Chapter 11:
Gases
Chem 1110
Figures: Basic Chemistry 3rd Ed., Timberlake and Timberlake
Forces in Matter
We know that matter is held together by a variety
of forces:
• Electrons are attracted to protons
• Cations (+) are attracted to anions (-)
• Atoms in molecules are held together by bonds
sharing electrons
• Bonds and molecules may have a net dipole
moment which sets up positive and negative
areas that are attracted to each other
Forces in Matter
Intermolecular forces are attractive forces
between molecules:
1. Weaker than bonds
2. Help to determine the state of matter:
• Solid, Liquid, or Gas
Intermolecular forces which organize matter
are opposed by the motion of molecules
which disorganize matter
Kinetic Molecular Theory of Gases
Five statements which explain the physical
behavior of solids, liquids, and gases
• Primary Idea: particles are always in motion
and contain two types of energy:
• Kinetic Energy: energy matter possesses due
to particle motion: DISRUPTIVE!
• Potential Energy: stored energy in matter due
to composition and structure: ORGANIZING!
Kinetic Molecular Theory of Gases:
The Five Statements
1. Particles are in rapid straight line motion
•
Possess kinetic energy
2. Assume NO attractive forces between gas particles
3. Particles are far apart
4. Particles have very small volumes compared to
the volume of the container they occupy
5. Kinetic Energy increases with increasing
temperature
Atmospheric Pressure
Air in the atmosphere exerts a pressure:
• We have lived under this pressure always and
do not recognize or “feel” it, but it is there
• At sea level, a column of air (1 in2) = 14.7 lbs
• We say the pressure is 14.7 lbs/in2 or psi
• At higher altitudes (Denver, Mt Everest) have
a shorter column of air, therefore less weight
• Low altitudes (Death Valley) - more weight
• Scuba divers under 10 m water ≈ 14.7 psi
Atmospheric Pressure
• The air in the
atmosphere exerts a
pressure (force) on
everything below it
• We have lived under
this pressure always,
but we do not
recognize or “feel” it
Measuring Atmospheric Pressure
• We use a barometer to
measure pressure
• Invented by Evangelista
Torricelli in 1600s
• Pressure of atmosphere
is compared to pressure
of a mercury column
Learning Check
A. The downward pressure of the Hg in a barometer
is _____ than (as) the pressure of the atmosphere:
1) greater
2) less
3) the same
B. A water barometer is 13.6 times taller than a Hg
barometer (DHg = 13.6 g/mL) because:
1) H2O is less dense (less heavy)
2) H2O is heavier
3) Air is more dense than H2O
Units of Pressure
Pressure is the force acting on a specific area:
Pressure (P) = force
area
Measuring Atmospheric Pressure
The height of a column of mercury may be
measured in many units (Sea Level, 0 °C):
 1 atmosphere
 14.7 psi
 760 mm Hg
 760 torr
 76.0 cm Hg
 29.9 inches of Hg (weather stations)
• Each of these are conversion factors!!
Learning Check
How many atmospheres if the pressure is
measured as 635 torr? in Hg?
Ideal Gases
1. Gases are mostly space with a large
separation between atoms
2. Atoms move rapidly in straight line motion,
change direction on collision
3. No intermolecular attractive forces in a gas
4. Collisions do not change net energy of the
gas but transfer energy
5. Individual particles move at different speeds
Real Gases
Real gases approach IDEAL behavior when
we ignore intermolecular forces:
• Low Pressure
o Lots of space so can “ignore” each other.
• High Temperature
o Moving quickly so whiz past each other
Properties of Gases
Gases are described by Four Properties:
Pressure (P)
Temperature (T)
Volume (V)
Amount(n)
Gas Laws
Gases are the most compressible state of
matter as there is mostly space between
particles
• Let’s look at how a sample of gas responds
to changes in pressure and temperature:
o Pressure
o Temperature
o Volume
Ideal Gas Law Relationships
P and V are inversely related
• As pressure increases, volume decreases
• As pressure decreases, volume increases
P is directly related to T
• As temperature increases, pressure increases
• As temperature decreases, pressure decreases
V is also directly related to T
• As temperature increases, volume increases
• As temperature decreases, volume decreases
Ideal Gas Law
PV = nRT
P = pressure in atmospheres
V = volume in liters
T = temperature in Kelvin
n = moles of gas
R = gas constant = 0.0821 L∙atm
mole∙K
Learning Check
How many moles of gas are present if a sample of
some unknown gas at 2.00 atm and 0°C takes up
1.12 liters volume?
