Real Gases
• Ideal gas equation is only accurate for
moderate P and T
•At high Pressures, V is small and density rises
• Space occupied by molecules is no longer
negligible
• Attractive forces between molecules are important
Volume of molecules and thus size of
intermolecular forces are different for each gas
•Correction factors can be applied
to P and V for real gases
Intermolecular Forces I
• Three types of attractive forces exist between
neutral molecules
Dipole-Dipole Forces
• Electrostatic forces of attraction that act between
polar molecules, e.g. HCl
Intermolecular Forces II
Dispersion (London) Forces
• Weak attractive forces arising from temporary
dipoles induced in atoms or molecules
• Accounts for intermolecular forces between nonpolar species
• Polarisability is the ease with which the electron
distribution can be distorted
(Increases with atomic/molecular mass)
•boiling points of inert gases: He 4 K
Ar 87 K
Xe 131 K
• The principal intermolecular force in liquids and
accounts for boiling points
Dipole-dipole forces and London dispersion forces
are collectively called van der Waal’s Forces
Intermolecular Forces III
Hydrogen Bonding
• Strong dipole-dipole forces between H atom in a
polar molecule and a very electronegative atom in
another molecule
Only occurs for O, N and F
• Hydrogen bonding accounts for unusually
high boiling points of HF, NH3 and H2O
• Hydrogen bonding accounts for helix
structure in DNA
What type of intermolecular
forces are present?
Liquids
•Physical properties can be understood in terms of
intermolecular forces and molecular motion
Surface Tension
• Energy required to increase surface area of a liquid
by a unit amount
strong intermolecular forces = high surface tension
Viscosity
• The resistance to flow
(Viscosity increases as temperature decreases)
strong intermolecular forces = high viscosity
Solids
Type
Molecular
Metallic
Structural
Attractive
Melting
Examples
Units
Forces
Point
Atoms /
Inter -
Low
Ne, H2O, CO2
molecules
molecular
Atoms
Metallic
Variable
Fe, Cu, Ag
High
NaCl, ZnS
Very High
Diamond,
Bonding
Ionic
Ions
Ionic
Bonding
Covalent
Atoms
Covalent
Bonding
Graphite
Understand MACROSCOPIC properties on the
MOLECULAR scale
Metallic
bonding
in Copper
Ionic
bonding
in NaCl
Covalent
bonding
in Carbon
States of Matter
Physical Properties
State
Volume/Shape Density Compressibility
Molecular
Motion
Gas
Volume and
Low
High
shape of
Very free
motion
container
Liquid Definite volume
High
Slight
Shape of
Slide past each
other freely
container
Solid
Definite volume
and shape
High
None
Vibrate about
fixed positions
Understand MACROSCOPIC properties on
The MOLECULAR scale