The Nature of Gases
Gases:
Have mass
Can be compressed
Fill their containers
Diffuse
Exert pressure
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 have mass.
It’s this very low density that allows us
to be able to walk through the room
without concerning ourselves with
air resistance.
Since it is so easy to “swim” across the
room we don’t put much thought
into the mass of a gas.
Gases can be compressed
A gases low density
means there to is a lot
of empty space
between gas
molecules
If you squeeze a gas
(increase pressure), its
volume can be
reduced considerably
Engine
We can use this ability of a gas to do
work for us.
Think of a shocks on a car. You really
are riding on a pillow of air.
A bump in the road compresses
the gas in the shocks until the
bump’s energy is absorbed.
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
(the same) throughout the entire
space.
This is why air is all around you.
Gas particles are in constant random
motion.
Gas particles move in a straight line until
they collide with other particles or the
sides of the container, which causes
them to change directions until they
collide with something else.
Bouncing off everything around them
spreads the particles out until they are
uniform throughout the entire
container.
Motion states of matter
Gases diffuse
If I opened up a bag of popcorn in front of
the class you would soon be able to
smell it in the back.
The popcorn smell is a high energy
molecule or group of molecules that is
in the gas state.
Diffusion
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 diffuse
Gas molecules are in constant random
motion they will mix with other gases
uniformly (evenly).
Some gases diffuse faster then other gases
based on their size and their energy.
Diffusion explains why we can all
breath oxygen anywhere in the
room.
It also helps us minimize potential
odoriferous problems.
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.
Imagine a gas in a container as a room of
hard rubber balls.
The collisions of the balls bouncing
around exert a force on the object that
with which they collide.
The definition of a pressure is a force per
unit area – so the total of all of the tiny
collisions makes up the pressure
exerted by the gas.
The gases push against the walls of their
containers with a force.
The pressure of gases is what keeps our
tires inflated, makes our basketballs
bounce, makes hairspray come out of
the can, etc.
Kinetic Molecular Theory
A theory used to explain the behaviors
and characteristics of gases.
KMT Assumptions
1) A gas is made up of many small particles that
move constantly and randomly in straight lines.
2) The molecules in a gas occupy no volume.
3) When gas molecules collide, they don’t lose
energy due to friction or gain energy either.
4) Gas molecules are not attracted to each other
at all.
5) The kinetic energy of gas molecules depends
only on the temperature of the gas.
KMT Assumptions
Ideal gases would be exactly like the description on the
previous slide. It is useful to use them as a model.
However, they do not actually exist.
Real gases :
1) really are small, constantly moving particles
2) but, the molecules do have some volume
3) and, they do lose energy due to friction in collisions
4) and, they are slightly attracted to each other
5) their energy is really only dependent on
temperature
If a Real gas is at a high temperature and low
pressure, it behaves very much like an Ideal gas.
At high temperatures, the
molecules have a lot of energy –
hard to notice really small
losses and they can escape any
Ideal Gas attraction to another molecule.
At low pressures, the molecules Real Gas
are not forced close to each other
– so volume doesn’t matter and
they are not close enough to be
attracted very much to each
other.
Gas Variables
The amount (moles not volume) of a
gas.
The volume (in liters).
The pressure
The temperature
Amount (n)
The quantity of gas in a given sample is
expressed as moles of gas.
1 mole = molar mass = 6.02 x 1023
molecules
Volume
The volume of the gas is simply the
volume of the container it is contained in.
The metric unit of volume is the liter (L)
Pressure
The pressure of a gas is the force exerted
on the wall of the container a gas is
trapped in.
There are several units for pressure
depending on the instrument used to
measure it including:
Atmospheres, kiloPascals, and millimeters
of Mercury
Pressure Units
Atmospheres – atm
kiloPascals – kPa
millimeters of Mercury – mm Hg
1 atm = 101.3 kPa = 760 mm Hg
Temperature
The temperature of a gas is generally
measured with a thermometer in Celsius.
All calculations involving gases should be
made after converting the Celsius to
Kelvin temperature.
Kelvin = C° + 273
Gas Laws
PV PV
nT nT
The four gas variables are related
through this equation.
Boyle’s Law
Pressure and Volume
Robert Boyle determined the
relationship between pressure and
volume of a gas.
Moles of gas and temperature of the
system were kept constant
What happens to the air in a bicycle
pump if you push down?
Boyles Law
As the pressure
increases
Volume
decreases
Boyles Law
Boyle’s Law
PV PV
nT nT
Moles and temperature were kept
constant during the experiment.
How do Pressure and Volume of gases
relate graphically?
Inverse
relationship
PV = k
Volume
Pressure
Guy Lussac’s Law
Pressure and Temperature
Guy Lussac determined the relationship
between temperature and pressure of a gas.
Moles and volume were kept constant during
the experiments.
What happens if you heat up the gas in a
closed container?
Today’s Today’s
temp: 35°F temp: 85°F
Pressure
Gauge
Guy Lussac’s Law
Pressure and Temperature
PV PV
nT nT
Moles (n) and volume are constant
The pressure increases when
temperature increases because the
molecules are moving with greater
speed and colliding against the sides
of their containers more often.
Therefore, the pressure inside that
container is greater, because there
are more collisions.
How do Pressure and Temperature of
gases relate graphically?
P/T = k
Pressure
Temp
Charles’s Law
Volume and Temperature
Jacques Charles determined the
relationship between temperature and
volume of a gas.
During his experiments pressure of the
system and amount of gas were held
constant.
If you have a balloon that is beginning
to deflate, what would happen if you
put it in a hot car?
Charles Law
Volume of balloon Volume of
at room balloon at 5°C
temperature
Charles Law
Volume and Temperature
PV PV
nT nT
Moles (n) and pressure are constant
How do Temperature and Volume of
gases relate graphically?
Direct
relationship
V/T = k
Volume
Temp