Chem 1721
Intermolecular Forces
Intermolecular Forces of attraction are the forces that hold individual particles in a bulk sample together as a unit. They are all electrostatic in nature. There are 4 types of intermolecular forces:
1.
Ion-Dipole Forces
➛
exist between ions and polar molecules
➛
ion-dipole forces increase in strength with increasing
ionic charge and increasing molecular polarity
3.
London Dispersion Forces (LDF)
➛
exist between neutral nonpolar molecules
➛
nonpolar molecules can have an instantaneous dipole
resulting from electron movement that results in short-lived
electron concentration on one side of molecule
➛
an instantaneous dipole existing in one molecule can induce
an instantaneous dipole in a neighboring molecule
➛
LDF increase in strength with increasing polarizeability; increasing
size in electron cloud; increasing molecular size and molar mass
➛
LDF present in all substances
2.
Dipole-Dipole Forces
➛
exist between neutral polar molecules
➛
polar molecules must be close together
➛
dipole-dipole forces increase in strength with increasing
molecular polarity (larger µ or Δχ) and decreasing size
4.
Hydrogen Bonding
➛
special type of dipole-dipole interaction
➛
exists between a H atom in a polar bond (especially with
N, O, or F) and an unshared electron pair on a nearby small
electronegative atom (especially N, O, or F)
➛
stronger than other intermolecular forces
➛
results in properties like bp, ΔH°vap, specific heat being
greater than predicted for molecules that can H-bond
➛
There is a hierarchy of strength for these intermolecular forces. From strongest to weakest they are: ion-dipole forces > H-bonds> dipole-dipole forces > LDF
➛
Remember: all intermolecular forces are much weaker than intramolecular forces (i.e. chemical bonds); on the order of only 15% as strong as covalent, ionic or
metallic bonds.
➛
To determine what intermolecular forces are present in a substance use the following flow chart:
interacting molecules or ions
NO
ions involved?
YES
polar molecules and ions?
polar or nonpolar molecules?
YES
POLAR
NO
NONPOLAR
Any H's bonded to N, O, or F?
London Dispersion
Forces only
ex. I 2 , Ar
NO
Dipole-Dipole Forces
+ London Dispersion Forces
ex. H2 S, CH3Cl
Ion-Dipole Forces
+ LDF
ex. KCl in H2O
YES
H-bonding
+ LDF
ex. NH 3, H2O, HF
Ionic Bonding
+ LDF
ex. NaCl,
NH4 NO3