Gas Laws
Molecules In The Air
How do we know there are molecules
in the air?
 Has anyone ever had wind burn?
 How about smelled a hamburger when
you drove by McDonalds?
 These examples demonstrate that air
is made up of constantly moving
molecules.

Kinetic Molecular Theory

Kinetic energy is perfectly maintained in
“elastic” molecular collisions.

Molecules are in constant Motion.
– Vibrational motion.
– Translational motion.

Tiny particles called “molecules” make up all
matter.
Kinetic Molecular Theory

Describes only “Ideal Gases”
– Ideal gases follow the gas laws in all
conditions of pressure and temperature.
Ideal gases assume particles of gas
have no volume or intermolecular
attraction.
 Ideal gases have perfectly elastic
molecular collisions.

In REALITY!



Real gases have
volume.
Real gases have some
intermolecular
attraction.
Real gases can be
liquefied and solidified
by cooling and applying
pressure.

In reality,
experimentation shows
us that real gases do
not follow “Ideal
Behavior”
Gases Are All Around Us!
Many gases are invisible, but some we
can see.
 Iodine Vapor is Pink.
 Chlorine Gas is Yellow-Green.
 Smoke, clouds, and fog are NOT
gases!!

Gases Are All Around Us!




Some gases we can
smell.
Hydrogen Sulfide
smells like rotten eggs.
In natural gas, a smell
has been added for
our protection.
Other gases we can’t
smell at all.
Gases Are All Around Us!

Gases have mass.

Balloons that are
filled with air
weigh more than
those that are unfilled.

When air moves it
can do work. For
example tornadoes
or windmills.
Gases Are All Around Us!

Gases occupy
space, or have
volume.

Examples:
– Inflated Balloon
– Our Lungs
– Scuba Tanks
Gas Behaviors

Compressibility
– The volume of a gas can be decreased by
increasing the pressure.

Permeability
– The mixing of molecules in a container.

Diffusion
– The ability to spread from a high concentration
to a lower concentration.

Expansibility
– The ability of a gas to expand and fill a
container of any size.
Under Pressure
What causes pressure?
 Pressure is caused by the collision of
gas molecules.
 The molecules can collide with each
other or with the walls of their
container.

Atmospheric Pressure

Air exerts pressure on earth.
– Gravity holds air molecules in the earth’s
atmosphere.
Atmospheric Pressure occurs when
those air molecules collide with each
other or other objects.
 Atmospheric pressure decreases as
you move to a higher elevation.

Gas Laws


There are several
different gas law
equations.
Variables:
–
–
–
–
–
–
–
V
P
T
n
R
d

Variables





V= volume in liters.
T= temp in Kelvins.
P= pressure in atm,
mmHg, or kPa.
n= number of moles
of gas.
R= the universal
gas constant.
d= gas density in
g/L.
 = molar mass of a
gas in g/mol.

Units of Pressure

Pressure is defined as a force over a
specific area.
– P=F/A

English system:
– lbs/in2 = PSI

Metric system:
– N/M2=Pascal
Boyle’s Law
Robert Boyle
(1627-1691)
 At a constant
temperature,
volume and
pressure of a gas
are inversely
proportional.

Boyle’s Law
As pressure increases, volume
decreases, and vice versa.
 V1P1=V2P2
 V œ 1/P


Remember the volume of a gas can
NOT be squeezed down to zero!
Charles’s Law
Jacques Charles
(1746-1823)
 At constant
pressure, the
volume of a gas and
temperature are
directly
proportional.

Charles’s Law
As temperature increases, volume
increase, and vice versa.
 V1/T1=V2/T2
VœT
 Gases expand as they get warm , and
contract as they cool.

Gay-Lussac’s Law
Joseph Gay-Lussac
(1778-1850)
 At constant
volume, gas
temperature and
pressure are
directly
proportional.

Gay-Lussac’s Law
As temperature increases, pressure
increases, and vice versa.
 P1/T1=P2/T2
TœP
 This is why aerosol cans say “Do not
incinerate!”

Combined Gas
To simplify life, Boyle’s Law, Charles’s
Law and Gay-Lussac’s Law have been
put together into one mathematical
expression.
 Combined Gas Law
V1P1 = V2 P2
T1
T2
