It’s a Gas
Air is a gas
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It is a mixture of several gases.
It surrounds you all of the time.
It inflates tires
It provides cushioning in an air mattress
It transmits sound waves so you can hear
So, how do you know it exists???
Gases
What gases are important for each of the
following: O2, CO2 and/or He?
A.
B.
C.
D.
Gases
•
What gases are important for each of the
following: O2, CO2 and/or He?
• A. CO2
B. O2/CO2
C. O2
D. He
The Nature of Gases
The systematic study of gases began 300
years ago. What did they learn?
What did my high
school chemistry
teacher say?
1. Gases have mass
• Proof: Weigh a basketball deflated and
inflated. The mass increases. The increase
is due to air molecules.
+
Mass: 2567 g
What was the mass of gas added?
Mass: 2571 g
2. It is easy to compress a gas
• Why can you put more air in a tire,
but can’t add more water to a glass full
of water?
• If you squeeze a gas, its volume can be
reduced considerably.
• This is why gases are used as shock
absorbers and in air bags.
3. Gases completely fill
their containers
• This property explains why nowhere
around you is there an absence of air.
• Air in a balloon is distributed evenly
throughout the balloon not just on the
bottom
4. Different gases can move
through each other quite rapidly
• The movement of one substance through
another is called DIFFUSION.
• Therefore, gases diffuse easily through each
other.
• You observe diffusion when smell popcorn
at the theater or when a skunk is nearby.
5. Gases exert pressure
• You have experienced the effects of
changing air pressure when your ears “pop”
• You have observed air pressure when you
inflate a balloon.
Pressure = Force
Area
Units of Pressure
1 atm = 760 mm Hg
1 atm = 760 torr
1 atm = 101.325 kPa
Barometer
10 miles
4 miles
Sea level
0.2 atm
0.5 atm
1 atm
6. The pressure of a gas
depends on its temperature.
• The higher the temperature of a gas, the
higher the pressure.
• Your tire pressure can become dangerously
high on hot southwest summer days.
Summary of Gas Properties
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Gases have mass.
It is easy to compress a gas.
Gases completely fill their containers.
Different gases can move rapidly through each
other.
• Gases exert pressure.
• The pressure of a gas depends on temperature.
Kinetic-Molecular Theory
• All of the gas properties covered are
explained by the kinetic-molecular theory.
• Kinetic means motion
• Molecular means molecules
• Therefore, Kinetic-Molecular means motion
of molecules.
K-M Theory
• Gases consist of discrete molecules that
have mass.
• Every molecules is independent of other gas
molecules.
K-M Theory
• Individual molecules are small and far apart
compared to their size.
• This assumption explains why gases can be
so easily compressed.
K-M Theory
• Gas molecules are in
continuous, random,
straight line motion
with varying
velocities.
• This explains why
gases immediately
fill their containers.
K-M Theory
• Gases exert pressure
because their
particles frequently
collide with the wall
of the container in
which they are held.
K-M Theory
• Collisions are elastic.
• Collisions occur without any loss of energy
(speed)
K-M Theory
• Gas molecules exert no attraction
• or repulsion force on one another.
Repulsion
K-M Theory:
Temperature
Temperature is a
measure of the amount of the average kinetic energy
of the particles in matter. The more kinetic energy
the particles have, the higher the temperature. The
temperature of particles are typically recorded in
one of three ways:
1. Fahrenheit (ºF)
2. Celsius (ºC)
3. Kelvin (K)
Do you remember which
is the standard unit????
KMT: Billiard analogy
•In this analogy each billiard ball represent different
gas molecules moving in random motion. Pressure
result from collision of each between ball and the
boundary. Each collision is perfectly elastic with
each ball exhibiting no attractive or repulsive force
between each other.
•Motion (energy) → Temperature (K)
•Collision (impact) → Pressure (atm)
•Boundary (container size) → Volume (L)
Summary of KMT Postulates
• Gas particles are in constant random motion.
• Gas particles occupy no volume.
• Collisions between gas particles are perfectly
elastic: there is no loss of kinetic energy.
• There are neither attractive nor repulsive forces
between gas particles.
• The higher the absolute temperature, the higher
the average kinetic energy of the gas.
Boyle’s Law
• At constant temperature, the volume of a
gas varies inversely with its pressure.
• In other words, as the pressure increases,
the volume of the gas decreases.
• As the pressure decreases, the volume of
the gas increases.
• UP → DOWN and DOWN → UP
The Math of Boyle’s Law
• P1V1 = P2V2
• 10 liters of air at 1 atm is compressed to a
pressure of 4 atm. What is the volume of the
compressed air.
• (1 atm)(10 L) = (4 atm)V2
• (1 atm)(10 L) = (4 atm)V2
(4 atm)
(4 atm)
The Math of Boyle’s Law (continued)
• P1V1 = P2V2
• (1 atm)(10 L) = (4 atm)V2
(4 atm)
(4 atm)
• V2 = (1 atm)(10 l)
(4 atm)
• V2 = 2.5 L
Kinetic Molecular Theory and
Boyle’s Law
• In a smaller volume, the number of collisions
between the particles and the walls of the
container is concentrated on a smaller area
(think high heels), so the pressure is greater.
• In a larger volume, the number of collisions
between the particles and the walls of the
container is spread out over a larger area (think
snowshoes), so the pressure is less.
Avogadro’s Hypothesis
• At constant temperature and pressure,
equal volumes of gases contain equal
numbers of particles.
• Or restated in mole-speak, at constant
temperature and pressure, equal
volumes of gases contain equal
numbers of moles.
The Math of Avogadro’s Hypothesis
• V1 =
n1
V2
n2
Where V is Volume and n
is moles.
• One mole of ozone gas (O3) occupies 22.4 L.
The ozone decomposes to 1.5 moles of
molecular oxygen (O2). What is the volume
of the resulting molecular oxygen?
• 22.4 L =
V2___
1 mol
1.5 mol
The Math of Avogadro’s Hypothesis
(continued)
• 22.4 L =
V2___
1 mol
1.5 mol
• 22.4 L ( 1.5 mol ) =
1.0 mol
• V2 = 33.6 L
V2
Kinetic Molecular Theory and
Avogadro’s Hypothesis
• As the number of gas particles increases, the
frequency of collisions with the walls of the
container must increase. This, in turn, leads to
an increase in the pressure of the gas. Flexible
containers, such as a balloon, will expand until
the pressure of the gas inside the balloon once
again balances the pressure of the gas outside.
Thus, the volume of the gas is proportional to
the number of gas particles.
Summary
• Boyle’s Law: P1VI = P2V2
• Avogadro’s Hypothesis:
V1 = V2
n1
n2
• Put the right numbers in the right
places.
• Isolate the unknown variable and
solve.