Chapter 8 Lecture
Fundamentals of General,
Organic, and Biological
Chemistry
7th Edition
McMurry, Ballantine, Hoeger, Peterson
Chapter Eight
Gases, Liquids, and Solids
Julie Klare
Gwinnett Technical College
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8.2 Intermolecular Forces
Intra- vs. inter(within)
(between)
Liquids and Solids
• There are two major types of intermolecular forces
(IMF) in liquids and solids:
– dipole–dipole forces – polar substances
• Permanent dipole–dipole attractions
• Relatively strong type of IMF
• Hydrogen bonding is especially strong and biologically
crucial
– London dispersion forces – nonpolar substances
• Temporary or “fleeting” charge imbalances
• Relatively weak type of IMF
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(Permanent) Dipole-Dipole Forces
• Dipole moment: property of a molecule with
permanent positive & negative charge centers
– Dipoles occur in polar molecules
• The positive and negative ends of different
polar molecules are permanently attracted to
one another in condensed phases
Water is polar because it has a
very large dipole moment
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5
The resulting balance of forces
• Individual dipole-dipole forces are much weaker
than ‘true’ covalent bonds (intramolecular forces)
• Effects of many dipole–dipole forces are immense
• That’s why polar molecules have high boiling points
bp = −0.5oC
nonpolar:
No permanent IMFs
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bp = +56.2oC
polar:
molecules attract
permanently
Hydrogen Bonding
• Very strong (permanent) dipole-dipole attraction
• First, think of water
– 2 negative poles (d-) – the lone pairs
– 2 positive poles (d+) – H bonded to electronegative O
• Explains why water is a great polar solvent
– Each water molecule makes 4 hydrogen bonds…
Hydrogen bonding for non-water
molecules
• Typically, an attraction between H+ and OH-think of water as (H-OH)
• Formally, an attraction between a hydrogen
atom which is bonded to an O, N, or F atom…
wth any nearby O, N, or F atoms
focus on ethanol alone
can two ethanol molecules
hydrogen bond to each other?
focus on ethanol alone
can two ethanol molecules
hydrogen bond to each other? YES
focus on dimethyl ether alone
can two dimethyl ether molecules
hydrogen bond to each other?
NO
because there are no O-H bonds
(and of course, no N-H or F-H bonds)
Can ethanol hydrogen bond with dimethyl ether?
Yes!
There are two oxygens present, and one is an O-H
nitrogen
can two ammonia molecules
hydrogen bond to each other
YES
Because there are N-H bonds present
nitrogen
can two trimethyl amine molecules
hydrogen bond to each other?
NO
because there are no N-H bonds
nitrogen and oxygen together
Can trimethyl amine
hydrogen bond to ethanol?
YES
because there is an O-H bond
Can ammonia
hydrogen bond to dimethyl ether?
YES
Because there is an N-H present
Effects of hydrogen bonding
• Hydrogen bonding affects physical properties
– Boiling point
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18
19
Concept Check
Below are two Lewis structures for the
formula C2H6O
How would their boiling points differ?
H H
H
H C C O H
H C O
H H
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H
20
H
C H
H
Shown below is a model of liquid water. What
is the major intermolecular force of attraction
responsible for water being a liquid?
a.
b.
c.
d.
Covalent bonding
Dipole–dipole
Hydrogen bonding
London dispersion
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Which of the following is not capable of
hydrogen bonding?
a.
b.
c.
d.
HF
H2O
NH3
CH4
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Methane has H atoms, but none of
them are connected to an O, N, or F
London Dispersion Force
How non-polar molecules condense
• Nonpolar molecules are only really nonpolar on
average (over time)
• At any instant, however, fleeting charge
imbalances occur
– The electrons are constantly moving
– Occasionally, more end up at one side of molecule
– Higher molecular weight (more electrons) and
surface area favor more London forces
Time-averaged
charge distribution
is uniform, as in
the non-polar
(symmetric) Lewis
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structure
An instantaneous charge
distribution with
electrons shifted to the
left (d-). They would also
shift right just as often
(not shown).
