Lesson 7: Intermolecular Forces

• Intermolecular Forces (IMF)

  Relatively weak forces of attraction that occur between molecules, weaker than intramolecular forces (covalent bonds)
• Water has a high surface tension due to its strong hydrogen bonds
• Intermolecular Forces


  • Directly related to physical properties like melting, boiling and freezing points, as well as state of matter
  • The closer atoms are together, the stronger the intermolecular forces
• Changes in state of molecular substances are due to changes in the forces among molecules, not within molecules
• When water changes phase

  1. H2O(s) -> H2O(l) -> H2O(g)

  2. Covalent bonds are not broken, rather IMF are overcome

  3. As it changes phase, the intermolecular forces are overcome

  4. Intramolecular forces do not change, more energy is needed to break intramolecular forces
• Dipole-Dipole Forces


  The attraction between oppositely-charged ends of polar molecules, only occur within polar substances
• Solubility effects of "like dissolves like"

  • Polar solutes dissolve well in polar solvents
  • Nonpolar solutes dissolve well in nonpolar solvents
  • Ionic compounds dissolve well in polar solvents, due to ion-dipole attractions
• Unlike substances do not dissolve well in each other, due to lack of attractions
• Solute

  What gets dissolved
• Solvent

  Dissolves the solute
• Ionic compounds dissociate in water to form ions, and ions are attracted to the positive and negative dipoles of water
• Hydrogen Bonds


  Very strong dipole-dipole forces between molecules containing H-F, H-O or H-N bonds, due to the high electronegativity of F, O and N
• Hydrogen Bonds


  • Result from the attraction of a covalently-bonded hydrogen to the lone pair of electrons on F, O or N
  • Hydrogen bonds are the strongest intermolecular force due to the high electronegativities
• Physical Properties of Hydrogen Bonding

  • Responsible for the high melting and boiling points of water
  • Explains why ice is less dense than water, as the water molecules spread out to maximize hydrogen bonding
  • Responsible for freezing (liquid to solid), with water having a freezing point of 0 Celsius, unlike other compounds that get denser when frozen
• London Dispersion Forces (LDF)


  The attraction between molecules (or atoms) due to the formation of temporary dipoles, the only IMF between nonpolar molecules and noble gas atoms

  
• London Dispersion Forces


  • On average, bonding electrons in nonpolar molecules are equally shared, but at any given instant, they may be closer to one nucleus, creating a temporary dipole
  • This temporary dipole can induce a temporary dipole in an adjacent molecule, resulting in temporary dipole-dipole attractions
• Effects of London Dispersion Forces

  • LDFs increase as molecular mass increases, because larger molecules have more electrons and a greater chance of temporary dipoles
  • LDFs are greater in straight-chain molecules than in more spherical, branched molecules, as the straight-chain has more surface area for interactions
  • In large molecules, LDFs are quite significant and more important than dipole-dipole forces
• Intermolecular Forces are Additive
• Comparing isoelectronic substances

  • If one substance is polar, it will have a higher boiling point than a nonpolar, isoelectronic substance, since it has 2 types of IMFs: London dispersion and dipole-dipole
  • If one substance has hydrogen bonding, its boiling point will be highest, since it has all 3 IMFs
• Polar substances will have a higher boiling point due to more intermolecular forces
• more electrons usually mean a higher boiling point because more electrons typically mean a greater molecular mass, so there is a greater chance of temporary dipole induction
• Intermolecular forces can explain:

  • melting/boiling points
  • surface tension
  • capillary action
  • vapor pressure
  • miscibility
  • differences between ideal and real gases
• the surface tension of a substance depends on the strength of intermolecular forces. therefore, a substance with stronger intermolecular forces will have a higher surface tension
• capillary action is the movement of a liquid up a tube. a substance with stronger adhesive forces will be attracted to the tube and move up the tube more than a substance with weaker adhesive forces. since water has stronger adhesion than mercury, it has a greater capillary action
• vapor pressure is measured in a closed system, so if a liquid is in a closed system, it will be the amount of pressure exerted by the gases, so it will jump from a liquid to a gas state. the higher the vapour pressure is, the more volatile the liquid is. for example, alcohol is very volatile because a lot of it jumps out of the liquid phase and becomes a gas due to its weaker intermolecular forces. TLDR: weaker intermolecular forces, higher vapor pressure