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Chapter 11
Intermolecular Forces, Liquids,
and Solids
Jeremy Wolf
3 Common States of Matter
Intermolecular Forces
• Forces that hold together molecules and
atoms (both electrically neutral species)
are referred to collectively as van der
Waals forces
– Dipole-dipole attraction
• Hydrogen bonding with O-H, N-H, and H-F
– London dispersion force
Dipole-Dipole
• Results from permanent dipoles
• OH, NH, and HF are particularly strong
London Dispersion Force
• Caused by chance instant where more electrons
are located on one side of the atom
• Leads to an induced dipole in neighboring atom
• Instantaneous dipole
Liquids
• Viscosity is a fluid's resistance to flow
– Strong attraction = high viscosity
– Low temperature = high viscosity
• Surface tension is a cohesive intermolecular
attraction that causes a liquid to minimize its
surface area
– intermolecular forces that bind similar molecules
together are called cohesive forces
– Intermolecular forces that bind a substance to a
surface are called adhesive forces
– Adhesive forces are responsible for liquids rising up
very narrow tubes (and water rising up the stems of
plants) through capillary action
Phase Changes
Energy changes vs. state of matter
Heating curve for water
The energy associated with melting a solid is called the heat of fusion; the energy
associated with vaporizing a liquid is called the heat of vaporization; and the
energy associated with the sublimation of a solid is called the heat of sublimation.
For a given substance, the heat of vaporization will always be greater than the
heat of fusion. Imparting enough energy to molecules to allow them to move past
one another takes less heat than separating the molecules completely
Vapor Pressure
• A substance with
relatively weak
intermolecular forces
will have a greater
tendency to escape
into the vapor phase
and will exhibit a higher
vapor pressure then a
substance with
stronger intermolecular
forces.
• A substance with a high
vapor pressure is said
to be volatile
Structures in Solids
• Amorphous solids do not have any order
and are heterogeneous in their
intermolecular forces
• Crystalline solids are highly ordered
– highly ordered network of a repeating unit
called a unit cell
– A crystal lattice is a three-dimensional array of
points called lattice points, each of which is
identical
– Primitive, Body-centered, Face-centered
Comparison of Packing
Efficiency
Packing Scheme
Coordination Number
Percent of Space Filled
Hexagonal close packing
12
74
Cubic close packing
12
74
Body-centered cubic
8
68
Primitive cubic
6
52
Bonding in Solids
• Molecular solids compounds held together by
dipole-dipole interactions and dispersion forces
• Covalent-network solids - giant molecules held
together by covalent bonds
• Ionic solids consist of ions held together by
electrostatic attractions
• Metallic solids consist of metal atoms held
together by a "sea" of delocalized (same energy
level) valence electrons