Intermolecular Bonds

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  • Created by: bob
  • Created on: 12-05-13 18:38

Permanent Dipoles

  • Formed when two atoms in a bond have substantially different electronegativites
  • The more electronegative atom attracts the electrons more, so it becomes slightly negative.
  • The other atom in the bond becomes slightly positive due to the absence of electrons
  • This forms a polar bond.
  • This can then go on to attract other permanent dipoles, forming a permanent dipole - permanent dipole intermolecular bond.
  • A dipole in a molecule could also induce a dipole in a neighbouring molecule.
  • The delta negative part of the molecule repels the electrons on the neighbouring molecule away, so causing it to be slightly positive.
  • The slightly positive end of one molecule and slighly negative end of the other molecule are then attracted towards each other.
  • This forms a permanent dipole - induced dipole bond.
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Instantaneous Dipoles

  • Electrons in a molecule randomly move around the molecule.
  • At one point they may be unevenly distributed - one side may be more concentrated with electrons than the other
     - this causes that side to be slightly negative and the other side to be slightly positive. 
  • At this instant, this creates a dipole in the molecule
  • This can then induce a dipole in a neighbouring molecule (the delta negative side repels electrons on the neighbouring molecule so making it slightly positive)
  • These two molecules then attract each other, forming an instantaneous dipole - induced dipole bond.
  • Larger atoms form stronger instantaneous dipole - induced dipole bonds
  • They have more electrons so can create more positive/negative areas of charge
  • The induced dipole is therefore also stronger
  • This leads to greater attraction between the molecules, so take more energy to break.
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Hydrogen Bonds

  • Forms when a hydrogen is attached to a small and electronegative N/O/F atom, which draws the bonding electrons towards itself (due to it's electronegativity), meaning it becomes quite negative (a high delta negative charge)
  • This causes the hydrogen to become very positive.
  • The delta positive hydrogen atom is then attracted to a lone pair on a delta negative N/O/F atom attached to a neighbouring molecule.

  • Hydrogen bonding is the strongest type of intermolecular bonding (apart from giant covalent network/ionic lattice), and so molecules with hydrogen bonds will have high boiling points.
  • Molecules which can form hydrogen bonds with water are usually soluble
    - too few OH groups = cannot form enough hydrogen bonds to break the molecules apart
    - too many OH groups = water cannot break so many hydrogen bonds apart so it is insoluble.
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