14.1 Properties of Gases
The Nature of Gases
• Gases have some interesting
characteristics that have fascinated
scientists for 300 years.
• The first gas to be studied was air & it was
a long time before it was discovered that
air was actually a mixture of particles
rather than a single gas.
The Nature of Gases
• But this realization did not make the study of
gas behavior more difficult.
• Although air is a mixture of several different
gases, it behaves much the same as any single
gas.
• Regardless of their chemical identity, gases
tend to exhibit similar physical behaviors
Variables that describe a Gas
• The four variables and their common
units:
1. pressure (P) in kilopascals
2. volume (V) in Liters
3. temperature (T) in Kelvin
4. amount (n) in moles
• The amount of gas, volume, and
temperature are factors that affect
gas pressure.
1. Amount of Gas
• When we inflate a balloon, we are
adding gas molecules.
• Increasing the number of gas
particles increases the number of
collisions
–thus, the pressure increases
• If temperature is constant, then
doubling the number of particles
doubles the pressure
Pressure and the number of
molecules are directly related
• More molecules means more
collisions, and…
• Fewer molecules means fewer
collisions.
• Gases naturally move from areas of
high pressure to low pressure,
because there is empty space to
move into – a spray can is example.
Common use?
• A practical application is Aerosol
(spray) cans
–gas moves from higher pressure to
lower pressure
–a propellant forces the product out
–whipped cream, hair spray, paint
• Fig. 14.5, page 416
• Is the can really ever “empty”?
2. Volume of Gas
• In a smaller container, the
molecules have less room to
move.
• The particles hit the sides of the
container more often.
• As volume decreases, pressure
increases. (think of a syringe)
–Thus, volume and pressure are
inversely related to each other
3. Temperature of Gas
• Raising the temperature of a gas increases
the pressure, if the volume is held constant.
(Temp. and Pres. are directly related)
– The molecules hit the walls harder, and
more frequently!
• Should you throw an aerosol can into a
fire? What could happen?
• When should your automobile tire pressure
be checked?
Kinetic Molecular Theory
• There is a theory that modern day chemist’s
use to explain the behaviors and
characteristics of gases - the Kinetic Molecular
Theory of Matter.
• The word kinetic refers to motion.
• The word molecular refers to molecules
Kinetic Theory and a Model for Gases
• Kinetic Theory and a Model for Gases
What are the three assumptions of
the kinetic theory as it applies to
gases?
Kinetic Theory and a Model for Gases
•The kinetic theory as it applies to gases
includes the following fundamental
assumptions about gases.
The particles in a gas are considered to be
small, hard spheres with an insignificant
volume.
– Within a gas, the particles are relatively far apart
compared with the distance between particles in a
liquid or solid.
– Between the particles, there is empty space.
– No attractive or repulsive forces exist between the
particles.
Kinetic Theory and a Model for Gases
•The kinetic theory as it applies to gases
includes the following fundamental
assumptions about gases.
The motion of particles
in a gas is rapid,
constant, and random.
– Gases fill their containers
regardless of the shape and
volume of the containers.
– An uncontained gas can spread
out into space without limit.
Bromine
molecule
Kinetic Theory and a Model for Gases
•The kinetic theory as it applies to gases
includes the following fundamental
assumptions about gases.
All collisions between particles in a gas are
perfectly elastic.
– During an elastic collision, kinetic energy is
transferred without loss from one particle to another.
– The total kinetic energy remains constant.
• The theory states that the tiny particles in all
forms of matter in all forms of matter are in
constant motion.
• This theory is used to explain the behaviors
common among gases
• There are 3 basic assumptions of the KMT as it
applies to gases.
The Nature of Gases
• Gas particles can be monatomic (Ne), diatomic
(N2), or polyatomic (CH4) – but they all have
these characteristics in common:
Gases Have Mass
• Gases seem to be weightless, but they are
classified as matter, which means they have
mass.
• The density of a gas – the mass per unit of
volume – is much less than the density of a
liquid or solid, however.
Gases Are Compressible
• If you squeeze a gas, its
volume can be reduced
considerably
• A gases low density
allows for there to a lot
of empty space
between gas
molecules.
Gases Fill Their Containers
• Gases expand until they take up as much room
as they possibly can.
• Gases spread out to fill containers until the
concentration of gases is uniform throughout
the entire space.
• This is why that nowhere around you is there
an absence of air.
Gases Diffuse
• Gases can move through each other rapidly.
• The movement of one substance through
another is called diffusion.
• Because of all of the empty space between
gas molecules, another gas molecule can pass
between them until each gas is spread out
over the entire container.
Gases Exert Pressure
• Gas particles exert
pressure by colliding
with objects in their
path.
• The sum of all of the
collisions makes up the
pressure the gas exerts.
Pressure is Dependent on Temperature
• The higher the temperature of a gas -the
higher the pressure that the gas exerts
• The reverse of that is true as well, a the
temperature of a gas decreases – the pressure
decreases.
• Think about the pressure of a set of tires on a
car
Summary
• You should know that:
• There are several behaviors common to all
gases despite chemical makeup
• Six properties of gases
• The three assumptions of the KMT regarding
gases.