Slide 1 / 92
Slide 2 / 92
Intermolecular Forces
Intermolecular forces are the piece we need to add to the
puzzle to explain the world around us.
Intermolecular Forces,
Liquids, and Solids
We first explained atoms, and how to build up the
periodic table from quantum numbers.
Then we explained how atoms combine to form
molecules: the most common way we find most atoms in
nature.
Now, we're going to use intermolecular forces to combine
molecules to create the common states of matter.
Without intermolecular forces, we wouldn't have tables,
lakes, wall...or even our bodies. Intermolecular forces
shape our world.
Slide 3 / 92
Slide 4 / 92
States of Matter
States of Matter
The fundamental differences between states of matter is:
While there are many states of matter, the
three common states that dominate our world
are gases, liquids and solids.
· the distance between particles
· the particles' freedom to move
These are the states of matter we'll be
studying.
We won't be discussing more exotic states
such as plasma, nuclear matter, etc.
cool or
increase pressure
cool
heat or
decrease pressure
heat
Gas
Liquid
Slide 5 / 92
Liquid
Solid
Assumes the shape of its container
Expands to the volume of its container
Is compressible
Flows easily
Diffusion within a gas is rapid
Assumes the shape of the part of a container it occupies
Does not expand to the volume of its container
Is virtually incompressible
Flows easily
Diffusion within a liquid is slow
Retains its own shape, regardless of container
Does not expand to the volume of its container
Is virtually incompressible
Does not flow
Diffusion within a solid is very very slow
orderd arrangement,
particles are in fixed
positions,
close together
Slide 6 / 92
Characteristics of the States of Matter
Gas
Crystalline solid
disorder, freedom,
free to move relative
to each other,
close together
Particles are far apart,
total freedom,
much of empty space,
total disorder
Condensed Phases
In the solid and liquid states particles are closer
together, we refer to them as condensed phases.
Gas
Particles are far apart,
total freedom,
much of empty space,
total disorder
cool or
increase pressure
cool
heat or
decrease pressure
heat
Liquid
disorder, freedom,
free to move relative
to each other,
close together
Crystalline solid
orderd arrangement,
particles are in fixed
positions,
close together
Slide 7 / 92
1
Which of the following is not a type of
solid?
A
ionic
B
molecular
C
covalent-network
D
E
Slide 8 / 92
2
Which of the below is a
characteristic of a gas?
A
It fills only a portion of its container.
B
Its molecules are in relatively rigid
positions.
C
It takes on the shape of its container.
supercritical
D
It is not compressible.
metallic
E
Diffusion is very slow within it.
Slide 9 / 92
3
Which of the below is a
characteristic of a liquid?
Which of the below is a
characteristic of a solid?
It fills only a portion of its container.
A
It fills all of its container.
B
Its molecules are in relatively rigid
positions.
B
Its molecules are in relatively rigid
positions.
C
It takes on the shape of its container.
C
It takes on the shape of its container.
D
It is compressible.
D
It is compressible.
E
Diffusion is very rapid within it.
E
Diffusion is very rapid within it.
Together, liquids and solids, constitute
__________ phases of matter.
A
4
A
Slide 11 / 92
5
Slide 10 / 92
the compressible
B
the fluid
C
the condensed
D
all of the above
E
the disordered
Slide 12 / 92
States of Matter
The state of a substance at a particular temperature
and pressure depends on two opposing properties:
· Intermolecular Forces: the strength of the
attractions between the particles, which pulls them
together
· the kinetic energy of the particles, which pulls
them apart
Without intermolecular forces, all molecules would
be ideal gases...there would be no liquids or solids.
Slide 13 / 92
Slide 14 / 92
Intermolecular Forces & Boiling
Points
Intermolecular Forces
Boiling represents a transition from a liquid to a gas.
To make that transition, molecules in the liquid must
break free of the intermolecular forces that bind them.
H
Slide 15 / 92
Slide 16 / 92
Intermolecular Forces
+
-
+
· These forces are only
important when the molecules
are close to each other.
Slide 17 / 92
-
+
+
-
+
The interaction between any
two opposite charges is
attractive ( red)
Slide 18 / 92
Dipole-Dipole Interactions
The polarity of a molecule is measured by its dipole
moment, m. The more polar the molecule, the greater its
dipole moment.
