Intermolecular Forces

Section 1: Understanding Intermolecular Forces

  • Intermolecular Forces (IMFs) are the forces of attraction between molecules.
  • These forces are weaker than intra molecular forces, i.e., the forces holding atoms together within a molecule.
  • There are three types of IMFs, namely London dispersion forces, dipole-dipole interactions, and hydrogen bonding.

Section 2: London Dispersion Forces

  • London dispersion forces, also known as Van der Waals forces, are the weakest type of IMFs.
  • They exist between all types of molecules, polar or non-polar.
  • These forces arise due to temporary fluctuations in electron distribution, which create temporary dipoles.
  • Greater the molar mass of a molecule, stronger the London dispersion forces as more electrons mean greater fluctuations in electron distribution.

Section 3: Dipole-Dipole Interactions

  • Dipole-dipole interactions occur between molecules that have a net dipole moment, i.e., between polar molecules.
  • These forces arise due to the electrostatic attraction between the positive end of one molecule and the negative end of another molecule.
  • Dipole-dipole forces are stronger than London Dispersion forces but weaker than hydrogen bonding.

Section 4: Hydrogen Bonding

  • Hydrogen bonding is the strongest type of IMF and it occurs in molecules in which hydrogen is bonded to a highly electronegative atom (e.g., N, O or F).
  • It results from a large difference in electronegativity values causing a larger dipole moment.
  • Hydrogen bonds are responsible for the unique properties of water such as high boiling point and high heat capacity.

Section 5: Understanding the Impact of IMFs on Molecular Properties

  • The type and strength of IMFs affect various physical properties of substances like boiling and melting points, viscosity, and surface tension.
  • Higher the strength of IMFs, higher the boiling and melting points, as more energy is required to break the attractions between molecules.
  • Substances with stronger IMFs tend to have higher viscosities and surface tensions as the molecules are more strongly attracted to each other.

Remember, a good grasp of intermolecular forces will allow you to predict and explain the physical properties of molecules and how these properties influence a substance’s behaviour.