Condensed Phases
Explaining the physical properties of
gases was simplified since the
interactions between the gas molecules
could be ignored.
However the particles in a liquid or solid
are fairly close together and their mutual
interactions cannot be ignored.
Liquids and solids are called
CONDENSED PHASES
and intermolecular forces will play a
major role in explaining their physical
properties.
Non-Bonding Interactions
Chemical Bonds:
Intramolecular interactions
Covalent: Sharing of atomic electrons
Ionic: Transfer of atomic electrons and
subsequent attraction
Non-Bonding interactions do not result
in the formation of a chemical bond.
These are intermolecular interactions
arising from intermolecular forces.
Ion-dipole
Dipole-dipole
London dispersion forces
Hydrogen bonding
Ion-Dipole Interactions
A Coulombic Interaction between an ion
and the dipole moment of a molecule.
Dipole moments occur when the charge
distribution in a molecule is asymmetric.
Molecules with a dipole moment are
called polar molecules.
Dipole-Dipole Interactions
Occur between two polar molecules
Not as strong as ion-dipole interaction
Depends on the magnitude of the dipole
moments
Manifested in the boiling points of polar
molecules
London Dispersion Forces
Accounts for intermolecular forces in
atoms and non-polar molecules
Attractive force between the induced
dipole moments
Strength depends on the polarizability of
the atom or molecule
Polarizability depends on the size of the
atom or molecule
Hydrogen Bonding
Strongest of the intermolecular forces
Occurs when a hydrogen atom lies
between two strongly electronegative
atoms
Results when the positive charge on the
hydrogen atom is attracted to the lone
pair electrons on the electronegative
atom
The electronegative atoms are usually
F, O, N
Boiling Point
The temperature at which the vapor
pressure of the liquid is equal to the
external pressure acting on the liquid
Properties of Liquids
Viscosity: a measure of resistance that one
part of a fluid offers to the flow of another
part
Depends on the magnitude of the attractive
intermolecular forces in the liquid
Measured in units of poise or centipoise
Inversely proportional to temperature
Surface Tension: energy required to increase
the surface area of a liquid by a given
amount.
Measured in J/m2
Depends on the magnitude of the attractive
intermolecular forces in the liquid
Adhesion: interaction between the
molecules of the liquid and the surface
of the container
Depends on the magnitude of the
intermolecular forces between the
molecules in the liquid and the atoms in
the surface of the container
Shape of meniscus is the result of a
competition between the attractive forces
between the fluid molecules and those
between the fluid and the surface of the
container
Water: meniscus curved upwards
Mercury: meniscus curved downwards
Capillary action
Vapor Pressure
Vapor pressure of a liquid is the pressure
exerted by the vapor in equilibrium with the
liquid. Equilibrium is dynamic.
Vapor pressure
Increases with increasing temperature
Liquids that evaporate readily are
volatile
Phase Changes
Associated energy changes
Enthalpy of Vaporization
∆HVap = HVapor − H liquid
Enthalpy of Fusion
∆H Fusion = H liquid − H solid
Clausius-Claperon Equation
ln P = −
∆HVap
RT
+ cons tan t
∆HVap = − slope x R
Vapor pressure vs T data for ethanol
∆H Vap ( CH 3CH 2OH ) = +38.56kJ / mol
Phase Diagrams
Graphical summary of the T&P at which
a given phase of a substance is stable
AB
Vapor pressure curve of liquid
AC
Vapor pressure curve of solid
AD
Variation of melting point with
Pressure
Point A Triple point
Structure of Solids
Crystalline: Atoms, ions or molecules are
ordered in well defined arraignments
External morphology: Flat surfaces or
faces that make definite angles with one
another
Quartz, Diamond, Amethyst, etc.
Amorphous: Atoms, ions or molecules
have no orderly relationship to one
another
Glass, rubber, etc.
Unit Cell
The order of a crystalline solid allows
one to describe it in terms of
Crystal Lattice: a three dimensional array
of points, each of which represents a
identical environment in the crystal
Unit Cell: the repeating unit of the
crystal
Form the entire crystal by repeating the
contents of the unit cell on the crystal
lattice
Unit Cells
Only 7 types allowed by symmetry
Simplest is cubic
All sides are of the same length and all
faces make angles of 90 degrees with
one another
Three types of cubic unit cells
Primitive cube
Body-centered cubic
Face-centered cubic
Unit Cells (Continued)
Some atoms in a unit cell are shared
between adjacent cells
Crystal Structure of NaCl
Density of LiF
Face centered cubic unit cell
Sides are 4.02 Angstroms
What is the density of solid LiF?