Intermolecular forces

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  • Created by: Jasmin
  • Created on: 23-01-14 09:52
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  • Intermolecular Forces
    • Forces between molecules
    • Much weaker than covalent bonds
    • London Forces
      • Van der Waals forces or dispersion forces
      • Exist between all molecular substances an also in monotomic rare gases like helium
      • Caused by instantaneous unequal electron distribution in an atom, which gives rise to a temporary or instantaneous dipole
      • Origin: instantaneous sipole-induced dipole interactions
      • Electrons in a covalent molecule osciallte within the covalent bonds
        • Causes an induced dipole in the opposite direction on a neighbouring atom, so the two dipoles attract
          • Induced dipole can cause further induced dipoles on the neighbouring atoms. Net result is a weak attractive force
            • More electrons in a molecule, the larger the induced dipoles and so the larger the London forces
            • Overall effect is for the atoms to be attracted to each other
              • Molecules with greater SAs have stronger London forces
              • Stronger London forces means HMP/HBP
                • Long chain alkanes have higher BP than short-chain alkanes
                • Straight chain alkanes have higher BP than branched alkanes
    • Permanent dipole-dipole forces
      • Occur between permanent dipoles in neighbouring polar molecules
        • Polar molecules have small opposite charges S+ and S-
        • The S+ on one molecule attracts the S- on another polar molecule
      • The S+ and S- charges on polar molecules cause weak electrostatic forces of attraction between molecules
    • Hydrogen bonds
      • H must be bonded directly to a highly electronegative atom - F, O, N
      • For H bonds to form there must be a lone pair of electrons on the highly electronegative atom
      • H-F, H-O and H-N bonds are strongly polar
      • S+ H atom is within the orbital of the non-bonding pair of electrons on the O atom. As if the H has formed a second weak covalent bond - H bond is at 180 degrees to the normal covalent bond between O and H
      • Strongest force
      • Higher BP and MP because of extra energy needed to break the H bonds
      • H20 molecules in ice are further apart on average, making ice less dense than liquid water
      • H-F-H bond angle 109.5 degrees F-H-F 180 degrees


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