Download Hydrogen Bonding

Intermolecular Forces
What are intermolecular
forces?
 NOT chemical bonds, less strength
 Attractive forces between molecules
 Molecular level, not individual atoms
 Types
 Dipole-Dipole
 London Dispersion
 Hydrogen Bonding
1. Dipole-Dipole Forces
 Found in polar molecules
 Dipole—equal and opposite charge separation over a short distance
in a molecule
 Charge separation occurring within a molecule, force of attraction
between molecules
 Acts on adjacent molecules, opposite charges on “neighboring”
molecules attract
Ex. HCl
2. London Dispersion Forces
 TEMPORARILY uneven charge distribution
 Found between ALL atoms/molecules at varying
degrees, some occur more frequently than others
 At SOME point in time, electron density is greater
around one atom than the other
 Induces temporary dipoles in adjacent molecules
London Dispersion Forces
(cont.)
 Mobile electron density
 Molecular shape influences the strength of dispersion
forces within a molecule
 Melting and boiling points indicative of intermolecular
force strength
Penny Lab !
3. Hydrogen Bonding
 Strong dipole-dipole force
 Hydrogen attached to electronegative atom is attracted
to UNSHARED electron pairs of another nearby
electronegative atom (book definition)
 Dotted lines represent hydrogen bonding
Hydrogen Bonding (cont.)
 Considered a FORCE, not a type of bond
 Connects 2 electronegative atoms together
 Force holding water molecules together
 Results from a negative charge on an atom and a positive
charge on hydrogen
 H-X format
 Ex. Hydrogen bonding between DNA base pairs of double
helix
Example: H2O
Unique Properties of Water
 Freezes and expands as solid
 Solid is less dense than liquid (ex. Ice floats)
 High melting point
 High boiling point
 Highest surface tension (Hg only exception)
 Other compounds can easily dissolve in it
 High specific heat
ALL DUE TO HYDROGEN
BONDING !