Concepts
Topic
12
Explaining the behavior of gases
• Compression and expansion A gas will
expand to fill its container. Thus, the density
of a sample of gas will change with the
volume of the container it is placed in. The
gas will become more dense as it is
compressed into a smaller container. The gas
will become less dense as it expands in a
larger container.
Kinetic Theory: Basic Concepts
Topic
12
Explaining the behavior of gases
• Diffusion Gas particles flow past each other
easily because there are no significant forces
of attraction between them.
• Diffusion refers to the movement of one
material through another, such as when one
gas flows into a
space already
occupied by
another gas.
Kinetic Theory: Basic Concepts
Topic
12
Explaining the behavior of gases
• Effusion If you have ever seen a tire deflate
from a puncture, you are familiar with
effusion. Effusion is the escape of a gas
through a small opening in its container.
Kinetic Theory: Basic Concepts
Topic
12
Gas pressure
• When gas particles collide with the walls
of their container, they exert pressure on
the walls.
• Pressure is force per unit area.
• The pressure exerted by the particles in the
atmosphere that surrounds Earth is called
atmospheric pressure, or air pressure.
• Air pressure varies at different locations on
Earth.
Kinetic Theory: Basic Concepts
Topic
12
Gas pressure
• At Earth’s surface, air pressure is
approximately equal to the pressure exerted by
a 1-kilogram mass on a
square centimeter.
• Air pressure at higher
altitudes, such as on a
mountaintop, is slightly
lower than air pressure
at sea level.
Kinetic Theory: Basic Concepts
Topic
12
Gas pressure
• Air pressure is
measured using a
barometer.
• A barometer consists
of a thin tube closed
on one end and filled
with mercury.
Kinetic Theory: Basic Concepts
Topic
12
Gas pressure
• The tube is placed so
that the level of the
mercury is determined
by air pressure.
• The mercury rises
when the air pressure
increases and falls
when the air pressure
decreases.
Kinetic Theory: Basic Concepts
Topic
12
Forces of Attraction
• The attractive forces that hold particles
together in ionic, covalent, and metallic
bonds are called intramolecular forces.
• Intermolecular forces, which are weaker
than intramolecular forces, also can hold
particles together.
• Three types of intermolecular forces are
described below: dispersion forces, dipole–
dipole forces, and hydrogen bonds.
Kinetic Theory: Basic Concepts
Topic
12
Dispersion forces
• Weak forces that result from temporary
shifts in the density of electrons in electron
clouds are called dispersion forces, or
London forces.
• When two nonpolar molecules are in close
contact, the electron cloud of one molecule
repels the electron cloud of the other
molecule.
Kinetic Theory: Basic Concepts
Topic
12
Dispersion forces
• As a result, the electron density in each
electron cloud is greater in one region of
the cloud.
• Two temporary dipoles form.
Kinetic Theory: Basic Concepts
Topic
12
Dispersion forces
• Weak dispersion forces exist between
oppositely charged regions of the dipoles.
• Dispersion forces, which are the weakest
intermolecular forces, are important only
when no stronger forces are acting on the
particles.
• Dispersion forces are noticeable between
identical nonpolar molecules as the number
of electrons involved increases.
Kinetic Theory: Basic Concepts
Topic
12
Dipole–dipole forces
• Attractions between oppositely charged
regions of polar molecules are called
dipole–dipole forces.
• Polar molecules have a permanent dipole and
orient themselves so that oppositely charged
regions match up.
• Dipole–dipole forces are stronger than
dispersion forces as long as the molecules
being compared are similar in mass.
Kinetic Theory: Basic Concepts
Topic
12
Hydrogen bonds
• A hydrogen bond is a
dipole–dipole attraction
that occurs between
molecules containing a
hydrogen atom bonded
to a small, highly
electronegative atom
with at least one lone
electron pair.
Kinetic Theory: Basic Concepts
Topic
12
Hydrogen bonds
• The hydrogen must be
bonded to a fluorine, an
oxygen, or a nitrogen
atom.
• Hydrogen bonds explain
why water is a liquid at
room temperature, while
compounds of comparable
mass are gases.
Kinetic Theory: Basic Concepts
Topic
12
Liquids and Solids
• The kinetic-molecular theory also explains
the behavior of liquids and solids.
• However, the forces of attraction between
particles in liquids and solids must be
considered as well as their energy of motion.
Kinetic Theory: Basic Concepts
Topic
12
Liquids
• Liquids conform to the
shape of their container
but have a fixed volume.
• The particles in a liquid
maintain a fixed
volume because the
forces of attraction
between them limit
their range of motion.
