Reading Assignments:
Chapter 11 in
R. Chang, Chemistry, 8th Ed., McGraw-Hill,
2005
Or Related topics in other textbooks.
Intermolecular Forces and
Liquids and Solids
Consultation outside lecture room:
Office Hours:
Tuesday & Thursday
10 am -12 pm
Wednesday
1-4 pm
@Room 313-3 or by appointment
Chemistry for Engineers, SCS126
Intermolecular Forces
Chemistry for Engineers, SCS126
Intermolecular Forces
Intermolecular forces are attractive forces between
molecules.
Intramolecular forces hold atoms together in a molecule.
Dipole-Dipole Forces
Attractive forces between polar molecules
Intermolecular vs Intramolecular
Orientation of Polar Molecules in a Solid
•
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
Chemistry for Engineers, SCS126 11.2
Chemistry for Engineers, SCS126 11.2
Intermolecular Forces
Intermolecular Forces
Dispersion Forces
Ion-Dipole Forces
Attractive forces between an ion and a polar molecule
Attractive forces that arise as a result of temporary
dipoles induced in atoms or molecules
Ion-Dipole Interaction
ion-induced dipole interaction
dipole-induced dipole interaction
Chemistry for Engineers, SCS126 11.2
Chemistry for Engineers, SCS126 11.2
Intermolecular Forces
What type(s) of intermolecular forces exist between
each of the following molecules?
Dispersion Forces
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
CH4
Dispersion
forces usually
increase with
molar mass.
SO2
Chemistry for Engineers, SCS126 11.2
Chemistry for Engineers, SCS126 11.2
Intermolecular Forces
Water is a Unique Substance
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
Maximum Density
40C
Density of Water
H…A
A & B are N, O, or F
Ice is less dense than water
Chemistry for Engineers, SCS126 11.2
Why is the hydrogen bond considered a
“special” dipole-dipole interaction?
Decreasing molar mass
Decreasing boiling point
Chemistry for Engineers, SCS126
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
Chemistry for Engineers, SCS126 11.2
Chemistry for Engineers, SCS126 11.3
Properties of Liquids
Properties of Liquids
Cohesion is the intermolecular attraction between like molecules
Adhesion is an attraction between unlike molecules
Adhesion
Viscosity is a measure of a fluid’s resistance to flow.
Strong
intermolecular
forces
Hg
water
High
viscosity
Cohesion
Chemistry for Engineers, SCS126 11.3
Chemistry for Engineers, SCS126 11.3
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
Chemistry for Engineers, SCS126 11.4
Chemistry for Engineers, SCS126 11.4
Chemistry for Engineers, SCS126 11.4
Chemistry for Engineers, SCS126 11.4
Shared by 8
unit cells
Chemistry for Engineers, SCS126 11.4
Shared by 2
unit cells
Chemistry for Engineers, SCS126 11.4
Chemistry for Engineers, SCS126 11.4
Chemistry for Engineers, SCS126 11.5
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?
nλ = 2d sin θ
Extra distance = BC + CD = 2d sinθ = nλ
(Bragg Equation)
Chemistry for Engineers, SCS126
11.5
Chemistry for Engineers, SCS126 11.5
Types of Crystals
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
Covalent Crystals
• Lattice points occupied by atoms
• Held together by covalent bonds
• Hard, high melting point
• Poor conductor of heat and electricity
carbon
atoms
CsCl
ZnS
CaF2
Chemistry for Engineers, SCS126 11.6
diamond
Types of Crystals
Chemistry for Engineers, SCS126 11.6
graphite
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
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-
Chemistry for Engineers, SCS126 11.6
Chemistry for Engineers, SCS126 11.6
Types of Crystals
An amorphous solid does not possess a welldefined 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
Chemistry for Engineers, SCS126 11.6
Chemistry for Engineers, SCS126 11.7