Transcript 3.2 – The Gas Laws - Mrs. Norton's New Scientists

3.2 – The Gas Laws
The Gas Laws
 When you inhale, the
volume of your chest cavity
increases, and air moves
into your lungs. When you
exhale, the volume of your
chest cavity decreases,
and air is pushed out of
your lungs. Changes in the
volume, the temperature,
the pressure, and the
number of particles have
predictable effects on the
behavior of a gas.
Pressure
What causes gas pressure in a closed
container?
 Pressure is the result of a force
distributed over an area.
Collisions between particles of a gas
and the walls of the container cause
the pressure in a closed container of
gas.
Pressure
 A moving hockey puck exerts pressure on
any object it hits. A layer of shatterproof
glass protects spectators.
 The faster the puck is traveling, the greater
the force of the puck on the glass. A greater
force means more pressure.
 The smaller the area of impact is, the greater
the pressure. If the edge of the puck hits the
glass, it exerts more pressure than if the face
of the puck hits the glass.
Pressure
 The SI unit of pressure is derived from SI
units for force and area.
 Force is measured in newtons (N).
 Area is measured in square meters (m2).
 The SI unit for pressure, the pascal (Pa), is
shorthand for newtons per square meter.
 Scientists often express larger amounts of
pressure in kilopascals. One kilopascal (kPa) is
equal to 1000 pascals.
Pressure
 The helium atoms in a balloon are
constantly moving. There are more than
1022 helium atoms in a small balloon.
 When many particles collide with the walls of
a container at the same time, they produce a
measurable pressure.
 The more frequent the collisions, the greater
the pressure is.
 The speed of the particles and their mass also
affect the pressure.
Factors That Affect Gas Pressure
What factors affect gas pressure?
Factors that affect the pressure of an
enclosed gas are its temperature, its
volume, and the number of its
particles.
Temperature
Raising the temperature of a gas
will increase its pressure if the
volume of the gas and the number
of particles are constant.
Temperature
 The firefighter is using
a pressure gauge to
check the air pressure
in a tire on a firetruck.
 If he checks the tire
pressure again after a
long drive on a
highway, he will find
that the pressure has
increase
Temperature
 The motion of tires on the highway heats the
tires and increases tire pressure.
 As the temperature rises, the average kinetic energy
of the particles in the air increases.
 With increased kinetic energy, the particles move
faster and collide more often with the inner walls of
the tires.
 Faster-moving particles hit the walls with greater
force.
 More collisions and increased force cause the
pressure of the air in the tires to rise.
Volume
Reducing the volume of a gas
increases its pressure if the
temperature of the gas and the
number of particles are
constant.
Volume
 Twist the cap onto a plastic bottle and
then squeeze it. What happens?
 The volume of the plastic bottle begins to
decrease.
 As the volume decreases, the particles of
trapped air collide more often with the walls
of the bottle.
 The pressure in the bottle increases.
Volume
 Movement of a muscle called the diaphragm
changes the volume of your chest cavity.
 The volume increases when you inhale. The pressure
decreases and air flows to your lungs.
 The volume decreases when you exhale. The pressure
increases and air flows from your lungs.
Exhaling
Inhaling
Diaphragm
contracts. Rib
cage is lifted
up and out.
Lungs
Rib Cage
Diaphragm
Diaphragm
relaxes. Rib
cage moves
down and in.
Number of Particles
 Increasing the number of particles will
increase the pressure of a gas if the
temperature and the volume are
constant.
 The more particles there are in the same
volume, the greater the number of
collisions and the greater the pressure.
Charles’s Law
French physicist Jacques Charles
collected data on the relationship
between the temperature and
volume of gases. The graph of the
data showed a direct relationship
between the volume of a gas and
the temperature of the gas.
Charles’s Law
 Charles extended
the graph beyond
the measured data
to find the
temperature that
would produce a
volume of 0 L.
 The temperature at
the point where the
line crossed the xaxis was –273.15°C.
Charles’s Law
This temperature is equal to
0 K on the Kelvin temperature
scale.
A temperature of 0 K is
called absolute zero.
Charles’s Law
Charles’s law states that the
volume of a gas is directly
proportional to its temperature
in kelvins if the pressure and the
number of particles of the gas
are constant.
Boyle’s Law
 Robert Boyle
described the
relationship between
the pressure and
volume of a gas. The
graph shows an
inverse relationship
between the volume
of a gas and the
pressure of the gas.
Boyle’s Law
 Boyle’s law states that the volume of a gas is
inversely proportional to its pressure if the
temperature and the number of particles are
constant.
 P1 and V1 represent the pressure and volume of a
gas before a change occurs. P2 and V2 represent
the pressure and volume of a gas after a change
occurs.
The Combined Gas Law
 The relationships described by Boyle’s law and
Charles’s law can be described by a single law.
The combined gas law describes the
relationship among the temperature, volume,
and pressure of a gas when the number of
particles is constant.
The Combined Gas Law
A cylinder that contains air at a
pressure of 100 kPa has a volume
of 0.75 L. The pressure is
increased to 300 kPa. The
temperature does not change.
Find the new volume of air.
The Combined Gas Law
 Read and Understand
 What information are you given?
 P1 = 100 kPa
 P2 = 300 kPa
 V1 = 0.75 L
The Combined Gas Law
 Plan and Solve
 What unknown are you trying to calculate?
 What expression can you use?
The Combined Gas Law
 Plan and Solve
 Cancel out the variable that does not change and
rearrange the expression to solve for V2.
 Replace each variable with its known value.
The Combined Gas Law
 Look Back and Check
 Is your answer reasonable?
 Volume should decrease as pressure increases. The
pressure tripled from 100 kPa to 300 kPa. The answer,
0.25 L, is one third the original volume, 0.75 L.
The Combined Gas Law
1. A gas has a volume of 5.0 L at a pressure of 50
kPa. What happens to the volume when the pressure
is increased to 125 kPa? The temperature does not
change.
The Combined Gas Law
2. Gas stored in a tank at 273 K has a pressure of 388
kPa. The safe limit for the pressure is 825 kPa. At what
temperature will the gas reach this pressure?
The Combined Gas Law
3. At 10ºC, the gas in a cylinder has a volume of 0.250
L. The gas is allowed to expand to 0.285 L. What must
the final temperature be for the pressure to remain
constant? (Hint: Convert from degrees Celsius to
kelvins using the expression ºC + 273 = K.)