Notes: Intermolecular Forces and Properties
How do intermolecular forces affect the properties of a
substance?
 Intermolecular forces change the properties of a substance.
 There are three types of intermolecular forces:
o Hydrogen bonding
o Dipole-dipole forces
o Dispersion forces
Do intermolecular forces have different strengths?
 The strength of the intermolecular forces changes the properties of a substance.
o Hydrogen bonds are the strongest of the intermolecular forces.
o Dipole-dipole forces have medium strength.
o Dispersion forces are the weakest of the intermolecular forces.
How does the strength of an intermolecular force impact the substance’s properties?
 The stronger the force, the higher the melting and boiling point.
o Substances that have more intermolecular forces are going to have higher melting or boiling
points.
 The weaker the force, the lower the melting and boiling point.
o Substances that have less intermolecular forces are going to have lower melting or boiling
points.
PRACTICE: for each pair of substances circle the one that would have the highest boiling point.
HF (hydrogen bonding, dispersion, and dipole-dipole)
or
HBr (dispersion and dipole-dipole)
NH3 (hydrogen bonding, dispersion, and dipole-dipole)
or
Cl2 (dispersion)
PRACTICE: for each pair of substances circle the one that would have the lowest melting point.
F2 (dispersion)
or
HCl (dispersion and dipole-dipole)
H2O (hydrogen bonding, dispersion, and dipole-dipole)
or
SO2 (dispersion and dipole-dipole)
How do we predict the boiling point or melting point of a substance?
 Step 1: Is the molecule covalent?
o If yes, move on to step 2.
o If no, you cannot use these rules! Ionic compounds, network covalent substances, and
substances held together with metallic bonds do not apply here.
 Step 2: Does the compound contain hydrogen bonds?
o If yes, it will have the highest melting or boiling point.


Hydrogen bonds form between H and F, O or N. Hydrogen bonds have the highest
melting and boiling points.
o If no, move on to step 3.
Step 3: Is the compound polar or nonpolar?
o If the compound is polar, it will have a higher melting or boiling point.
 Polar compounds are ones that are trigonal pyramidal shaped, bent shaped, or are
asymmetrical. Polar compounds have the second highest melting and boiling points
because of dipole-dipole forces.
o If the compound is nonpolar, then it will have the lowest melting or boiling point.
 Nonpolar compounds are ones that are symmetrical and not trigonal pyramidal or bent
shaped. Nonpolar compounds have the lowest melting and boiling points because they
only experience dispersion forces.
PRACTICE: circle the substance in each group that would have the highest boiling point or melting point.
OR
OR
PRACTICE: circle the substance in each group that would have the lowest boiling point or melting point.
OR
OR
PRACTICE: for each of the following substances determine the type of intermolecular force that would have to
be overcome to get a sample of the substance to melt.