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Permanent Dipole-Dipole

Permanent Dipole-Dipole

  • A molecule with a permanent dipole will have a weak electrostatic force of attraction between the δand>δ charges on the neighboring molecules, otherwise known as a permanent dipole-dipole force
  • Permanent dipole-dipole is the 2nd strongest intermolecular force, with Hydrogen bonds being the strongest, and Van der Waals being the weakest. It only acts between certain types of molecules
  • Molecules with a permanent dipole will experience dipole-dipole forces
  • It is found between molecules with a differing electronegativity, such as HCl. In HCl, the chlorine is more electronegative and thus will have a negative charge cloud around it (compared to the hydrogen atom) The result of this is that the strong negative chlorine will be attracted to the positive nucleus of the neighbouring Hydrogen in the HCl molecule.Dipole moments
    • Molecules that have polar bonds may also have a dipole moment
    • In molecules where there are more than one polar bond, the result of this is that the bonds may cancel each other out and thus there is no dipole moment. The effect also may add up and therefore each individual bond reinforces each other
    • It is greatly determined on the shape of the molecule
Van der Waals

Van der Waals

  • Van Der Waals is the weakest of all the intermolecular forces
  • All atoms and molecules have a positive and a negative charge despite being neutral overall
  • These charges result in the electrostatic attraction between all atoms and molecules, otherwise known as Van der Waals (VdW)
  • Larger molecules have larger electron clouds and therefore can create a larger dipole difference meaning there is a stronger Van der Waals
  • the shape of the molecule can also effect the Van der Waals, for example a long straight molecule can lie alongside another similar molecule thus the area of attraction is greater. This differs to a branched molecule as there is less surface area for attraction

Forming Van der Waals

  • The random movement of electrons in their charge clouds means naturally for a moment of time one side of a molecule will have more electrons compared to the other
  • The molecule now has a temporary dipole
  • The shift in charge causes another neighbouring molecule to be attracted to it cause it to have a temporary dipole, however in the opposite direction (as electrons repel from each). The result is a positive and a negative end on a molecule, with the positive end being attracted to the temporary dipole negative end.
  • The chain continues with the 2nd dipole going on to effect other molecules in the same fashion
  • As the movement of electrons is random the dipoles are created and destroyed all of the time, despite this the molecules still remain attracted to one another