Kinetic Theory: Basic Concepts
Topic
12
Density and compression
• The density of a liquid is much greater than
that of its vapor at the same conditions.
• The higher density is due to intermolecular
forces, which hold the particles together.
Kinetic Theory: Basic Concepts
Topic
12
Fluidity
• Fluidity is the ability to flow.
• Liquids are less fluid than gases.
Kinetic Theory: Basic Concepts
Topic
12
Viscosity
• A measure of the resistance
of a liquid to flow is called
viscosity.
• The stronger the intermolecular
forces, the higher is the viscosity.
• Viscosity also increases with the
mass of a liquid’s particles and
the length of molecule chains.
Kinetic Theory: Basic Concepts
Topic
12
Surface tension
• The energy required to increase the surface
area of a liquid by a given amount is called
surface tension.
• Surface tension is a measure of the inward
pull by particles in the interior of the liquid.
Kinetic Theory: Basic Concepts
Topic
12
•
•
•
•
Capillary action
The movement of a liquid up a narrow glass
tube is called capillary action, or capillarity.
Capillary action occurs when adhesive
forces are greater than cohesive forces.
Adhesion is the force of attraction between
molecules that are different, such as water
molecules and the molecules of silicon
dioxide in glass.
Cohesion is the force of attraction between
identical molecules, such as water molecules.
Kinetic Theory: Basic Concepts
Topic
12
Solids
• Strong attractive forces between the particles
in a solid limit the movement of the particles
to vibrations around fixed locations.
• Thus, solids have a definite shape and volume.
• Because solids are so dense, ordinary
amounts of pressure will not compress them
into a smaller volume.
Kinetic Theory: Basic Concepts
Topic
12
Crystalline solids
• A solid whose atoms, ions, or molecules are
arranged in an orderly, geometric, threedimensional structure (lattice) is called a
crystalline solid.
Kinetic Theory: Basic Concepts
Topic
12
Crystalline solids
• The individual pieces of a crystalline solid
are called crystals.
• Crystalline solids are divided into five
categories based on the types of particles
they contain:
• atomic solids
• molecular solids
• covalent network solids
• ionic solids
• metallic solids
Kinetic Theory: Basic Concepts
Topic
12
Crystalline solids
• Noble gases are atomic solids whose
properties reflect the weak dispersion forces
between the atoms.
• Molecular solids are held together by
dispersion forces, dipole–dipole forces, or
hydrogen bonds.
• Elements that are able to form multiple
covalent bonds, such as carbon and silicon,
are able to form covalent network solids.
Kinetic Theory: Basic Concepts
Topic
12
Crystalline solids
• The type of ions and
the ratio of ions
determine the structure
of the lattice and the
shape of the crystal in
an ionic solid.
• Metallic solids consist of positive metal ions
surrounded by a sea of mobile electrons.
Kinetic Theory: Basic Concepts
Topic
12
Crystalline solids
• Not all solids are crystalline.
• The particles in an amorphous solid are not
arranged in a regular, repeating pattern and
do not form crystals.
Kinetic Theory: Basic Concepts
Topic
12
Crystalline solids
• Examples of amorphous solids
include glass, rubber, and many
plastics.
Kinetic Theory: Basic Concepts
Topic
12
Phase Changes
• Most substances can exist in three states—
solid, liquid, and gas—depending on the
temperature and pressure.
Kinetic Theory: Basic Concepts
Topic
12
Phase Changes
• States of substances are called phases when
they coexist as physically distinct parts of a
mixture, such as ice water.
• When energy is added to or taken away from
a system, one phase can change into another.
Kinetic Theory: Basic Concepts
Topic
12
Phase changes that require energy
• The amount of energy required to melt one
mole of a solid depends on the strength of
the forces keeping the particles together.
• The melting point of a crystalline solid is the
temperature at which
the forces holding the
crystal lattice together
are broken and the
solid becomes a liquid.
Kinetic Theory: Basic Concepts
Topic
12
Phase changes that require energy
• If a substance is usually a liquid at room
temperature (as water is), the gas phase is
called a vapor.
• Vaporization is the process by which a
liquid changes into a gas or vapor.
• When vaporization occurs only at the
surface of a liquid, the process is called
evaporation.
Kinetic Theory: Basic Concepts
Topic
12
Phase changes that require energy
• Vapor pressure is the pressure exerted by a
vapor over a liquid.
• As temperature increases,
water molecules gain
kinetic energy and vapor
pressure increases.
Kinetic Theory: Basic Concepts
Topic
12
Phase changes that require energy
• When the vapor pressure of a liquid equals
atmospheric pressure, the liquid has
reached its boiling point, which is 100°C
for water at sea level.