The more polar the molecule, the higher its boiling point.
That's because the attraction between the dipoles holds
the molecules together, not letting them boil away.
3.9
2.7
1.9
1.3
0.1
-
· London dispersion forces
-
· The positive end of one is
attracted to the negative end
of the other and vice-versa.
· Hydrogen bonding
41
44
50
46
44
The interaction between
any two like charges is
repulsive (black)
· Molecules that have
permanent dipoles are
attracted to each other.
+
· Dipole-dipole interactions
Dipole-Dipole Interactions
-
There are three types of Intermolecular
Forces: they are sometimes called van der
Waals Forces
Acetonitrile, CH3CN
Acetaldehyde, CH3CHO
Methyl chloride, CH3Cl
Dimethyl ether, CH3OCH3
Propane, CH3CH2CH3
Intermolecular
attraction ( week)
They are, however, strong enough to control
physical properties such as boiling and melting
points, vapor pressures, and viscosities.
The boiling point is a measure of the strength of the
intermolecular forces: the higher the boiling point - the
stronger the intermolecular forces.
Dipole
Moment u ( D)
Cl
The attractions between molecules, intermolecular
forces, are not nearly as strong as the
intramolecular attractions that hold compounds
together.
The temperature where the molecules' energy
overcomes intermolecular forces is called the boiling
point.
Molecular
Weight (amu)
H
Covalent bond
(strong)
The kinetic energy of the molecules is proportional to the
temperature: as temperature rises, so does kinetic
energy.
Substance
Cl
Boiling
Po i n t ( K)
355
294
249
248
231
6
Which of the below
molecules will have the
highest boiling point?
A
CH3CH2CH3
B
CH3OCH3
C
CH3Cl
Substance
Molecular Dipole
Moment
Wt.
D
CH3CHO
CH3CH2CH3
44
0.1
E
CH3CN
CH3OCH3
46
1.3
CH3Cl
50
1.9
CH3CHO
44
2.7
CH3CN
41
3.9
Slide 19 / 92
7
Slide 20 / 92
London Dispersion Forces
Which of the below
molecules will have the
lowest boiling point?
A
CH3CH2CH3
B
CH3OCH3
C
CH3Cl
D
CH3CHO
E
CH3CN
London Dispersion Forces occur between all molecules.
Substance
Molecular Dipole
Moment
Wt.
CH3CH2CH3
44
0.1
CH3OCH3
46
1.3
CH3Cl
50
1.9
CH3CHO
44
2.7
CH3CN
41
3.9
They result from the fact that electrons are in constant
motion and sometimes are the same side of the molecule.
When they are on one side, the molecule is polarized: one
side is negative and the other is positive; the molecule
acts like a dipole.
That creates an electric field that oppositely polarizes
nearby molecules...leading to an attraction.
Let's see how that works using Helium as an example.
Slide 21 / 92
Slide 22 / 92
London Dispersion Forces
London Dispersion Forces
e-
While the electrons in helium repel each other, they
occasionally wind up on the same side of the atom.
2+
e-
Helium atom
At that instant, the helium atom is polar, with an excess
of electrons on one side and a shortage on the other.
These forces are present in all molecules, whether they are polar or nonpolar.
·
The tendency of an electron cloud to distort in this way is called polarizability.
·
The larger the molecule, the more polarizable it is...and the stronger the London
Dispersion Force.
Another helium atom nearby, has a dipole induced in it,
as the electrons on the left side of the first atom repel
the electrons in the second.
London dispersion forces, or dispersion forces, are
attractions between an instantaneous dipole and an
induced dipole.
·
δ
-
δ+
·
That means, the higher the molecular weight of a molecule, the more London
Dispersion Force it experiences.
electrostatic attraction
e-
e-
2+
2+
e-
e-
Helium atom 1
δ-
Helium atom 2
δ+
δ-
δ+
Slide 23 / 92
Slide 24 / 92
London Dispersion Forces
· The strength of dispersion forces tends to
increase with increased molecular weight.
· Larger atoms and molecules have larger electron
clouds which are easier to polarize.
