Chemical Bonding - Intermolecular and Intramolecular Forces

Intermolecular vs. Intramolecular Forces

Inter vs. Intra Prefixes

  • Inter: Between two things (e.g., interstate - between states).
  • Intra: Within one thing (e.g., intramurals - within a school).

Intermolecular Forces

  • Forces between molecules.
  • Do not create new compounds.
  • Affect the properties of a compound (e.g., boiling point, melting point).
  • Generally weaker than intramolecular forces.
  • Stronger intermolecular forces lead to higher boiling and melting points and vice versa.
  • Example: Attraction between two water molecules.

Intramolecular Forces

  • Forces within molecules or formula units.
  • Involve covalent and ionic bonding.
  • Form compounds.

Example

  • Intermolecular force: Attraction between two separate molecules.
  • Intramolecular force: Forces holding atoms together to form a molecule.

Types of Intermolecular Forces

1. Dipole Forces

  • Occur between polar molecules.
  • Polar molecule: Has a positive and negative end (like a AA battery).
  • Water (H_2O) as an example:
    • Bent shape due to non-bonding pairs.
    • Oxygen (O) side acts like it has a partial negative charge (\delta^-(delta negative)).
    • Hydrogen (H) side acts like it has a partial positive charge (\delta^+(delta positive)).
    • Water has two polar bonds and is a polar molecule.
  • Dipole force: Attraction between the partial positive end of one polar molecule and the partial negative end of another.
    • Example: Attraction between the O (\delta^-) of one water molecule and the H (\delta^+) of another.
  • Also explains why ionic compounds dissolve well in water (attraction between ions and polar water molecules).

2. Hydrogen Bonds

  • A special type of dipole force.
  • Occurs between a hydrogen atom bonded to nitrogen (N), oxygen (O), or fluorine (F) and another N, O, or F atom.
  • Example: Ammonia (NH3) dissolved in water (H2O).
    • Attraction between the negative O in water and the positive H in ammonia.
  • Requires a Hydrogen atom bonded to Fluorine, Oxygen, or Nitrogen (FON).
  • Dipole vs. Hydrogen Bond example: Hydrochloric acid (HCl) in water.
    • Although a dipole force exists (partial charges), it is not a hydrogen bond because the H is not bonded to F, O, or N.

3. London Dispersion Forces (Dispersion Forces / van der Waals Forces)

  • Occur between nonpolar molecules.
  • Weakest type of intermolecular force.
  • Involves temporary, induced dipoles due to electron movement.
  • Analogy: Magnet attracting metal pins.
    • Pins become temporarily magnetic when near a magnet.
  • Example: Octane (C8H{18}) in gasoline.
    • Electrons moving in nonpolar C-H bonds.
    • Temporary attraction between molecules due to momentary shifts in electron distribution.
    • The bigger the molecules, the stronger the London dispersion forces, due to increased electron movement.

Strength of Intermolecular Forces (In decreasing order)

  1. Hydrogen Bond (strongest).
  2. Dipole Force.
  3. van der Waals / London Dispersion Forces (weakest).
  • Hydrogen bonds are special dipole forces.
  • van der Waals forces get stronger with larger molecules.