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