Intermolecular Forces
and
Liquids and Solids
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
A phase is a homogeneous part of the system in
contact with other parts of the system but
separated from them by a well-defined boundary.
2 Phases
Solid phase - ice
Liquid phase - water
11.1
Intermolecular Forces
Intermolecular forces are attractive forces between molecules.
Intramolecular forces hold atoms together in a molecule.
Intermolecular vs Intramolecular
•
41 kJ to vaporize 1 mole of water (inter)
•
930 kJ to break all O-H bonds in 1 mole of water (intra)
“Measure” of intermolecular force
Generally,
intermolecular
forces are much
weaker than
intramolecular
forces.
boiling point
melting point
DHvap
DHfus
DHsub
11.2
Intermolecular Forces
Dipole-Dipole Forces
Attractive forces between polar molecules
Orientation of Polar Molecules in a Solid
11.2
Intermolecular Forces
Ion-Dipole Forces
Attractive forces between an ion and a polar molecule
Ion-Dipole Interaction
11.2
11.2
Intermolecular Forces
Dispersion Forces
Attractive forces that arise as a result of temporary
dipoles induced in atoms or molecules
ion-induced dipole interaction
dipole-induced dipole interaction
11.2
Intermolecular Forces
Dispersion Forces Continued
Polarizability is the ease with which the electron distribution
in the atom or molecule can be distorted.
Polarizability increases with:
•
greater number of electrons
•
more diffuse electron cloud
Dispersion
forces usually
increase with
molar mass.
11.2
What type(s) of intermolecular forces exist between
each of the following molecules?
HBr
HBr is a polar molecule: dipole-dipole forces. There are
also dispersion forces between HBr molecules.
CH4
CH4 is nonpolar: dispersion forces.
S
SO2
SO2 is a polar molecule: dipole-dipole forces. There are
also dispersion forces between SO2 molecules.
11.2
Intermolecular Forces
Hydrogen Bond
The hydrogen bond is a special dipole-dipole interaction
between they hydrogen atom in a polar N-H, O-H, or F-H bond
and an electronegative O, N, or F atom.
A
H…B
or
A
H…A
A & B are N, O, or F
11.2
Hydrogen Bond
11.2
Properties of Liquids
Surface tension is the amount of energy required to stretch
or increase the surface of a liquid by a unit area.
Strong
intermolecular
forces
High
surface
tension
11.3
Properties of Liquids
Cohesion is the intermolecular attraction between like molecules
Adhesion is an attraction between unlike molecules
Adhesion
Cohesion
11.3
Properties of Liquids
Viscosity is a measure of a fluid’s resistance to flow.
Strong
intermolecular
forces
High
viscosity
11.3
Water is a Unique Substance
Maximum Density
40C
Density of Water
Ice is less dense than water
11.3
Types of Crystals
Ionic Crystals
• Lattice points occupied by cations and anions
• Held together by electrostatic attraction
• Hard, brittle, high melting point
• Poor conductor of heat and electricity
CsCl
ZnS
CaF2
11.6
Types of Crystals
Covalent Crystals
• Lattice points occupied by atoms
• Held together by covalent bonds
• Hard, high melting point
• Poor conductor of heat and electricity
carbon
atoms
diamond
graphite
11.6
Types of Crystals
Molecular Crystals
• Lattice points occupied by molecules
• Held together by intermolecular forces
• Soft, low melting point
• Poor conductor of heat and electricity
11.6
Types of Crystals
Metallic Crystals
• Lattice points occupied by metal atoms
• Held together by metallic bonds
• Soft to hard, low to high melting point
• Good conductors of heat and electricity
Cross Section of a Metallic Crystal
nucleus &
inner shell emobile “sea”
of e-
11.6
Types of Crystals
11.6
An amorphous solid does not possess a well-defined
arrangement and long-range molecular order.
A glass is an optically transparent fusion product of inorganic
materials that has cooled to a rigid state without crystallizing
Crystalline
quartz (SiO2)
Non-crystalline
quartz glass
11.7
The boiling point is the temperature at which the
(equilibrium) vapor pressure of a liquid is equal to the
external pressure.
The normal boiling point is the temperature at which a liquid
boils when the external pressure is 1 atm.
11.8
The melting point of a solid
or the freezing point of a
liquid is the temperature at
which the solid and liquid
phases coexist in equilibrium
Freezing
H2O (l)
Melting
H2O (s)
11.8
PHASE TRANSISTIONS
ALSO CALLED CHANGES OF STATE, HAPPENS BY CHANGING THE TEMPERATURE
AND/OR PRESSURE OF A SUBSTANCE.