• At this point, molecules throughout the
liquid have the energy to enter the gas or
vapor phase.
Kinetic Theory: Basic Concepts
Topic
12
Phase changes that require energy
• The process by which a solid changes directly
into a gas without first becoming a liquid is
called sublimation.
• Solid air fresheners
and dry ice are
examples of solids
that sublime.
Kinetic Theory: Basic Concepts
Topic
12
Phase changes that require energy
• At very low temperatures, ice will sublime in
a short amount of time.
• This property of ice
is used to preserve
freeze-dried foods.
Kinetic Theory: Basic Concepts
Topic
12
Phase changes that release energy
• Some phase changes release energy into their
surroundings.
• For example, when a vapor
loses energy, it may change
into a liquid.
• Condensation is the
process by which a gas
or vapor becomes a
liquid. It is the reverse
of vaporization.
Kinetic Theory: Basic Concepts
Topic
12
Phase changes that release energy
• Water vapor undergoes condensation when
its molecules lose energy, their velocity
decreases, and hydrogen bonds begin to
form between them.
• When hydrogen bonds form, energy is
released.
Kinetic Theory: Basic Concepts
Topic
12
Phase changes that release energy
• When water is placed in a freezer, heat is
removed from the water.
• When enough energy has been removed, the
hydrogen bonds keep the molecules frozen
in set positions.
• The freezing point is the temperature at
which a liquid becomes a crystalline solid.
Kinetic Theory: Basic Concepts
Topic
12
Phase changes that release energy
• When a substance changes from a gas or
vapor directly into a solid without first
becoming a liquid, the process is called
deposition.
Kinetic Theory: Basic Concepts
Topic
12
Phase diagrams
• Temperature and pressure control the phase
of a substance.
• A phase diagram is a graph of pressure
versus temperature that shows in which phase
a substance exists under different conditions
of temperature and pressure.
• A phase diagram typically has three regions,
each representing a different phase and three
curves that separate each phase.
Kinetic Theory: Basic Concepts
Topic
12
Phase diagrams
• The points on the curves indicate conditions
under which two phases coexist.
• The phase diagram for each substance is
different because the normal boiling and
freezing points of substances are different.
Kinetic Theory: Basic Concepts
Topic
12
Phase diagrams
• The triple point is the point on a phase
diagram that represents the temperature and
pressure at which three phases of a substance
can coexist.
• All six phase changes can occur at the triple
point: freezing and melting, evaporation and
condensation, sublimation and deposition.
• The critical point indicates the critical
pressure and the critical temperature above
which a substance cannot exist as a liquid.
Basic Assessment Questions
Topic
12
Question 1
Classify each crystalline solid as molecular,
ionic, covalent network, or metallic.
A. NaCl
C. Fe
B. SiO2
D. H2O
Basic Assessment Questions
Topic
12
Answers
A. NaCl
ionic
B. SiO2
covalent network
C. Fe
metallic
D. H2O
molecular
Basic Assessment Questions
Topic
12
Question 2
Classify each of the following phase changes.
Basic Assessment Questions
Topic
12
Question 2a
dry ice (solid carbon dioxide) to carbon
dioxide gas
Answer 2a
sublimation
Basic Assessment Questions
Topic
12
Question 2b
ice to liquid water
Answer 2b
melting
Basic Assessment Questions
Topic
12
Question 2c
liquid bromine to bromine vapor
Answer 2c
vaporization
Basic Assessment Questions
Topic
12
Question 2d
moth balls giving off a pungent odor
Answer 2d
sublimation
Basic Assessment Questions
Topic
12
Question 2e
liquid water to ice
Answer 2e
freezing
Basic Assessment Questions
Topic
12
Question 2f
water vapor to liquid water
Answer 2f
condensation
Kinetic Theory: Additional Concepts
Topic
12
Additional Concepts
Kinetic Theory: Additional Concepts
Topic
12
Diffusion
• The motions of particles of a gas cause them
to spread out to fill the container uniformly.
• Diffusion is the process by which particles
of matter fill a space because of random
motion.
Kinetic Theory: Additional Concepts
Topic
12
Diffusion
• If you have seen dye such as food coloring
spreading through a liquid, you have
watched diffusion.
• Your sense of smell depends on diffusion and
air currents for you to detect molecules of a
gas that waft by your nose.
Kinetic Theory: Additional Concepts
Topic
12
Diffusion
• Diffusion is slow, but in your lungs,
oxygen reaches your blood rapidly enough
by diffusion.
• Oxygen diffuses across the walls of tiny blood
vessels called capillaries from the air sacs of
your lungs that fill with air each time you
inhale.