Halogen Molecular
Weight
( amu)
Boiling Noble
Point (K) gas
F2
38.0
85.1
He
4.0
4.6
Cl2
71.0
238.6
Ne
20.2
27.3
Br2
159.8
332.0
Ar
39.9
87.5
I2
253.8
457.6
Kr
83.8
120.9
Xe
131.3
166.1
Molecula Boiling
rWeight point (K)
(amu)
8
Which of the molecules
below will have the highest
boiling point?
A
F2
B
Cl2
C
Br2
D
I2
Slide 25 / 92
9
What intermolecular force is responsible for ice being
less dense than liquid water?
A
London Dispersion Forces
B
Dipole-Dipole Forces
C
Ion-Dipole Forces
D
Hydrogen Bonding
E
Ionic Bonding
Slide 26 / 92
10
Intermolecular force(s) responsible for the fact that
CH4 has the lowest boiling point in the set CH4, SiH4,
GeH4, SnH4 is/are __________.
A
Hydrogen Bonding
B
Dipole-Dipole Interactions
C
London Dispersion Forces
D
Mainly H2 Bonding with some dipole-dipole interactions
E
mainly London Dispersion Forces with dipole-dipole interactions
Slide 27 / 92
11 Which of the below
molecules will have the
lowest boiling point?
A
F2
B
Cl2
C
Br2
D
I2
A
13 Which of the below
molecules will have the
lowest boiling point?
He
B
Ne
C
Ar
D
Kr
E
Xe
12 Which of the below
molecules will have the
highest boiling point?
B
Slide 29 / 92
A
Slide 28 / 92
He
Ne
C
Ar
D
Kr
E
Xe
Slide 30 / 92
Which Have a Greater Effect?
Dipole-Dipole Interactions or Dispersion
Forces
· If two polar molecules are of comparable size,
dipole-dipole interactions are the dominating force.
· If one molecule is much larger than another,
dispersion forces will likely determine its physical
properties.
· If molecules are nonpolar, dispersion forces will
dominate, since all molecules experience dispersion
forces.
Slide 31 / 92
Slide 32 / 92
Hydrogen Bonding
Hydrogen Bonding
In these graphs, boiling points increase
with the mass of the molecules...as we'd
expect: larger dispersion forces due to
larger masses.
· The nonpolar series (CH4 to SnH4 )
follow the expected trend.
· The polar series follows the trend from
H2Te through H2S,
· But water defies the trend. It is the
lightest in its series, but has the highest
boliing point.
H
· The dipole-dipole interactions
experienced when H is bonded to N,
O, or F are unusually strong.
H
N
H
O
H
H
· We call these interactions hydrogen
bonds.
H
O
· These powerful bonds occur only
when H is bonded directly to N, O or
F.
O
H
H
H
H
H
H
N
H
O
H
Slide 33 / 92
Slide 34 / 92
Hydrogen Bonding
Hydrogen Bonding
Hydrogen bonding arises in part from the high
electronegativity and small radius of nitrogen,
oxygen, and fluorine.
When hydrogen is bonded to one of those very
electronegative elements, the hydrogen nucleus is
exposed.
Ice is the only solid that floats in its liquid form.
If it didn't, life on Earth would be very different. For instance,
lakes would freeze from the bottom and fish couldn't survive
winters.
Hydrogen bonding creates the space in ice that explains its low
density.
Water-Ice transition simulation.htm
Slide 35 / 92
A
HF
B
HCl
HBr
HI
· There is a fourth molecular force that will be important
as we explore solutions later this year: Ion-dipole
interactions are not considered a van der Waals force.
· The strength of these forces are what make it possible
for ionic substances to dissolve in polar solvents.
-
C
D
Ion-Dipole Interactions
-
+
14 Which of the following
molecules has hydrogen
bonding as one of its IM forces?
Slide 36 / 92
+
All of the above.
-
+
+
+
-
_
+
+
-
+
+
-
-
-
E
Anion- dipole attractions
Cation -dipole attractions
Slide 37 / 92
Slide 38 / 92
Summarizing Intermolecular Forces
Interacting
molecules or ions
Are polar
molecules involved?
NO
Are ions involved?
YES
YES
NO
15 Which of the following
molecules has London
dispersion as its only IM force?
Are polar
molecules and ions
both present?
