Intermolecular
Forces
By Peter Demkowicz
Why?
Intermolecular Forces
• In simple, forces with which molecules
attract each other.
Intermolecular Forces Properties
Solids/Liquids
Boiling/Melting Point
Surface Tension
Capillary Action
Vapor Pressure
Miscibility
Intermolecular Forces
• Hydrogen Bonds
• Dipole-Dipole interactions
• London Dispersion
Dipole-Dipole
• Molecules can have a separation of charge
• Molecules are attracted to each other in a
compound by the valence electrons:
Positive & Negative
• Stronger than London Dispersion
Orientation of Polar Molecules
in a Solid
Dipole
• A polar molecule that has two poles.
Hydrogen Bonding
• Hydrogen bonding is the attraction
between a hydrogen atom of a molecule to
an unshared pair of electrons in another
molecule.
• Hydrogen bonding occurs in molecules
where hydrogen is covalently bonded to a
very electronegative element.
• Hydrogen bonding occurs in molecules
containing N, O, F.
Hydrogen Bonding, Continued
• Hydrogen bonds are the strongest of all
intermolecular forces.
• Hydrogen bonds are responsible for the
physical properties of many biological
substances and, more importantly, water.
Hydrogen
Interactions
Which of these are Hydrogen Bonds?
H20
CH3F
N2
H2CO
London Dispersion
London forces are named after Fritz London
(also called van der Waal forces)
London forces are due to small dipoles that
exist in non-polar molecules
Since electrons are moving around in atoms
there will be instants when the charge
around an atom is not symmetrical
The resulting tiny dipoles cause attractions
between atoms/molecules
(London) Van der Waals Forces Dispersion
(continued)
Increase as the number of electrons
increases.
Weakest of all intermolecular forces.
Put the bonds in strongest order
Ionic/covalent
Dipole-Dipole
London Dispersion
Hydrogen Bonding
Which ones are Dipole, London Dispersion, and
Hydrogen Bonding?
C2H6
HCl
CH3OH
H2
H2O