Download Intermolecular forces

Intermolecular Forces and
Liquids and Solids
Chapter 11
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Acknowledgement
Thanks to The McGraw-Hill Companies, Inc. for allowing usage in the classroom.
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
Kinetic-Molecular Description
Intermolecular forces are attractive forces between molecules
Intramolecular forces hold atoms together in a molecule.
(covalent bond)
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
Generally,
force
intermolecular
boiling point
forces are much
melting point
weaker than
intramolecular
DHvap
forces.(only about
DHfus
15% as strong)
11.2
Types of Intermolecular Forces (IMF)
• Should actually be called Interparticulate Forces (molecules, ions, and/or
atoms)
• Ion - ion forces
• Ion-dipole forces
• Dipole-dipole forces
• Dispersion Forces
• Hydrogen Bonds
IMF determines the boiling point (b.p.), melting point(m.p.), vapor pressure,
solubility, viscosity, surface tension, etc.
Greater IMF greater b.p., m.p., solubility, surface tension , and viscosity; lower
vapor pressure.
Ion - ion forces: (lattice energy-ionic compound)
• Remember Chapter 9
• Force depends on the charge on the ions and
the distance separating the ions
• (STRONG FORCES)
E=k
Q +Qr
Intermolecular Forces
1. Ion-Dipole Forces
Attractive forces between an ion and a polar molecule
Example: ions in solution
Ion-Dipole Interaction
11.2
The strength of the interaction depends on the charge and
size of the ion and on the magnitude of the dipole moment
and size of the molecule.
Water molecules
Higher charge, smaller size (that is, higher charge density)  strong interaction
11.2
Intermolecular Forces
2. Dipole-Dipole Forces (permanent dipole moment)
Attractive forces between polar molecules
Orientation of Polar Molecules in a Solid
11.2
Intermolecular Forces
3. Temporary dipoles (Dispersion forces)
What attractive force occurs in nonpolar substances?
Attractive forces that arise as a result of temporary dipoles
induced in atoms or molecules; based on molar mass; the weakest
intermolecular force.
– Ion induced
– Dipole induced
– Instantaneous dipole
ion-induced dipole interaction
The electron distribution of atom is distorted
by the force exerted by the ions or polar
molecules.
dipole-induced dipole interaction
11.2
The likelihood of a dipole moment being induced depends not only
on the charge of the ion or the strength of the ddipole but also on
the polarizability of the atom or molecules.
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 (more
electrons), or size of
the atom.
Melting point increases as
the number of electrons in
the molecule increase.
11.2
London Dispersion Force (Dispersion force)
Polarizability allows gases containing atoms or nonpolar
molecules (e.g.He,N2) to condense.
At any instant it is likely that the atom has a dipole moment
created by a specific positions of electrons. This dipole
moment is called instantaneous dipole.
Induced dipoles interacting with each other. This type of
interaction produces dispersion forces, which arise as a result of
temporary dipoles induced in atoms or molecules, is
responsible for the condensation of nonpolar gases.
**Dispersion force exists between all species as long as they
have mass.
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
4. 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
H…A
A & B are N, O, or F
11.2
Hydrogen Bonds:
• Strength of H bonds: up to 40 kJ/mol
• Lots of H bonds = strong
• compare with strength of typical covalent bonds:
250 kJ/mole)
Why is the hydrogen bond considered a
“special” dipole-dipole interaction? Why water
has a high boiling point?
Decreasing molar mass
Decreasing boiling point
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
The intermolecular attraction tend
to pull molecules into liquids and
cause the surface to tighten like a
11.3
plastic film.
Properties of Liquids
Viscosity is a measure of a fluid’s resistance to flow.
Strong
intermolecular
forces
High
viscosity
CH2-OH
CH-OH
CH2-OH
11.3
Solid---crystal structure
A crystalline solid possesses rigid and long-range order. In a
crystalline solid, atoms, molecules or ions occupy specific
(predictable) positions. (e.g. NaCl)
An amorphous solid does not possess a well-defined
arrangement and long-range molecular order. (e.g. glass)
Structures of crystalline solids:
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
11.4
Figure 11.15
11.4
Corner atom
Shared by 8
unit cells
edge atom
Shared by 4
unit cells
face centered
atom
Shared by 2
unit cells
11.4
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)
11.4
11.4
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
1 mole Ag
107.9 g
-22 g
x
m = 4 Ag atoms x
=
7.17
x
10
mole Ag 6.022 x 1023 atoms
7.17 x 10-22 g
m
3
=
=
10.5
g/cm
d=
V
6.83 x 10-23 cm3
11.4
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
sp2
sp3
diamond
allotrope
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
amorphous solid
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
On top of a mountain
• Water boils at a lower temp. on top of a mountain than
at sea level.
• Why? because the external pressure (atmospheric
pressure) is less on top of a mountain.
In an autoclave
• An autoclave is used to sterilize medical instruments
• The pressure is often 2 atmospheres.
• Will water inside the autoclave boil at 100°C?
Molar heat of vaporization (DHvap) is the energy required to
vaporize 1 mole of a liquid at its boiling point.
Clausius-Clapeyron Equation
DHvap
ln P = +C
RT
P = (equilibrium) vapor pressure
T = temperature (K)
R = gas constant (8.314 J/K•mol)
Vapor Pressure Versus Temperature
11.8
Molar heat of fusion (DHfus) is the energy required to melt
1 mole of a solid substance at its freezing point.
11.8
Heating Curve
• Heat of fusion = energy
needed to convert a solid
to a liquid
• Heat of vaporization =
energy needed to
convert a liquid to a gas
• Be able to draw a
heating curve
Energy is needed to
• heat a solid
• heat a liquid
• heat a gas
• convert a solid to a
liquid
• convert a liquid to a gas
• convert a solid to a gas
11.8
phase diagram
A phase diagram summarizes the conditions at which a
substance exists as a solid, liquid, or gas.
Phase Diagram of Water
Triple point
All three
phases are in
equilibrium
Solid line:
Two phases are
in equilibrium
11.9