GASES
Behavior of Gases
 Gases behave much differently than liquids
and solids and thus, have different laws.
 Because gas molecules have no forces
keeping them together, they fill whatever
container in which they are kept.
 This distance between the molecules also
means they can be compressed.
Kinetic Molecular Theory of
Gases
 Model explaining gas behavior.
1) A gas consists of small particles (atoms or
molecules) that move randomly with high
velocities.
2) The attractive forces between gases can be
ignored.
Kinetic Molecular Theory of
Gases
3) The actual volume that the gas molecules occupy
is usually much smaller than the volume of the
container.
- Therefore, gases can be easily compressed
because of all the empty space.
4) The average kinetic energy (temperature) of gas
molecules is proportional to the Kelvin
temperature.
Kinetic Molecular Theory of
Gases
5) Gas particles are in constant motion,
moving rapidly in straight paths until collide
with another gas molecule and change
direction.
- When they hit each other or the walls of
the container, they create gas pressure.
- As collisions increase, pressure increases.
Properties of Gases
 The following properties are used to describe
gases:
 Pressure
 Volume
 Temperature
 Amount of gas
Pressure of a Gas
 Pressure of a gas occurs when gas molecules
strike each other as well as the sides of a
container.
As collisions increase between molecules,
pressure increases.
The pressure of the air molecules around us is
called atmospheric pressure.
Atmospheric pressure –
decreases with altitude
because there are fewer air
molecules present
Common Units of Pressure
 Atmosphere (atm)
 Millimeters of Mercury (mmHg)
 Torr (torr)
 Pounds per square inch (lb/in2 )
 Pascal (Pa)
Barometric Pressure
-As air pressure increases
(downward) the barometer
reading (Hg) rises upward.
Volume of a Gas
 The volume of a gas equals the size of the
container in which the gas is placed.
 Volume of gases are generally measured in
liters (L) or cubic decimeters (dm3 ) which
equal the same amount.
 Also, 1mL = 1 cm3
Temperature of a Gas
 Temperature reflects the average kinetic
energy due to the movement of a gas.
 Measured in Kelvin (K)
Amount of a Gas
 Uses moles to describe the relative quantity
of molecules of a gas that are present.
What happens to the pressure
of a gas when volume
decreases?
1. Pressure decreases
2. Pressure increases
3. No change
4. Unable to tell
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What is the relationship
between pressure and volume?
1. Direct
2. Inverse
3. Exponential
4. No relationship
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Boyle’s Law
 Boyle’s Law – If temperature is held
constant, gas pressure is inversely
proportional to the volume of the gas.
Boyle’s Law: P1 V1 = P2 V2
What happens to the volume
of a gas when temperature
increases?
1. Volume decreases
2. Volume increases
3. No change
4. Unable to tell
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What is the relationship
between temperature and
volume?
1. Direct
2. Inverse
3. Exponential
4. No relationship
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Charles’ Law
 As temperature increases, average kinetic
energy of gas molecules increases.
 As the molecules move faster, volume
increases.
 Charles’ Law – If pressure is held constant,
there is a direct relationship between volume
and temperature.
Charles’ Law
V1
T1
=
V2
T2
Note: Temperature is measure in Kelvin
Kelvin = Celsius + 273
What happens to the pressure
of a gas when temperature
increases?
1. Pressure decreases
2. Pressure increases
3. No change
4. Unable to tell
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Pr
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Pr
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What is the relationship
between temperature and
pressure?
1. Direct
2. Inverse
3. Exponential
4. No relationship
0%
0%
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No
Ex
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Gay – Lussac’s Law
 As temperature increases, collisions increase
and pressure increases as well.
 Gay – Lussac’s Law – If volume is held
constant, there is a direct relationship
between pressure and temperature.
Gay – Lussac’s Law
P1
T1
=
P2
T2
Gay-Lussac’s Law is true if number of moles of
the gas as well as the volume is held constant.
Note: Temperature is measure in Kelvin
Kelvin = Celsius + 273
Combined Gas Law
P 1 V1
T1
=
P 2 V2
T2
The combined gas law applies as long as the
number of moles of a gas are held constant.
Note: Temperature is measure in Kelvin
Kelvin = Celsius + 273
What happens to the volume
of a gas when the number of
atoms increases?
1. Volume decreases
2. Volume increases
3. No change
4. Unable to tell
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What is the relationship
between volume and moles of
a gas?
1. Direct
2. Inverse
3. Exponential
4. No relationship
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No
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Avogadro’s Law
V1
n1
=
V2
n2
Avogadro’s Law is true if pressure and
temperature are held constant.
Note: n = moles of the gas
Avogadro’s Law
V1
n1
=
V2
n2
The combined gas law applies as long as the
number of moles of a gas are held constant.
Note: Temperature is measure in Kelvin
Kelvin = Celsius + 273
True or False: Increasing
the volume of a gas
increases the pressure.
1. True
2. False
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True or False: Increasing
the pressure of a gas
increases the temperature.
1. True
2. False
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True or False: 1 mole of
iron metal has a volume of
22.4L at STP.
1. True
2. False
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True or False: Decreasing
the temperature of a gas
increases the pressure.
1. True
2. False
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True or False: Decreasing
the temperature of a gas
increases the volume.
1. True
2. False
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True or False: Increasing
the moles of a gas increases
the pressure.
1. True
2. False
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STP
 STP – Standard Temperature and Pressure
 STP: T = 273K (or 0o C)
P = 1 atm or 760 mmHg
Molar Volume
***** At STP, according to Avogadro, any gas
has a molar volume of 22.4L.
Conversion factor: 1 mole (of any gas) = 22.4L
Ideal Gas Law – used for
conditions not at STP
 PV = nRT
 R = Ideal Gas Constant
 Rearranged, R = PV/nT
 At STP for one mole, calculate R values for
units of atm and mmHg.
Ideal Gas Law
 At 2 atm pressure, what is the volume of 10
moles of hydrogen gas at 200K?
 What pressure would 4.04g of hydrogen gas
(H2) create at 100oC?
Law of Partial Pressures
 The Law of Partial Pressures states that the
total pressure of a gas is equal to the sum of
all of the individual pressures.
 Ptot = P1 + P2 + … Pn
Volume
(mL)
Volume (L)
of gas at STP
1000mL = 1 L
Volume (L)
Molarity
Molar Mass
6.02 x 1023
Mass (g)
Moles of A
Mass (g)
Mole Ratio
6.02 x 1023
Moles of B
Molar Mass
Molarity
Volume (L)
Volume (L)
of gas at STP
1000mL = 1 L
Volume
(mL)
Atoms or
Molecules
Atoms or
Molecules