Concept Check
Consider the following compounds:
NH3 CH4
H2
How many of the compounds above exhibit
London dispersion forces?
a) 0
b) 1
c) 2
d) 3
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25
fyi: Grease Molecule
•
•
•
•
lots of electrons
lots of molecular surface area
so there are numerous London ‘sites’
resulting in a semi-solid at room temperature
fyi: Hexane Molecule
•
•
•
•
fewer electrons
very little molecular surface area
resulting in fewer London sites
resulting in a liquid at room temperature
Like dissolves Like
• London forces allow nonpolar molecules to
“stick” together as liquids or solids
• Permanent dipoles, especially hydrogen
bonds, allow polar molecules to “stick”
– Polar molecules do also experience London forces,
but they are much weaker and irrelevent
• Nonpolar and polar substances will not “stick”
to each other
– Oil (nonpolar) and water (polar) don’t mix
fyi
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8.3 Gases and the Kinetic
Molecular Theory
An ideal gas obeys all the assumptions of the kinetic–molecular
theory
1. The ideal gas consists of hypothetical particles moving
perfectly at random with zero attractive forces between them
2. The volume of the ideal gas particle is insignificant compared
with the volume of its container – a gas is mostly empty space
3. The average kinetic energy of ideal gas particles is proportional
to the Kelvin temperature
4. All collisions are perfectly elastic. Gasses exert pressure.
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Postulates
of the Kinetic Molecular Theory
1. The ideal gas consists of particles moving
perfectly at random with no attractive forces
between them
IMF = intermolecular force
Postulates
of the Kinetic Molecular Theory
2. The volume of the ideal gas particle is
insignificant compared with the volume of its
container
Solid and liquid particles are
closely packed by touching
each other.
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A gas is mostly empty space.
The particles do not touch,
except when colliding, upon
which they bounce (elasticity).
Postulates
of the Kinetic Molecular Theory
3. The average kinetic energy of ideal gas
particles is proportional to the Kelvin
temperature
Volume will increase with temperature as gas speeds up
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33
Postulates
of the Kinetic Molecular Theory
4. All collisions of ideal gas particles are perfectly
elastic in other words…
the total kinetic energy of a pair of colliding
particles is unchanged following impact
collisions with
other particles or with
the container create a constant
pressure force equal in all directions
Higher T  faster collisions  higher P
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Concept Check
Which gas would behave more ideally at the
same conditions of P and T?
CO
or
Why?
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N2
8.4 Pressure
=
𝑁𝑒𝑤𝑡𝑜𝑛𝑠
𝑚2
or Pascals (Pa)
• Gasses exert pressure forces uniformly in 3D space
• Gasses exert this same pressure force on the 2D surface of a
container or wall
• English unit is pounds (of force) per square inch, or psi
Atmospheric (air) pressure
Gravity ‘pulls down’ on the
gas envelope surrounding our
planet, creating pressure
at the surface
= 14.7 pounds per square inch
= 101,325 Pascals
= 1 atmosphere
= 760 mm Hg
= 760 Torr
= 14.696 psi (English unit)
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Simple barometer
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Simple manometer
• Gas pressure in a container is
measured using a manometer
• The difference between the
mercury levels indicates the
difference between gas
pressure and atmospheric
pressure
• Pressure is given in the SI
system by the pascal (Pa)
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Conversions
• 1 atm = 760. mm Hg
– we will frequently use: 1 atm = 760. mm Hg
– for conversions
• 1 mm Hg = 1 torr
Oxygen gas is contained in the apparatus shown
below. What is the pressure of the oxygen in the
apparatus?
a.
b.
c.
d.
500 mm Hg
725 mm Hg
775 mm Hg
1000 mm Hg
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The Gas Laws
• All gas laws can be formulated with
four variables
–Volume
–Pressure
–Temperature (Kelvins only!)
–n (number of moles of gas)
8.5 Boyle’s Law:
• Boyle’s law: The volume of a gas at constant
temperature decreases proportionally as its
pressure increases
– Smaller V  more collisions with wall  higher P
– e.g. double the pressure, half the volume
P1V1 = P2V2
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The volume of a balloon is 2.85 L at a pressure
of 763 mm Hg. What will the volume of the
balloon be when the pressure is decreased to
755 mm Hg at a constant temperature?
a.
b.
c.
d.
2.82 L
0.355 L
2.88 L
0.347 L
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8.6 Charles’s Law:
The Relation between Volume and Temperature
V1 V2
=
T1 T2
Charles’s law: The volume of a gas at constant pressure is
directly proportional to its Kelvin temperature.