• The rate of diffusion of a gas depends upon
its kinetic energy, that is, on the mass and
speed of its molecules.
Kinetic Theory: Additional Concepts
Topic
12
Diffusion
• The rate of diffusion depends mostly on
the mass of the particles.
• Lighter particles diffuse more quickly than
heavier particles.
• Because lighter particles have the same
average kinetic energy as do heavier particles
at the same temperature, lighter particles must
have, on average, a greater velocity.
Kinetic Theory: Additional Concepts
Topic
12
Effusion
• Graham’s law of effusion states that the rate
of effusion for a gas is inversely proportional
to the square root of its molar mass.
Kinetic Theory: Additional Concepts
Topic
12
Effusion
• Using Graham’s law, you can also compare
the rates of diffusion for two gases.
Kinetic Theory: Additional Concepts
Topic
12
Finding the Ratio of Diffusion Rates
• The molar mass of helium is 4.00 g/mol;
the molar mass of air is 29.0 g/mol.
• What is the ratio of their diffusion rates?
Which gas diffuses faster?
Kinetic Theory: Additional Concepts
Topic
12
Finding the Ratio of Diffusion Rates
• The ratio of the diffusion rates is 2.69.
Helium diffuses about 2.7 times faster
than air does.
Kinetic Theory: Additional Concepts
Topic
12
•
•
•
•
•
Gas pressure
The pascal (Pa) is the SI unit of pressure.
One pascal is equal to a force of one newton
per square meter.
Some scientists use other units of pressure.
For example, engineers use pounds per
square inch.
Barometers and manometers measure
pressure in millimeters of mercury (mm Hg).
A unit called the torr is equal to 1 mm Hg.
Kinetic Theory: Additional Concepts
Topic
12
Gas pressure
• Air pressure is often reported in a unit called
an atmosphere (atm).
• One atmosphere is equal to 760 mm Hg,
760 torr, or 101.3 kilopascals (kPa).
• These are all defined units; therefore, they
have as many significant figures as needed
when used in calculations.
Kinetic Theory: Additional Concepts
Topic
12
Dalton’s law of partial pressures
• Dalton found that each gas in a mixture exerts
pressure independently of the other gases.
• Dalton’s law of partial pressures states
that the total pressure of a mixture of gases
is equal to the sum of the pressures of all the
gases in the mixture, as shown below.
Kinetic Theory: Additional Concepts
Topic
12
Dalton’s law of partial pressures
• The portion of the total pressure (Ptotal)
exerted by one of the gases is called its partial
pressure (Pn).
• The partial pressure of a gas depends on the
number of moles of the gas, the size of the
container, and the temperature of the mixture.
• The partial pressure of one mole of any gas is
the same at a given temperature and pressure.
Kinetic Theory: Additional Concepts
Topic
12
Finding the Partial Pressure of a Gas
• Air is made up of four main gases: N2, O2, Ar,
and CO2.
• Air pressure at sea level is approximately
760 mm Hg.
• Calculate the partial pressure of oxygen, given
the following partial pressures: N2, 594 mm
Hg; Ar, 7.10 mm Hg; and CO2, 0.27 mm Hg.
Kinetic Theory: Additional Concepts
Topic
12
Finding the Partial Pressure of a Gas
• Use Dalton’s law of partial pressures to
solve the problem.
Kinetic Theory: Additional Concepts
Topic
12
Finding the Partial Pressure of a Gas
• The partial pressure of oxygen is about
159 mm Hg.
Additional Assessment Questions
Topic
12
Question 1
Calculate the ratio of diffusion rates for neon
and helium. Which gas diffuses faster? About
how much faster?
Additional Assessment Questions
Topic
12
Answer
He; about 2.25 times faster
Additional Assessment Questions
Topic
12
Question 2
Calculate the ratio of diffusion rates for
ammonia (NH3) and carbon dioxide (CO2).
Which gas diffuses more rapidly?
Additional Assessment Questions
Topic
12
Answer
1.61; NH3
Additional Assessment Questions
Topic
12
Question 3
What is the partial pressure of oxygen gas
in a mixture of nitrogen gas and oxygen gas
with a total pressure of 0.48 atm if the
partial pressure of nitrogen gas is 0.24 atm?
Additional Assessment Questions
Topic
12
Answer
0.24 atm
Additional Assessment Questions
Topic
12
Question 4
Find the total pressure of a mixture that
contains three gases with the following partial
pressures: 6.6 kPa, 3.2 kPa, and 1.2 kPa.
Additional Assessment Questions
Topic
12
Answer
11.0 kPa
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