SOLID TO LIQUID: MELTING
LIQUID TO SOLID: FREEZING
GAS TO LIQUID: CONDENSATION
LIQUID TO GAS: EVAPORATION
SOLID TO GAS: SUBLIMATION
GAS TO SOLID: DEPOSITION
Intermolecular Forces
London Dispersion Forces:
Also called Induced
dipole forces. A dipole is a separation of charge. An
instantaneous dipole is created within the atom or
molecule via the instantaneous movement of the
electrons around the nucleus. All molecules have LDF.
Dipole-Dipole Forces:
The attractive force
between molecules due to the existence of an overall
dipole moment. Polar molecules have d-d forces.
Hydrogen Bonding:
The attractive force between
a highly electronegative atom of one molecule with the
hydrogen on another molecule also containing a very
electronegative atom. N, O, F are the electronegative
atoms.
Interrogating interactions
Why can bugs walk on water?
Why green strawberry plastic baskets
float on water despite the 1” open
square “holes”??
Properties dependent on the Intermolecular Forces
SURFACE TENSION: describes the resistance
that a liquid has to an increase in its own
surface area.
• Intermolecular forces create surface
tension – an invisible “skin” holding the
molecules together
• Force must be applied to break the “skin”
Properties dependent on the Intermolecular Forces
EVAPORATION:
Evaporation of a liquid occurs when the
average kinetic energy present within the
liquid is greater than the intermolecular forces
responsible for holding the substance in its
liquid state. When the particles have enough
kinetic energy to overcome these attractive
forces, the particles will escape from the
surface to become a gas.
Energy must be added to a system to overcome the attractive
forces that are exerted among liquid molecules. When an
equal quantity of vapor condenses to a liquid, an equal
amount of energy is released.
WHAT HAPPENS DURING BOILING??
Describe in terms of…
1) Molecular movement in liquids vs. gases?
2) Energy transfer
3) Forces (attractive and intermolecular forces)
Explain the physical process of boiling.
At room temperature the water molecules have
enough energy to allow the particles to move past each
other but not enough to escape the surface tension.
As the temperature of water increases, the heat energy
(from the burner) is transferred to kinetic energy (for
the molecules) leading to an increase in the molecular
motion of the molecules. This action results in an
increase in the vapor pressure above the surface of the
liquid.
When the vapor pressure of the water equals the external
pressure, boiling begins. Now a sufficient amount of
the molecules have enough energy to resist the
attractive forces. Bubbles of vapor are formed
throughout the liquid and these bubbles rise to the
surface to escape.
Properties dependent on the Intermolecular Forces
• VAPOR PRESSURE: When a liquid
evaporates in a closed container, the gaseous
vapor that forms at the surface of the liquid
eventually establishes an equilibrium with the
particles remaining in the liquid state. Equilibrium
is established when the rate of evaporation is
equal to the rate of condensation.
• A VOLATILE substance evaporates readily, has a low
surface tension, and a high vapor pressure at ambient
temperature. The Intermolecular forces are weak.
• A NONVOLATILE substance requires a large amount of
energy to evaporate, has a high surface tension, and a
low vapor pressure. The intermolecular forces are
strong.
THE DISSOLVING PROCESS
Intermolecular Forces: “Like dissolves like”

Miscible / Immiscible: Two liquids are miscible in each
other if they readily mix to form a uniform solution.
Two immiscible liquids will always separate out into
two distinct layers.
THE DICTIONARY GAME:
HYDRATES
HYGROSCOPIC
DELIQUESCENT
DESICCANTS
EFFLORESCENCE
HYDROPHOBIC /
HYDROPHILLIC
PROPERTIES ASSOCIATED WITH WATER
HYDRATES: Solids that contain water molecules as part of
their crystalline structure. The water in the hydrate is known
as the water of hydration or the water of crystallization.
HYGROSCOPIC: A substance is hygroscopic if it readily
absorbs water from the atmosphere and forms a hydrate.
DELIQUESCENT: A substance is deliquescent if it absorbs
water from the air until it forms a solution.
DESICCANTS: Compounds that absorb water and are used as
drying agents.
EFFLORESCENCE: The process by which crystalline
materials spontaneously lose water when exposed to air.
HYDROPHOBIC / HYDROPHILLIC: Hydrophobic refers to
nonpolar substances interacting with water. Hydrophillic
(water loving) is in reference to the interactions polar
substances have with water.
Some Physical Properties of Water
 Water is colorless, odorless, and tasteless.
 The normal boiling point is 100oC and the normal
melting point is 0oC.
 The heat of vaporization (DHvap) is 2259 J/g or 540
cal/g and the heat of fusion (DHfus) is 335 J/g or
80 cal/g.
