Intermolecular Forces

Intermolecular Forces vs. Intramolecular Forces
  • Inter = between
  • Intra = within

The majority of the unit has been focused on INTRAmolecular forces called ionic and covalent bonds.  These bonds exist between ATOMS and create compounds.  Now that we understand those forces, we can focus on how they affect interactions BETWEEN molecules.  INTERmolecular forces are interactions that occur between COMPOUNDS.

Types of Intermolecular Forces:
  1. Ion-Ion 
  2. Dipole-Dipole
  3. Hydrogen-bonding
  4. Dispersion forces (van der Waals forces)
Ion-Ion Interactions: (ATTRACTION OF OPPOSITE CHARGES)
These are the strongest of all intermolecular forces due to the strength of the positive and negative charges.  Ionic compounds are the only compounds that experience this type of interaction.

Dipole-Dipole Interactions: (ATTRACTION OF OPPOSITE PARTIAL CHARGES)
The next strongest intermolecular force forms between polar covalent compounds due to their partial positive and negative charges.  In this type of interaction, the partial positive pole of one molecule is attracted to the partial negative pole of another molecule.  

Hydrogen Bonding:  (A TYPE OF DIPOLE-DIPOLE INTERACTION)
Hydrogen bonding is a strong intermolecular force that forms between molecules that contain H bonded to small, electronegative atoms like N, O, and F.  Water experiences this type of dipole-dipole interaction since the H is bonded to O.

Dispersion forces:  (TEMPORARY SHIFTING OF ELECTRON CLOUD WITHIN NONPOLAR MOLECULES)
The weakest of all intermolecular forces occurs between molecules that do not have charges or partial charges.  Since no attractions really exist, the molecules do not interact strongly.  Instead, the molecules simply affect the shape of the electron cloud temporarily as the atoms or molecules pass near one another in a space.

Properties of Compounds due to Intermolecular Forces:

Many properties of compounds depend upon the intra- and intermolecular forces that exist.

Solubility and Miscibility
Compounds will dissolve into one another IF their particles are attracted to one another.
Since ionic compounds have charges, they will mix well with other ionic compounds AND with polar covalent compounds, which have partial charges.
Since nonpolar compounds have no charges or partial charges, they will NOT mix with ionic or polar covalent compounds.

Melting and Boiling Point
The stronger the intermolecular force between the molecules...the more energy is needed to melt or boil the compound.  So, the stronger the intermolecular force...the higher the melting and boiling point.

Ion-Ion  = Strongest = Highest melting and boiling point
    • Ionic compounds are the most difficult compounds to melt or boil because the particles are charged.  The oppositely charged particles are very attracted to one another, and these attractions require a LOT of energy to overcome.
Hydrogen bonding = Strongest of the dipole-dipole interactions = Highest melting and boiling points of covalent compounds
  • Polar covalent compounds that contain H bonded to N, O, or F are the next most difficult compounds to melt or boil. The partially charged particles are very attracted to one another, and these attractions require quite a bit of energy to overcome.  
  • Example: water
Dipole-Dipole = Strong interactions = high melting and boiling points 
  • Polar covalent compounds are difficult to melt and boil due to the partial charges.  The opposite partially charged particles are attracted to one another.
Dispersion forces = Weak interactions = low melting and boiling points
  • Nonpolar covalent compounds are easy to melt and boil relative to their polar covalent cousins.  There are no charges or partial charges to create attractions to overcome.
Additional Resources:
Click here to visit http://www.wwnorton.com/college/chemistry/chemistry3/chemtours.aspx, and select Chapter 10 Intermolecular Forces.