YES
NO
Are hydrogen
atoms bonded to
N,O,or F atoms?
NO
Dispersion
forces only
(induced dipoles)
Ar, I2
A
PH3
B
H 2S
C
HCl
D
SiH4
E
None of the above.
YES
Dipole-dipole
forces
H2S, CH3Cl
Hydrogen bonding
H2O, NH3
Ion- dipole forces
NaCl in H2O
Ionic bonding
NaCl, KI
Slide 39 / 92
16 How many of these substances
will have dipole-dipole
interactions? (How many are
polar molecules?)
H 2O
CO2
CH4
NH3
A
0
B
1
C
2
D
3
E
4
Slide 40 / 92
17 Which of the following
molecules will have the
highest boiling point?
A
H 2O
B
CO2
C
CH4
D
NH3
Slide 41 / 92
18 Which liquid will have the lowest freezing
point?
A
pure H2O
B
aq. 0.60 M glucose
C
aq. 0.60 M sucrose
D
aq. 0.24 M FeI3
E
aq. 0.50 M KF
Slide 42 / 92
19 Of the following diatomic
molecules, which has the
highest boiling point?
A
B
N2
Br2
C
H2
D
Cl2
E
O2
Slide 43 / 92
20 Of the following diatomic
molecules, which has the
lowest boiling point?
Slide 44 / 92
21 Which one of the following
derivatives of methane (CH4)
has the lowest boiling point?
A
N2
A
CBr4
B
Br2
B
CF4
C
H2
C
CCl4
D
Cl2
D
CI4
E
O2
Slide 45 / 92
22 Which one of the following
derivatives of methane (CH4)
has the highest boiling point?
Slide 46 / 92
23 For an aqueous solution of a nonvolatile
compound, the vapor pressure will be _______, the
boiling point will be ________, and the freezing
point will be _______ than pure water.
A
CBr4
B
CF4
A
lower, lower, lower
C
CCl4
B
lower, higher, lower
D
CI4
C
lower, higher, higher
D
higher, higher, lower
E
higher, lower, higher
Slide 47 / 92
Intermolecular Forces Affect
Many Physical Properties
Slide 48 / 92
Viscosity
· Resistance of a liquid to flow is called viscosity.
· It is related to the ease with which molecules can move past
each other.
The strength of the
attractions between particles
can greatly affect the
properties of a substance or
solution.
· Viscosity increases with stronger intermolecular forces and
decreases with higher temperature.
Substance
Hexane
Heptane
Octane
Nonane
Decane
Formula
Viscosity ( kg/m-s)
CH3CH2CH2CH2CH2CH3
CH3CH2CH2CH2CH2CH2CH3
CH3CH2CH2CH2CH2CH2CH2CH3
CH3CH2CH2CH2CH2CH2CH2CH2CH3
CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3
3.26 x 10-4
4.09 x 10-4
5.42 x 10-4
7.11 x 10-4
1.42 x 10-3
http://upload.wikimedia.org/wikipedia/commons/b/b7/Viscosity.gif
Slide 49 / 92
Slide 50 / 92
Surface Tension
Phase Changes
GAS
Vaporization
Sublimation
Energy of system
Surface tension results from the net inward
force experienced by the molecules on the
surface of a liquid.
Deposition
LIQUID
Melting
Freezing
SOLID
Slide 51 / 92
Energy Changes Associated
with Changes of State
Condensation
Slide 52 / 92
24 Which of the following is
not a phase change?
Chemical and physical changes are usually accompanied by
changes in energy.
· When energy is put into the system, the process is called
endothermic.
A
vaporization
B
effusion
C
melting
D
sublimation
· When energy is released by the system, the process is called
exothermic.
Slide 53 / 92
25 Of the following, _____________ is an exothermic
process?
Slide 54 / 92
26
The first molecules to evaporate from a liquid
are__________.
melting
A
Those with the lowest KE
B
subliming
B
Those farthest from the surface of the liquid
C
freezing
C
Those with the highest KE
D
boiling
E
all are exothermic
A
Slide 55 / 92
27
Slide 56 / 92
Boiling vs. Evaporation
The direct change of a substance from a solid to a
gas is called ___________.
A
boiling
B
evaporation
C
sublimation
D
condensation
Boiling and evaporation are two ways in which a liquid can
vaporize into a gas. However, there are important
distinctions between these processes.
