Intermolecular Forces
Intermolecular forces are those which occur between molecules. They effect many physical properties such as boiling and melting point.
Van der Waals (London Dispersion) Forces
These are the weakest intermolecular force that occurs between non-polar molecules. These are a result of the interaction of transient and/or induced dipoles. Larger molecules or atoms experience more pronounced London dispersion forces.
Dipole-Dipole Interactions
These are the intermolecular force with moderate (medium) strength and occurs between polar molecules (i.e. those with a permanent dipole).
Hydrogen Bonding
These are the strongest intermolecular force that occurs and are a special kind of dipole-dipole interaction. They occur between a hydrogen-bond donor (OH, NH, FH) and a hydrogen bond acceptor (O, N, F with one or more lone pairs).
Melting and Boiling Point Comparisons
Intermolecular forces dictate melting and boiling points. Melting and boiling involve phase changes (solid to liquid; liquid to gas) which means the bonds between molecules need to be broken. The stronger the intermolecular force, the higher the melting and boiling point.
Example:
How do you determine the relative boiling point if two molecules are similar?
First: Consider the number or atoms/electrons using the molecules below as examples.
In this case, both molecules are non-polar and therefore both only have London dispersion forces, but the boiling point of heptane is higher than that of butane. Heptane is larger and has more electrons and is more polarizable leads to stronger intermolecular forces.
Second: Consider the molecular shape using the molecules below as examples.
In this case, both molecules are non-polar and therefore both only have London dispersion forces, the boiling point of pentane (C5H12) is higher than that of isopentane (C5H12). Pentane is not branched (like isopentane) meaning there is more contact points leading to stronger intermolecular forces.
Example: Intermolecular Forces
For each of the molecules shown below, state the primary intermolecular forces at play. Rank the five molecules in order of increasing boiling point.
Molecule 1:
Molecule 2:
Molecule 3:
Molecule 4:
Molecule 5:
Molecule 4 (London Dispersion Forces, branching) < Molecule 2 (London Dispersion Forces, less branching) < Molecule 3 (hydrogen bonding only) < Molecule 5 (hydrogen bonding and dipole-dipole interactions) < Molecule 1 (hydrogen bonding)
Example: Intermolecular Forces
Determine the important intermolecular forces in the following molecules. Rank 1-3 in order of increasing boiling point.
Molecule 1: Ionic forces, dipole-dipole, VDF
Molecule 2: Dipole-dipole, VDF
Molecule 3: Hydrogen-bonding, dipole-dipole, VDF
Boiling points: Molecule 2 < Molecule 3 < Molecule 1