INTERMOLECULAR FORCES
States of matter
 Physical properties of liquids and solids are due to
intermolecular forces
 Gases are highly compressible and assume the shape and
volume of their container.
o Gas molecules are far apart and don’t interact much with
each other
 Liquids are almost incompressible and assume the shape but not
the volume of their container
o Liquid molecules are closer to each other and interact
 Solids are incompressible and have a definite shape and volume
 Solids and liquids are condensed phases
Intermolecular forces
 Covalent bonds are intramolecular forces
 Intermolecular forces are much weaker than intramolecular
forces
 When a substance melts or boils (changes state) intermolecular
forces are broken
 Boiling points reflect intermolecular force strength. A high
boiling point indicates strong forces
 Van der Waals forces are intermolecular forces between neutral
molecules
Ion dipole forces
 An interaction between an ion and the partial charge on a
polar molecule
 It is the strongest of all intermolecular forces
 NaCl in water is a good example
Dipole-Dipole forces
 Forces that exist between neutral polar molecules
 The partially positive area attracts the partially negative area
 Generally weaker than ion-dipole
 Higher dipole moment= higher boiling point
London dispersion forces
 Weakest of intermolecular forces
 Attraction of adjacent nonpolar molecules
 Electron distribution creates a temporary dipole( instantaneous).
This dipole induces an adjacent molecule. That then attract each
other
 London dispersion forces exist among all molecules
 Effected by:
o Proximity
o Polarizability is the eases that the electron cloud can be
deformed. The larger a molecule, the more polarizable
o Molecule shape
Hydrogen bonds
 A special type of dipole-dipole interaction
 H bonding requires:
o H bonded to an electronegative element(F,O and N)
o It also requires an unshared pair of electrons on a nearby
electronegative ion or element
 Electrons in the H-X bond lie closer to X
 Hydrogen bonds are very important in biological systems
Viscosity
 Resistance of a liquid to flow
 Depends on:
o Intermolecular forces. Higher = more viscous
o Tendency of molecules to become tangled
o Temperature lower = more viscous
Surface tension
 Bulk molecules (those in the liquid) are equally attracted to
their neighbors
 Surface molecules are attracted to the bulk molecules.
Therefore they are more closely packed. This causes the liquid
to behave like it has a skin.
 Surface tension is the amount of energy required to increase the
surface area of a liquid by a unit amount
 Strong intermolecular forces cause high surface tension. Water
with hydrogen bonding, Hg even stronger with metallic bonds
 Cohesive forces bind molecules to each other
 Adhesive forces bind molecules to a surface
 Meniscus is the shape of a liquid surface. If adhesive forces are
greater, the liquid will be more attracted to the container and
will form a U
 Capillary action is the movement up narrow tubes. Liquid will
climb until adhesive and cohesive forces are balanced by
gravity
Phase changes
 Phase changes are changes of state
o Sublimation: solid to gas, endothermic
o Melting or fusion: solid to liquid, endothermic, heat of
fusion
o Vaporization: liquid to gas, endothermic, heat of
vaporization
o Deposition: gas to solid, exothermic
o Condensation: gas to liquid, exothermic
o Freezing: liquid to solid, exothermic
 Critical temperature is the highest temp at which a substance
can exist as a liquid
 Critical pressure is the pressure required for liquefaction at the
critical temp
Vapor pressure
 Some molecules on the surface of a liquid have enough energy
to escape
 These molecules move to the gas phase
 As the gas phase increases, some molecules return to the liquid
 When the pressure becomes constant, this is the equilibrium
vapor pressure
 Liquids that evaporate easily are termed volatile
Phase diagrams
 Plot of temperature vs pressure
 Triple point is the temp and pressure at which all 3 phases are
in equilibrium
Bonding in solids
 There are 4 types of solids
o Molecular
o Covalent network
o Ionic
o Metallic
 Molecular
o Atoms or molecules held together by intermolecular
forces
o Weak forces yield low melting points
o Poor thermal and electrical conductivity
o CH4 , sucrose
 Covalent network
o Atoms held together in large networks by covalent bonds
o Higher melting points
o Diamond,graphite, quartz
 Ionic solids
o Ions held together by ionic bonds
o Hard, brittle, high melting point
o Sodium chloride
 Metallic solids
o Consist entirely of metal atoms
o Can be soft or hard
o High melting points
o Good conductors
o Ductile
o Al, Cu, Fe, Au
o Metal ions float in a sea of delocalized electrons