Evaporation
Boiling
occurs at a specific
temperature, the boiling point
(B.P.)
occurs below the boiling point
occurs throughout the entire
liquid
occurs only at the surface of a
liquid
achieved when atmospheric
pressure equals vapor pressure
(Patm = Pvap)
Slide 57 / 92
Slide 58 / 92
Vapor Pressure
Vapor Pressure
As more molecules escape the liquid, the
pressure they exert increases.
· At any temperature some molecules in a liquid have
enough energy to escape.
· As the temperature rises, the fraction of molecules that
have enough energy to escape increases.
The liquid and vapor reach a state of dynamic
equilibrium: liquid molecules evaporate and vapor
molecules condense at the same rate.
Slide 59 / 92
Vapor Pressure
· The boiling point of a liquid
is the temperature at which
it’s vapor pressure equals
atmospheric pressure.
· The normal boiling point is
the temperature at which its
vapor pressure is 760 torr.
(AKA 760 mm Hg = 1 atm)
Slide 60 / 92
28
When an aqueous salt solution is compared to
water, the salt solution will have
A
a higher boiling point, a lower freezing point, and a lower vapor pressure
B
a higher boiling point, a higher freezing point, and a lower vapor pressure.
C
a higher boiling point, a higher freezing point, and a higher vapor pressure
D
a lower boiling point, a lower freezing point, and a lower vapor pressure
E
a lower boiling point, a higher freezing point, and a higher vapor pressure
Slide 61 / 92
29 What is the normal boiling
point of ethanol?
A
45
Slide 62 / 92
30 What is the boiling point ( in 0 C) of diethyl ether at 200
torr?
A
-10
B
0
B
55
C
35
C
760
D
78.3
D
35
E
80
Slide 63 / 92
31 What is the boiling point of water at 300 torr?
A
200
B
50
C
100
D
75
E
Slide 64 / 92
Vapor Pressure
A liquid will boil when its vapor pressure equals atmospheric
pressure. A pressure cooker works by increasing the
"atmospheric" pressure inside it, so water will not boil at 100oC;
instead, it may be heated up to 120oC before turning to steam.
Raising the cooking temperature cuts cooking time drastically.
90
Slide 65 / 92
Vapor Pressure
Slide 66 / 92
Vapor Pressure
Recall that boiling occurs when
Pvap = Patm.
Since atmospheric pressure is
so low at high altitudes, (e.g.
top of Mount Everest) water will
boil at a much lower
temperature than here at
Bergen Tech.
http://www.washingtonpost.com/wp-dyn/content/graphic/2007/02/20/GR2007022000797.html
Patm = 33 kPa on Mt. Everest
Patm = 101.3 kPa at sea level
Slide 67 / 92
32 It will take longer to hard-boil
an egg
Slide 68 / 92
33
A volatile liquid is one that _________.
A
is highly flammable
A
on top of Mt. Everest
B
is highly viscous
B
here at Bergen Tech
C
cooking times are equal
C
is highly hydrogen-bonded
D
is highly cohesive
E
readily evaporates
Slide 69 / 92
34
If a liquid is sealed in a container and kept at
constant temperature, how does its vapor pressure
change over time?
Slide 70 / 92
35 Based on this figure, the boiling point of diethyl ether under an
external pressure of 1.32 atm is _______°C.
A
10
A
it rises at first, then remains constant
B
20
B
it rises as first, then falls
C
30
C
it remains constant
D
40
E
0
Slide 71 / 92
Phase Diagrams
Phase diagrams display the state of a substance at
various pressures and temperatures and the places
where equilibria exist between phases.
Slide 72 / 92
Volatility
The more volatile a liquid:
· The more quickly it evaporates
· The higher its vapor pressure at a given temperature
· The weaker its Intermolecular Forces
Slide 73 / 92
Phase Diagrams
· The circled line is the liquid-vapor interface.
· It starts at the triple point (T), the point at which all three
states are in equilibrium.
Slide 75 / 92
Phase Diagrams
· The circled line in the diagram below is the
interface between liquid and solid.
· The melting point at each pressure can be found
along this line.