 The vapor pressure of water at 20oC is 17.5 torr;
this is relatively low when compared to volatile ethyl
alcohol (43.9 torr) and very volatile ethyl ether (442.2 torr)
 The density of water at 4.0oC is 1.0 g/mL; the
density of ice at 0oC is 0.917 g/mL.
 The specific heat of water is 1.0 cal/g oC or 4.184
J/g oC.
The Unusual Properties of Water
 Water co-exists in all three states of matter naturally
on earth.
 The only common substance is a liquid at STP.
 As a solid, it is less dense than its liquid form, that is
“Ice floats”. Most substances contract upon
solidifying.
 It has a very high Heat Capacity. It stores a large
amount of energy with very little atomic or molecular
motion.
 It requires a lot of heat energy (enthalpy) to change
states.
 It has a high boiling point for such a low molecular
weight compound.
 It is a universal solvent, as a good dissolving medium
a large number of substances are soluble in water.
What do you and water have in
common??
Everyone is unique. What makes you so
UnUsUal?? Name one thing that makes you
different. How are you different than others??
http://www.humanmetrics.com/cgi-win/jtypes2.asp
Relate this to chemistry. Why are you different?
What determines your characteristics?
Relate this to water: What mAkEs WaTeR so
UnUsUal?? How does its chemical make-up
determine these characteristics?
Why is Water so unusual?
The fundamental explanation for water’s unusual properties relates to the
polarity of its bonds. Polarity describes the partial charge associated with
a bond or molecule. A polar bond or molecule has a charge distribution
present (one end positively charged and the other end negatively charged)
while a nonpolar bond or molecule has no distinct charge distribution
(neutral).
Water is composed of two polar covalent O-H bonds (the difference in
electronegativity is 1.4) arranged in a “bent” molecular geometry. Each
bond has a dipole moment pointing in an overall similar direction leading to
the existence of an overall dipole moment. The oxygen atom pulls the pair
of electrons closer towards itself (making it partially negative) and further
from the hydrogen atoms (making them partially positive).
-
+
This charge distribution allows the partially positive hydrogen atoms from one
molecule to be attracted to the partially negative oxygen atom of another
molecule. This strong interlocking network between neighboring molecules is
called HYDROGEN BONDING. The ability to form strong hydrogen bonds is
the main reason for water’s unusual properties.
Why does ice float in water?
Ice floats in its own liquid due to the intermolecular force,
hydrogen bonding. As water freezes, the molecular motion
of the molecules slow down and the partial positive end
(hydrogen) of one water molecule is attracted to the partial
negative end (oxygen) of another water molecule.
Combine this event with the bent shape of water and the
molecules become arranged in a 3-D hexagonal array.
This array creates pockets of vacuum (empty space) in the
lattice structure as well as a decrease in the number of
molecules per unit volume. The mass is directly related to
the number of molecules therefore, in the solid state, since
there are less particles then there must be less mass per
unit volume therefore the solid is less dense than the
liquid.
http://www.youtube.com/watch?v=HVT3Y3_gHGg
Explain if ice will float in ethyl alcohol
(d = 0.789 g/L)?
Ice would not float in pure ethyl alcohol
because the density of water is 1.000 g/mL
which is greater than 0.789 g/mL for ethyl
alcohol. Yet since ethyl alcohol also
undergoes a small degree of hydrogen
bonding, the sinking effect is not as
dramatic as it would be with a nonpolar
substance.
Why does water have a relatively high boiling
point?
Water has a relatively high boiling
point because of the amount of
intermolecular forces present. Water
experiences LDF (London Dispersion
Forces) and d-d (dipole-dipole) forces,
along with the additional attractive
force, Hydrogen bonding. A large
amount of heat energy is required to
break all of these forces in order for a
phase transition to occur, thus the high
boiling point.
Water as a universal solvent
• Water is called the universal solvent because of its ability to
dissolve many substances. The general solubility rule is “like
dissolves like”. Since water is a polar molecule it will dissolve other
polar substances as well as ionic compounds. Water will not dissolve
or mix with nonpolar substances therefore water is immiscible in
nonpolar substances.
• Description of how water dissolves an ionic salt (like
NaCl) on the molecular level?
Although the attractive force from the partial charge of a single
polar molecule is not as strong as the charge from an ion, it is
plausible that a multitude of polar molecules could react on a single
ion effectively. The positive end (H+) of several water molecules are
attracted to the negative end of the salt crystal (Cl-) while the
negative end of several water molecules (O2-) are attracted to the
positive end of the crystal (Na+). The ionic bonds of the crystal
are weakened by the solvating effect of the water molecules and
the ions break away from the bulk crystal. The large number of
water molecules in the container prevent the salt ions from recombining.