6.3 - Intermolecular forces

• Intermolecular forces are weak interactions between dipoles of different molecules. Intermolecular forces fall into 3 main categories:

  • Induced dipole-dipole interactions (London forces)
  • Permanent dipole-dipole interactions
  • Hydrogen bonding
• Out of the intermolecular force: Hydrogen is the strongest and London forces are the weakest
• London forces are weak intermolecular forces that exist between all molecule, polar or non-polar. They act between induced dipoles in different molecules
• • movement of electrons produces a changing dipole in a molecule
  • At any instant an instantaneous dipole will exist, but its position is always shifting
  • The instantaneous dipole induces a dipole on the neighbouring molecule
  • The induced dipole further induces dipoles on neighbouring molecules, which then attract one another
• London forces forming

  A) induced dipole

  B) induced dipole-dipole interaction

  C) instantaneous dipole

  
• Induced dipoles are only temporary. In the next instant of time, the induced dipoles may disappear, only for the whole process to take place amongst other molecules
• Induced dipoles result from interactions of electrons between molecules. The more electrons in each molecule:

  • The larger the instantaneous and induced dipoles
  • The greater the induced dipole-dipole interactions
  • the stronger the attractive force between molecules
• • Larger numbers of electrons mean larger induced dipoles
  • More energy is then needed to overcome the intermolecular forces, increasing B.P
• • Fluorine molecules are non-polar and only have London forces between molecules
  • Hydrogen Chloride molecules are polar and have London forces, and permanent dipole-dipole interactions between molecules
  • Extra energy is needed to break the additional permanent dipole-dipole interactions between hydrogen chloride molecules.
  • Therefore the boiling point go hydrogen chloride is higher then Fluorine

  
• A simple molecular substance is made up of small units containing a definite number of atoms with a definite molecular formula
• In the solid state, simple molecules form a regular structure called a simple molecular lattice:

  • the molecules are held in place by weak intermolecular forces
  • the atoms within each molecule are bonded together strongly by covalent bonds
• Non-polar simple molecular substances tend to be soluble in non-polar solvents
• Simple molcular substances tend to be insoluble in polar solvents
• Simple molecular structures are non-conductors of electricity