This is a proportional relationship, unlike the inverse
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Inc.
relationship
between P and V in Boyle’s Law.
8.7 Gay-Lussac’s Law:
• Gay-Lussac’s law: The pressure of a gas at
constant volume is directly proportional to its
Kelvin temperature
– As temperature goes up or down, pressure also
goes up or down
P1 P2
=
T1 T2
– Proportional relation between P and T
• Higher T  more energetic collisions  higher P
– It’s just a combination of Boyle’s and Charles’ Laws
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8.8 The Combined Gas Law
P1V1 P2V2
=
T1
T2
• Where the temperature remains constant,
this equation ‘reduces’ to Boyle’s law
• Where pressure is constant, it reduces to
Charles’ law
• Where volume is constant, it reduces to GayLussac’s law
A hot-air balloon has a volume of 960 L at 291 K.
The balloon is heated up and the volume
increases to 1070 L. What is the final
temperature (in °C) of the balloon?
a.
b.
c.
d.
50 °C
80 °C
110 °C
130 °C
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8.9 Avogadro’s Law
• Avogadro’s law: the volume of a gas is directly
proportional to its molar amount
• at a constant pressure and temperature
n1
n2
=
V1
V2
• More moles of gas  more volume
– Total moles of any gas, possibly combined species!!!
1.0 mol of gas at 25 °C and 0.90 atm is
contained in an apparatus with a movable piston.
Which set of conditions will result in a lower
piston position than shown in this drawing?
a. 1.0 mol of gas, 25 °C, and 1.80
atm
b. 1.0 mol of gas, 50 °C, and 0.90
atm
c. 2.0 mol of gas, 25 °C, and 0.45
atm
d. 2.0 mol of gas, 25 °C, and 0.90
atm
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If all other variables are held constant, which
change will NOT cause a change in the pressure
of oxygen in a mixture of oxygen and nitrogen?
a.
b.
c.
d.
Decreasing the moles of oxygen
Decreasing the temperature
Increasing moles of nitrogen
Increasing the volume
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At a constant volume, a flask is filled with helium
at 25 °C and 1.03 atm pressure, sealed, and
then heated to 98 °C. What is the pressure
inside the flask?
a.
b.
c.
d.
0.827 atm
1.21 atm
1.28 atm
0.916 atm
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8.10 The Ideal Gas Law: PV = nRT
• The ideal gas law defines a standard state
– STP = standard temperature and pressure
• 0 C (273.15 K exactly)
• 1 atm (760 mmHg by definition)
• The “standard volume” of 1 mole of (any) gas at STP
is the same.
– Standard volume for any gas at STP = 22.414 L
• R is the Universal Gas Constant
– It’s units must match the pressure and volume units
– R = 0.08206 L•atm/mol•K = 62.40 L•mm Hg/mol•K = …
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At STP, 22.414 L of an ideal gas
would just fit into this precision ball
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Which of the following condition(s) are NOT
considered to be standard temperature or
pressure?
a.
b.
c.
d.
e.
760 mm Hg
755 torr
0 °C
Both a and b
None of the above
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What is the volume of 3.86 g of nitrogen gas at
STP?
a.
b.
c.
d.
3.09 L
86.5 L
4.44 L
6.18 L
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8.11 Partial Pressure and Dalton’s Law
Each particle in a gas acts independently,
so its identity becomes irrelevant.
Thus, gas pressures add…
just like liquid volumes.
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• Mixtures of gases behave the same as a single pure gas
and obey the same laws
– Dry air is a mixture of 21% O2, 78% N2, and 1% Ar by
volume (and by mole percent)
• Partial pressure is the contribution of each gas in a
mixture to the total pressure (P).
– Proportional to volume (and mole percent)
– e.g. for air at STP (P = 1 atm), partial pressures are
• 0.21 atm for O2 ( = 21% of 1 atm total pressure)
• 0.78 atm for N2
• 0.01 atm for Ar
• Partial pressures add to the total pressure.
– P = P1 + P2 + P3 + …
– Above partial pressures for O2, N2 and Ar add to 1 atm
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• So for a mixture of ideal gases, it is the total
number of ‘particles’ that is important
– Not the identity or composition of the involved gases
2 atm gas A
+
5 atm gas B

7 atm total in
the mixture
Exercise
27.4 L of oxygen gas at 25.0°C and 1.30 atm, and
8.50 L of helium gas at 25.0°C and 2.00 atm were
pumped into a tank with a volume of 5.81 L at
25.0°C.
