Gases
The atomic equivalent to a
room full of preschoolers
and a birthday cake!!
(Get it? The preschoolers are full of energy and running
around lots because they are excited!!)
Properties of Gases
1. Gases have NO shape;
they fill the container
they are in.
2. Gases possess Kinetic energy – the
energy of motion. They are always
moving unless at absolute zero
(-273°C)
KE = ½ mv2
…more properties of GASES
3. Gases are
independent
molecules (unlike
solids and liquids
which are VERY
dependent on each
other).
4. Gases exhibit
Brownian
Motion –
movement due
to external
pressure.
Video of Brownian Motion
5. Gases transfer energy in elastic
collisions
• An elastic collision is where energy is
transferred from one object to another)
• Newton’s cradle or playing pool is a great
example of an elastic collision.
• DIFFUSION: the movement of molecules from areas of
high to low concentration until they are equally
distributed.
• Eating at Chipotle or Taco Bell helps fuel this property
7. Gases exert PRESSURE due to
collisions.
• Collisions are why a balloon remains inflated.
The gas molecules are colliding with the sides
of the balloon exerting a force on it and
keeping it that size.
• Draw a picture of a
balloon with all of the
gas molecules inside
creating pressure
BAROMETER
• A barometer is a device used to measure
barometric pressure
Important Pressure Constants
• These are called standard pressure (universally
used)
• 1 atmosphere (atm) = 760 mm Hg = 760 Torrs
(named after Galileo’s grad student who studied air pressure – Evangelista Torricelli) =
101, 325 Pascals = 29.92 inches = 14.7 psi
(pounds per square inch)
• You must KNOW these in order to convert
from one pressure unit to another.
8. Gases are Compressible
• Gases are affected by changes in pressure
• In other words, gases are the only state of
matter that can be squished into a smaller
space
• Greater pressure = smaller volume
• Smaller pressure = bigger volume
• This concept is how hydraulics and lots of toys
work because of the relationship between
pressure and volume.
9. Gases are affected by changes in
Temperature
• A higher temperature means more kinetic
energy and thus greater movement
• A lower temperature means less kinetic
energy and thus less (or smaller amounts of )
movement
• Kelvin = K = °C + 273
Temperature and Motion
Boiling point
100°C
Decreasing KE,
Freezing point
0°C
Absolute zero -273°C = 0 K
decreasing motion
NO molecular
motion
10. Gases have a MOLAR VOLUME
• 1 mole of any gas at STP
(standard temperature and
pressure) is equal to 22.4
Liters
• This is a constant in chemistry
like Avogadro’s number
STP – Standard Temperature and
Pressure
• Standard Temperature = 0°C or 273 Kelvin
• Standard Pressure = 1 atm = 760 mm = 760
Torrs = 29.92 in = 101,325 Pascals = 14.7 psi
• When the Temperature of anything is
measured, you are really just measuring the
average KINETIC ENERGY of the particles in
that substance.
Convert the following pressures:
Ie. A pressure of a gas from the mouths of
obnoxious juniors is 84 Torrs. Convert to
atmospheres and Pascals.
Some tires require a pressure of 35
psi. Convert this pressure to atm and
mm.
1. Boyle’s Law
• Boyle’s Law deals with pressures and
volumes
• Robert Boyle noticed that the pressure of a
gas is inversely proportional to its volume.
• In other words, as the volume of a gas goes
DOWN, the pressure of the gas will go UP.
• Or…as the volume goes up, the pressure will
go down.
What did the fish say when he ran into
a concrete wall?
Dam!
Boyle’s Law:
• Where P1 and V1 are the original and P2 and V2
are the NEW!
• An inverse relationship or proportion is where
one variable goes up (or down) and the other
one does the opposite. In other words, with
Boyle’s Law, as the volume gets smaller, the
pressure gets Bigger!
The volume of a stinky gas coming from the locker of a XHS girl
was found to have a volume of 30 mls and a pressure of 100
Torrs. If the pressure is suddenly changed to standard pressure,
find the new volume.
Why are 1980 pennies worth almost
$20?
1,980 pennies
equals $19.80,
which is almost
$20.
2. Charles’s Law
• Deals with temperatures and volumes
• Jacques Charles found the volume of a gas
varies directly with its Kelvin temperature.
• A direct proportion is where one variable
goes up causing the other one to go up OR
one variable goes down causing the second
one to go down.
Charles’ Law:
• Where T1 and V1 are original values
and T2 and V2 are new values
• DON’T FORGET: all gas law problems
MUST be solved in Kelvin
Kelvin =°C + 273
20 L of a gas is at a temperature of 20°C. If a chicken
patty sandwich from the XHS café is brought near the
gas, its temperature raises to 27°C. Find the new
volume.
3. Combined Gas Law
• Combines Boyle’s Law, Charles’ Law and GayLussac’s Law by including volumes, pressures
and temperatures.
A gas coming from the boy’s locker room at standard temperature
occupies 112 L at 2 atm of pressure. Find the volume of the gas if the
temperature is increased to 123°C and the pressure increased to 4
atm.
4. Ideal Gas Law
• The mother of all gas laws!
pV = nRT
• R = Universal gas constant = 0.0821 L-atm
/mole-K
• V = volume in Liters
• T = temperature in Kelvin
• n = number of moles (g/molar mass = moles)
• P = pressure in atmospheres (atm)
How many legs does this elephant
have?
16 g of oxygen is placed in a 10 Liter flask at 77°C.
What is the pressure inside the flask of this gas?
Convert the above pressure to Torrs.
Small grey dots should appear at the intersections of the
squares, but if you look directly at the intersection, the grey dots
will disappear.
Methane gas is pumped from Taco Bell into a 2.5 L flask at 27°C.
The barometer reads (can barometers really read?) 190 mm. How many
grams of CH4 are present?
Ninety (90) grams of magnesium chlorate is heated at 200°C
under 1.2 atm of pressure. How many liters of gas are
produced?
Derivations of the Ideal Gas Law
5. Derivation #1
6. Derivation #2
A 0.495 g sample of chloroform is collected to a volume of
127 mL at a temperature of 98°C and a pressure of 754 mm.
Calculate the formula weight of chloroform.
UF6 is the most dense and most dangerous gas know to man
What is the density of this gas at 100°C
and 1 atm?
(besides the gas coming from the Taco Bell customer!)
Two grams of ammonium nitrate explodes. What is the
volume of N2 gas collected at 527K and 745 Torrs?
2NH4NO3 → 4H2O + 2N2 + O2
7. Graham’s Law of Diffusion
• DIFFUSION: the movement of a gas from an
area of high concentration to an area of low
concentration until the gas molecules are
evenly distributed
• Under uniform conditions, the rates at which
different gases diffuse are inversely
proportional to the square root of their
formula weights (or molar masses)
• EFFUSION: the diffusion of a gas through a
small hole (medical term).
• Remember: when comparing two gases, the
lighter gas will always travel or diffuse faster
then the heavier gas.
A room is filled with 2 gases: He and
HBr.
• Which gas will travel or diffuse faster?
• How many times faster?