Liquids and Solids
• Intermolecular Forces
General Chemistry I
• Properties of Liquids
• Crystal Structure
2302101
• X‐ray Diffraction by Crystals
• Types of Crystals
Dr Rick Attrill
Office MHMK 1405/5
• Amorphous solids • Phase Changes and Phase Diagrams
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Intermolecular Forces
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.
Intermolecular forces are attractive forces between molecules.
Intramolecular forces hold atoms together in a molecule.
2 Phases
Intermolecular vs Intramolecular
Solid phase - ice
Liquid phase - water
•
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
Hvap
Hfus
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Hsub
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Intermolecular Forces
Intermolecular Forces
Dipole-Dipole Forces
Ion-Dipole Forces
Attractive forces between polar molecules
Attractive forces between an ion and a polar molecule
Orientation of Polar Molecules in a Solid
Ion-Dipole Interaction
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Intermolecular Forces
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Intermolecular Forces
Dispersion Forces
Dispersion Forces Due to Temporary Fluctuating Dipoles
A molecule approaches a molecule with
an instantaneous dipole.
Attractive forces that arise as a result of temporary
dipoles induced in atoms or molecules
The approaching molecule now has an
induced dipole due to the proximity of
the original temporary dipole.
ion-induced dipole interaction
An instant later the electrons in the left
hand molecule move to the other end.
They will then repel the electrons in the
right hand molecule.
dipole-induced dipole interaction
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If molecules are close enough together
this movement of electrons can occur
over a large number of molecules.
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Intermolecular Forces
What type(s) of intermolecular forces exist between
each of the following molecules?
Dispersion Forces Continued
Polarizability is the ease with which the electron distribution
in the atom or molecule can be distorted.
HBr
Polarizability increases with:
•
greater number of electrons
•
more diffuse electron cloud
HBr is a polar molecule: dipole-dipole forces. There are
also dispersion forces between HBr molecules.
CH4
CH4 is nonpolar: dispersion forces.
S
Dispersion forces
usually increase with
molar mass.
SO2
O
O
SO2 is a polar molecule: dipole-dipole forces. There are
also dispersion forces between SO2 molecules.
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Intermolecular Forces
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Why is the hydrogen bond considered a
“special” dipole-dipole interaction?
Hydrogen Bond
The hydrogen bond is a special dipole-dipole interaction
between the 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
Decreasing molar mass
Decreasing boiling point
H…A
A & B are N, O, or F
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Properties of Liquids
Properties of Liquids
Surface tension is the amount of energy required to stretch
or increase the surface of a liquid by a unit area.
Cohesion is the intermolecular attraction between like molecules
Adhesion is an attraction between unlike molecules
Adhesion
Strong
intermolecular
forces
High
surface
tension
Cohesion
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Properties of Liquids
Water is a Unique Substance
Viscosity is a measure of a fluid’s resistance to flow.
Maximum Density
40C
Strong
intermolecular
forces
Density of Water
Ice is less dense than water
High
viscosity
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A crystalline solid possesses rigid and long-range order. In a
crystalline solid, atoms, molecules or ions occupy specific
(predictable) positions.
An amorphous solid does not possess a well-defined
arrangement and long-range molecular order.
A unit cell is the basic repeating structural unit of a
crystalline solid.
At lattice points:
lattice
point
Unit Cell
Unit cells in 3 dimensions
•
Atoms
•
Molecules
•
Ions
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Shared by 8
unit cells
Shared by 2
unit cells
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Closest Packing
1 atom/unit cell
2 atoms/unit cell
4 atoms/unit cell
(8 x 1/8 = 1)
(8 x 1/8 + 1 = 2)
(8 x 1/8 + 6 x 1/2 = 4)
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Close packed layer
of spheres
(layer A)
Hexagonal closed
packed (hcp) structure
(ABA)
Cubic close-packed
(ccp) structure
(ABC)
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When silver crystallizes, it forms face-centered cubic
cells. The unit cell edge length is 409 pm. Calculate
the density of silver.
d=
m
V
V = a3 = (409 pm)3 = 6.83 x 10-23 cm3
4 atoms/unit cell in a face-centered cubic cell
m = 4 Ag atoms x
d=
1 mole Ag
107.9 g
x
= 7.17 x 10-22 g
mole Ag 6.022 x 1023 atoms
7.17 x 10-22 g
m
=
= 10.5 g/cm3
V
6.83 x 10-23 cm3
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Extra distance = BC + CD = 2d sin = n
(Bragg Equation)
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X rays of wavelength 0.154 nm are diffracted from a
crystal at an angle of 14.170. Assuming that n = 1,
what is the distance (in pm) between layers in the
crystal?
n = 2d sin 
d=
n=1
 = 14.170  = 0.154 nm = 154 pm
n
1 x 154 pm
2sin
=
2 x sin14.17
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
= 77.0 pm
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Types of Crystals
CsCl
ZnS
CaF2
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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
Molecular Crystals
• Lattice points occupied by molecules
• Held together by intermolecular forces
• Soft, low melting point
• Poor conductor of heat and electricity
carbon
atoms
diamond
graphite
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Types of Crystals
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-
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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
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T2 > T1
Condensation
Evaporation
Least
Order
Greatest
Order
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The equilibrium vapor pressure is the vapor pressure
measured when a dynamic equilibrium exists between
condensation and evaporation
H2O (l)
H2O (g)
Dynamic Equilibrium
Rate of
condensation
Rate of
= evaporation
Before
Evaporation
At
Equilibrium
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Molar heat of vaporization (Hvap) is the energy required to
vaporize 1 mole of a liquid.
Clausius-Clapeyron Equation
ln P = -
Hvap
RT
P = (equilibrium) vapor pressure
+C
T = temperature (K)
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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.
R = gas constant (8.314 J/K•mol)
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The critical temperature (Tc) is the temperature above which
the gas cannot be made to liquefy, no matter how great the
applied pressure.
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
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Freezing
H2O (l)
Melting
H2O (s)
The critical pressure
(Pc) is the minimum
pressure that must be
applied to bring about
liquefaction at the
critical temperature.
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Molar heat of fusion (Hfus) is the energy required to melt
1 mole of a solid substance.
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A phase diagram summarizes the conditions at which a
substance exists as a solid, liquid, or gas.
Phase Diagram of Water
Molar heat of sublimation
(Hsub) is the energy required
to sublime 1 mole of a solid.
Deposition
H2O (g)
Sublimation
H2O (s)
Hsub = Hfus + Hvap
( Hess’s Law)
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