Intermolecular forces - 2

  • Created by: Shannon
  • Created on: 08-05-14 15:40
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  • Intermolecular forces
    • Permanent dipoles
      • Polar molecules possess permanent dipoles
      • The slightly positive and slightly negative charges on the molecules cause weak electrostatic forces of attraction
        • Even though these forces are weak, they are stronger than London forces
    • Induced/ instantaneous dipoles (London forces)
      • Found between all atoms and molecules
      • Electrons in charge clouds are continuously moving
        • At any one time, they are more likely to be on one side of the atom than the other
          • This causes the atom to have a temporary dipole
            • This dipole can cause a temporary dipole in the opposite direction of a neighbouring atom
              • The two dipoles are then attracted to each other
      • London forces increase in strength as the molecule increases in size
        • Electron cloud increases in size
    • Hydrogen bonding
      • The strongest intermolecular force
      • Only happens when H covalently bonds to F.O.N
      • F.O.N are very electronegative
        • Pulls the bonding electrons away from the H
          • Polar bond formed
            • Hydrogen forms weak bonds with lone pairs of electrons on F.O.N atoms
              • Because charge density is high
    • Boiling and melting temperatures of alkanes with increasing chain length
      • Alkanes possess covalent bonds, which are held by London forces
        • The longer the carbon chain, the stronger the London forces
          • More molecular surface area and more electrons to interact
            • Electron cloud increases in size
        • Branched chain alkanes have smaller molecular surface areas, so the strenght of the London forces are reduced
    • High boiling temperatures of alcohols v alkanes
      • Alcohols will form hydrogen bonds
      • Hydrogen bonding increases boiling points because more energy is requires to break the interactions
    • Trends in boiling temperatures of hydrogen halides
      • H - F bond has the highest boiling temperature
        • The charge density is high, due to concentrated charges
          • Forces of attraction between neighbouring molecules is strong
            • Hydrogen bonding
      • Increases
        • Due to increasing strength on London forces
    • Solubility is affected by bonding
      • 2 types of solvent
        • Polar e.g water
        • Non-polar e.g hexane
      • Ionic substances dissolve in polar solvents, e.g water
        • Ions are attracted to oppositely charged ends
          • Pulled away from the ionic lattice, and become surrounded by water molecules
            • Hydration
      • Alcohols dissolve in polar solvents, e.g water
        • The polar -OH bond in the alcohol is attrached to the polar -OH bond in the water molecule
          • Hydrogen bonds form
      • Not all molecules with polar bonds dissolve in water
        • The dipoles within halogenoalkanes are not strong enough to form hydrogen bonds


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