Why are intermolecular forces important?

1 Answer
Jul 5, 2018

As mentioned here, intermolecular forces (IMFs) are important because they are the leading cause for differences in physical properties between similar molecules.

Be sure to read the linked answer to review if you are unfamiliar with IMFs.

Physical properties commonly discussed when relating to IMFs in pure substances are:

  • Melting and boiling points - when molecules go from solid to liquid or liquid to gas.
  • Vapor pressure - the pressure exerted by gases onto the walls of the container
  • Enthalpy of vaporization - energy needed at constant pressure to turn a liquid to gas
  • Viscosity - thickness of a liquid when it comes to fluid flow
  • Surface tension - resistance to distortion at the surface of the liquid from poking

The core principle is that the stronger the IMFs in the sample of molecules, the more strongly they interact, which means they stick together more.

That leads to the following trends:

  • Stronger IMF #-># Higher melting and boiling points (harder to melt and boil)
  • Stronger IMF #-># Lower vapor pressure (harder to boil)
  • Stronger IMF #-># Higher enthalpy of vaporization #DeltaH_(vap)# (takes more energy at constant atmospheric pressure to turn liquid to gas)
  • Stronger IMF #-># Higher viscosity (a thicker liquid, flows more like molasses)
  • Stronger IMF #-># Higher surface tension (more resistant to deformation from poking)