I.Intermolecular Forces

A.Intermolecular Forces are those that cause two or more molecules to stick together

Examples of molecules sticking together…

A stick of butter, clump of wax or drop of water

B.Intermolecular Forces are relativelyweak when compared to forces that hold atoms together in chemical bonds

Examples of atoms held together in a chemical bond…

  • Carbon and Oxygen atoms in Carbon Dioxide
  • Hydrogen and Oxygen atoms in a water molecule

Because intermolecular forces are weaker than chemical bonds, we can separate one molecule from another with relatively small amounts of energy, without taking atoms out of a molecule

What happens to butter when you warm it?

Are the atoms broken free, or molecules separated?

What happens to water when you boil it?

Are the atoms broken free, or molecules separated?

*Physical Changes involve Intermolecular Forces

*Chemical Changes involve Chemical Bonds

II.Types of Intermolecular Forces

A.Intermolecular Forces are often generally referred to as “van der Waals Forces”

van der Waals studied forces that bind molecules together(Nobel Prize in 1910)

  • van der Waals found that intermolecular forces involve Valence Electrons (like a chemical bond), but unlike chemical bonds, the electrons Can NotMove from one molecule to a different molecule.
  • Although Electrons stay within a single atom or molecule, they can still be attracted to other atoms or molecules. This is the reason for intermolecular attractions.

B.Four Specific Types of Intermolecular Forces

1.Dispersion Forces - a.k.a. London Forces (Weakest)

(named after Fritz London who first described these forces theoretically 1930)

Caused by the random motion of electrons within nonpolar molecules

  • ______regions of positive or negative charge
  • If many interactions are present, dispersion forces can be quite strong (paraffin wax)

2.Hydrophobic/Hydrophilic Interactions

  • Nonpolar molecules (oil) avoiding polar molecules (water)

3.Dipole Interactions(Stronger)

  • Caused by difference in “desire” for electrons among atoms
  • Results in a ______partial positive or negative charge in a molecule
  • partial positive ends attracted to partial negative ends of other molecules

4.Hydrogen Bonds (Strongest)

Hydrogen bonds take place in polar molecules. However molecules that undergo hydrogen bonding all contain a hydrogen atom that's bonded to a Nitrogen, Oxygen, or Fluorine atom. If there are no H-N, H-O, or H-F bonds, there is no hydrogen bonding. In a hydrogen bond, you'll notice that the atoms that hydrogen is bonded to each have lone pairs. Because the oxygen, fluorine, and nitrogen atoms all have these lone pairs, the lone pairs tend to be attracted toward the partial-positively charged hydrogen atoms on nearby molecules. Check out the diagram below:

III.Chemical Bonds are Stronger than Intermolecular Forces

Look at the energy required to break covalent vs. intermolecular forces…

Bond type / Relative Dissociation Energy
(kJ per mole)
Covalent / 400
Hydrogen bonds / 12–16
Dipole–dipole / 0.5–2
London (van der Waals) Forces / <1