• Calculate the new partial pressure of oxygen
6.13 atm
• Calculate the new partial pressure of helium
2.93 atm
• Calculate the new total pressure of both
9.06 atm
60
8.12 Liquids
• Liquids are usually in equilibrium with their vapor (gas)
– Some molecules have higher than average energy and
“escape” through the liquid-vapor surface into gas form
• Dynamic equilibrium is reached. Evaporation and
condensation occur constantly and at the same rate.
• Thus liquids exhibit a constant vapor pressure.
– It increases with T (higher average energy)
a) Liquid in initially
evacuated flask (no
pressure).
b) Liquid starts
evaporating.
c) Evaporation and
condensation reach
equilibrium. The gas
exerts vapor pressure
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over the liquid.
Definition
Normal boiling point is the boiling point
under a pressure of exactly 1 atmosphere
A liquid’s vapor pressure (and thus b.p.)
depends on temperature, atmospheric
pressure, and the chemical properties of
the liquid itself … eg, is it polar?
• All other things being equal, polar molecules
have lower vapor pressures, and thus higher
boiling points
Viscosity
• Many familiar properties of liquids can be
understood by studying intermolecular forces
• Viscosity
– some liquids flow easily like gasoline, and some are
sluggish like honey
– The measure of a liquid’s resistance to flow is called
viscosity
– Viscosity increases with increasing intermolecular
forces
Which would you expect to be more viscous?
nonpolar
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polar
• Surface tension is caused by the difference between
the intermolecular forces experienced by molecules
at the surface of the liquid and those experienced by
molecules in the interior. P239
– Liquid molecules exert more IMFs with each other than the
surrounding gas. The liquid surface pulls inward.
• This explains why gravity can make a puddle grow
only just so large
– Substances with high surface tension ‘bead up’
– Substances with low surface tension spread out thin
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8.13 Water: A Unique Liquid
• Liquid water is denser than solid water (ice) – so ice floats on
water
– Typically, a solid is denser than its liquid is denser than its gas
• The unique, extremely strong hydrogen bonding pulls the
liquid water molecules close together
– Closer than the lattice distance between the ice molecules in an ice
crystal, which has a regular spacing of the molecules
– Ice molecules are frozen in place, and hydrogen bonding cannot
pull them closer together as for the liquid
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8.14 Solids
• Metallic solids can be viewed as vast threedimensional arrays of metal cations immersed
in a sea of electrons.
– The electron sea acts as a glue to hold the cations
together and as a mobile carrier of charge to
conduct electricity.
– Bonding attractions extend uniformly in all
directions, so metals are malleable (bendable)
rather than brittle. When a metal crystal receives
a sharp blow, the electron sea adjusts to the new
distribution of cations.
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FYI
• An amorphous solid is one whose constituent particles
are randomly arranged and have no ordered long-range
structure
– It’s an extremely viscous liquid that appears like a
solid
– Amorphous solids often result when liquids cool
before they can achieve internal order (glass), or
when their molecules are large and tangled together
(viscosity modifiers in motor oil)
– Glass, tar, opal, and some hard candies are
amorphous solids
• Glass is really a liquid… in many thousand years it will form a
puddle.
• Compare amorphous (technically liquid) glass to the solid
phase crystal. Crystal is much more brittle.
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8.15 Changes of State
Heats of melting and boiling are technically called latent heats of fusion and
vaporization, respectively. They are measured in units of J/mol or cal/mol. Latent means
hidden, meaning that no temperature change is “sensed” during fusion or vaporization.
Vaporization requires considerably more energy
than melting… must overcome IMFs to separate
the liquid molecules into gas
ΔHvap water = 540. cal per gram
ΔHvap water = 9730. cal per mole
What is the boiling point of the substance having
the heating curve shown below?
a.
b.
c.
d.
–50 °C
12 °C
75 °C
125 °C
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What is the relationship between intermolecular
forces (IMF) and the boiling point of a liquid?
a.
b.
c.
d.
As IMF increase, boiling point increases.
As IMF increase, boiling point does not change.
As IMF increase, boiling point decreases.
None of the above
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