Assuming we have 5.8 grams of the gas, what is its
molecular weight?
Boyle’s Law
Boyle’s Law states:
• the pressure of a gas is
inversely related to its
volume (at constant T)
• if the pressure (P)
increases, then the
volume decreases
Boyle’s Law
P1V1 = P2V2
• Influenced by the work of Torricelli
o Creating a vacuum over mercury (barometer)
• Discovered the “Spring of Air”
o Gases behave elastically
o Spring back to original volume after compressed
Boyle’s Law
P1V1 = P2V2
• At constant T, the volume of a fixed sample
of gas (same n) varies inversely with the
pressure exerted
• Double the pressure → one half the volume
Learning Check
For a cylinder containing helium gas, indicate if
cylinder A or cylinder B represents the new volume
for the following changes (n and T are constant):
1) Pressure decreases
2) Pressure increases
Learning Check
A sample of helium gas in a balloon has a volume
of 6.4 L at a pressure of 0.70 atm.
At 1.40 atm (T and n constant), is the new volume
represented by A, B, or C?
P and V in Inhalation
During inhalation:
• the lungs expand
• the volume of the lungs
increases (↑V)
• pressure in the lungs
decreases (↓P)
• air flows toward the lower
pressure in the lungs
P and V in Exhalation
During exhalation:
• the lungs contract
• the volume of the lungs
decreases (↓V)
• pressure in the lungs
increases (↑P)
• air flows from the higher
pressure in the lungs
Charles’s Law
Charles’s Law states:
• the temperature of a gas
is directly related to the
volume (at constant P)
• when the temperature of
a gas increases, its
volume increases (at
constant P)
Charles’ Law
V1
V2
=
T1
T2
• Influenced by work with hot air balloons
• Discovered that hot air was less dense than cold air
– Heating air in balloon will allow it to rise
– Cooling the air will cause it to return to earth
Charles’ Law
V1
V2
=
T1
T2
• At constant pressure (P), the volume of a
fixed sample of gas (same n) is directly
proportional to its Kelvin temperature
• Doubling the Kelvin temperature → doubles
the volume of the gas
Gay-Lussac’s Law: P and T
Gay-Lussac’s Law states:
• the pressure exerted by a gas
is directly related to the Kelvin
temperature (at constant V)
• when the temperature of a gas
increases, its pressure
increases (at constant V)
Gay-Lussac’s Law
P1
P2
=
T1
T2
• Also based on work with hot air balloons
• At constant V, the pressure of a fixed sample of gas
(same n) is directly proportional to its temperature
• Doubling the Kelvin temperature → doubles the
pressure of the gas
• Also discovered that the composition of the
atmosphere does not change with decreasing
pressure (increasing altitude)
Learning Check
Use the gas laws to complete the following
statements with increases or decreases:
A. Pressure _______ when V decreases.
B. When T decreases, V _______.
C. Pressure _______ when V changes from 12 L
to 4 L.
D. Volume _______ when T changes from 15 °C to
45 °C.
Combined Gas Law
P1V1
P2V2
=
T1
T2
Can be derived directly from the
Ideal Gas Law: PV = nRT
Learning Check
A small cylinder of nitrogen gas (5.7 L) is under
5.03 atm of pressure. What would be the volume
at a pressure of 754 torr?
Learning Check
A helium balloon has a volume of 2.5 L at 25 °C.
What is the volume of the balloon in liquid
nitrogen at -196 °C?
Learning Check
A rubber glove is filled with carbon dioxide gas at a
volume of 1.73 L, 23 °C and 743 torr. What will the
temperature of the gas be (in °C) if the pressure is
increased to 1.71 atm and the volume is allowed to
increase to 2.00 L?
Take Home
Why do empty spray paint cans at 1.05 atm and
25 °C explode when put in a bonfire at 451°C?
Avogadro's Law: V and n
Avogadro’s Law states:
• the volume of a gas is directly
related to the number of moles
(n) of gas
• equal volumes of any gas at
the same temperature and
pressure have the same
number of molecule
Molar Volume and STP
Chemists have defined a specific set of
conditions known as standard temperature
and pressure (STP) as:
1 atmosphere and 0 °C
• Under these conditions (STP), 1 mole of
any gas will occupy exactly 22.4 L
• Consequently, Molar volume is defined as:
1 mole of gas = 22.4 L at STP
Molar Volume
What is the volume occupied by 2.75 mole of
N2 gas at STP?