Slide 77 / 92
Phase Diagram of Water
· Note the high critical temperature
and critical pressure.
· These are due to the strong van
der Waals forces between water
molecules.
Slide 74 / 92
Phase Diagrams
It ends at the critical point (C); above this critical
temperature and critical pressure the liquid and vapor
are indistinguishable from each other.
Slide 76 / 92
Phase Diagrams
· Below the triple point the substance cannot
exist in the liquid state.
· Along the circled line the solid and gas phases
are in equilibrium; the sublimation point at each
pressure is along this line.
Slide 78 / 92
Phase Diagram of Carbon Dioxide
Carbon dioxide cannot exist in the
liquid state at pressures below 5.11
atm; CO2 sublimes at normal
pressures.
· The slope of the solid-liquid line is
negative.
· This means that as the pressure
is increased at a temperature just
below the melting point, water goes
from a solid to a liquid.
The low critical temperature and
critical pressure for CO2 make
supercritical CO2 a good solvent for
extracting nonpolar substances (like
caffeine).
Slide 79 / 92
Phase Diagram of H2O and CO2
Slide 80 / 92
36
· The slope of the solid-liquid
line is negative.
· If you increase the pressure
at a given temperature, near
the freezing point, the ice will
melt.
· Water is the only known
substance with this behavior.
· The slope of the solid-liquid
line is positive, as it is for most of
the substances.
· if you increase the pressure at
a given temperature near -550C,
the liquid will freeze.
· Note that, since the Triple point
is at a high pressure, you will
see CO2 only subliming under
normal
condition.
Slide
81 atmospheric
/ 92
37
In this phase diagram, the substance is a
__________ at 25 °C and 1.0 atm
The phase diagram of a substance is shown to the
right. The region that corresponds to the solid phase
is _____.
A
w
B
x
C
y
D
x
E
x and y
Slide 82 / 92
38
On a phase diagram, the melting point is the same
as the __________
A
solid
A
triple point
B
liquid
B
critical point
C
gas
C
freezing point
D
supercritical fluid
D
boiling point
E
crystal
E
vapor-pressure curve
Slide 83 / 92
39
On the phase diagram shown to the right, segment
__________ corresponds to the conditions of
temperature and pressure under which the solid and
the gas of the substance are in equilibrium
A
AB
B
AC
C
AD
D
CD
E
BC
Slide 84 / 92
Solids
We can think of solids as falling into two groups
Crystalline, in which
particles are in highly
ordered arrangement.
Amorphous, in which
there is no particular order
in the arrangement of
particles.
Slide 85 / 92
Solids
Some examples of amorphous solids are: rubber, glass,
paraffin wax and cotton candy.
Slide 86 / 92
Types of Bonding in Crystalline
Solids
Crystalline solids include ionic compounds, metals and
another group called covalent-network solids. Crystalline
solids are categorized by bonding type as shown on the
next slide.
Slide 87 / 92
40 In a saturated solution of salt water, __________.
A
the rate of crystallization > the rate of solution
B
the rate of solution > the rate of crystallization
C
seed crystal addition may cause massive crystallization
D
rate of solution = rate of crystallization
E
addition of more water causes massive crystallization
Slide 88 / 92
Covalent-Network Solids
· Diamonds are an example of a covalent-network solid, in
which carbon atoms are covalently bonded to four other
carbon atoms.
· They tend to be hard and have high melting points.
Slide 89 / 92
Covalent-Network Solids
Slide 90 / 92
Metallic Solids
· Metals are not covalently
bonded, but the attractions
between atoms are too strong
to be van der Waals forces.
· Graphite is another example of a covalent-network
solid. Each carbon atom is covalently bonded to 3
others in layers of interconnected hexagonal rings.
· The layers are held together by weak dispersion
forces. The layers slide easily across one another, so
graphite is used as a lubricant as well as the "lead" in
pencils.
· In metals, valence electrons
are delocalized throughout the
solid. This means that the
"sea" of electrons moves freely
around all the nuclei.
Slide 91 / 92
Properties of Metallic Solids
The delocalized nature
of the electrons in
metals accounts for
many physical
properties.
For example, metals
are generally:
· good conductors of
heat and electricity
· malleable
· ductile, (i.e. may be
drawn into wires)
Slide 92 / 92