Learning Check
What is the density of SO2 gas at STP?
Gases in Chemical Equations
The volume (or amount) of a gas in a
chemical reaction can be calculated from:
• The ideal gas law
• Mole-mole factors from the balanced
equation
• The molar mass
Using the Ideal Gas Law
Within a Chemical Equation
What volume (in L) of Cl2 gas at 1.20 atm and 27 °C is
needed to completely react with 1.50 g of aluminum?
2 Al(s) + 3 Cl2(g)
2 AlCl3(s)
Learning Check
What volume (L) of O2 at 24 °C and 0.950 atm is
needed to react with 28.0 g of NH3?
4 NH3(g) + 5 O2(g)
4 NO(g) + 6 H2O(g)
Take Home
What mass of Fe will react with 5.50 L of O2 at STP?
4 Fe(s) + 3 O2(g)
2 Fe2O3(s)
Mixed Samples of Gases
Some gas samples are made up of more than
one type of gas:
• Air is a good example and includes:
N2, O2, CO2, H2O, Ar, traces of others
If we have a homogeneous mixture of gases,
we can determine the individual contributions
of pressures from each gas to the total…
Dalton’s Law of Partial Pressures
Dalton’s Law of Partial Pressures
Law of Partial Pressures
PTotal = Σ Peach gas
or
PTotal = Pgas1 + Pgas2 + Pgas3
PAir = PN2 + PO2 + PCO2 + …
Law of Partial Pressures
Understanding partial pressures of gases is
very important for medical applications:
• High PO2 can harm body tissues yet low
PO2 leads to other issues
• Dissolved nitrogen in the blood from deep
water or extended scuba diving leads to
decompression sickness or the bends
o Pressurized air is primarily N2 and O2
The Gases We Breathe
The air we breathe:
• is a mixture of gases
• contains mostly N2
and O2 as well as
small amounts of
other gases
Gas Exchange During Breathing
51
Blood Gases
• In the lungs, O2 enters the blood, while CO2
from the blood is released
• In the tissues, O2 enters the cells, while CO2
is released into the blood
Blood Gases
In the body:
• O2 flows into the tissues because the partial
pressure of O2 is higher in the blood and lower
in the tissues
• CO2 flows out of the tissues because the
partial pressure of CO2 is higher in the tissues
and lower in the blood
Scuba Diving
• When a scuba diver is below
the ocean surface, the
increased pressure causes
more N2 (g) to dissolve in the
blood
• If a diver rises too fast, the
dissolved N2 can form
bubbles in the blood, a
dangerous and painful
condition called "the bends”
• For deep descents, helium,
which does not dissolve in
the blood, is added to O2
Under water, the pressure
on a diver is greater than
the atmospheric pressure
Hyperbaric Chamber
Forces in Matter
So far we have focused on gases, specifically
Ideal Gases
Let’s revisit liquids and solids:
• We can no longer assume no attractions
between particles that are close together!
• We must re-examine intermolecular forces!
Forces in Matter Revisited
Intermolecular forces are attractive forces
between molecules:
1. Weaker than bonds
2. Help to determine the state of matter:
• Solid, Liquid, or Gas
Intermolecular forces which organize matter
are opposed by the motion of molecules
which disorganize matter
Boiling and Vapor Pressure
Boiling Point: a liquid boils when its vapor
pressure equals atmospheric pressure
• Vapor Pressure: the pressure exerted by a
vapor above a liquid when the liquid and
vapor are in an equilibrium state
• Water at sea level: Boiling Point = 100 °C
o Under column of air at 1 atm, 760 mm Hg,
14.7 psi, 760 torr…
Vapor Pressure of Water changes
based on Altitude (Pressure)
Boiling Point of Water changes
based on Altitude (Pressure)
Learning Check
In each of the following pairs of atmospheric and
vapor pressure, indicate if water will or will not boil:
Atmospheric
Pressure
Vapor
Pressure
A. 760 mmHg
760 mmHg
B. 960 mmHg
760 mmHg
C. 520 mmHg
620 mmHg
Boiling
Occurs?
Evaporation
Liquids evaporate when individual molecules reach
“escape velocity” to the gas phase:
o Reach their boiling point.
o Agrees with KMT – various speeds of particles
• Some liquids evaporate more easily than others
– Have a high vapor pressure – close to 1 atm
• Volatile liquids readily evaporate at room
temperature and have high vapor pressures
Evaporation