Handout 11-2:
IMF InClass Worksheet
Ok, now that we’ve seen the table of intermolecular forces, let’s work some examples! We’ll look
at each of the examples below using the 4-step process we’ve discussed. Here we go…!
Let’s look at the weakest of the intermolecular forces first: LONDON FORCES.
“London force is coming down, coming down, coming down. London force is coming down, my
fair lady!” Lame I know, but you need to remember that London forces are the smallest, the
weakest, of the intermolecular forces.
Looking at this backwards, if you can rule out EVERYTHING else, you still have London forces!
NO net dipoles, no hydrogen bonding, no ion interactions? Nothing? Zip? Nada? London forces!
N2
CH4
Since these ONLY have London forces present, they have low boiling points, low viscosity, and high
evaporation rates.
Next on our hit list is DIPOLE-DIPOLE or permanent dipoles. These compounds are
polar with net dipoles not equal to zero.
CH3Br
These compounds have slightly higher boiling points, slightly higher viscosity, and slightly lower
evaporation rates than those with London forces alone.
Handout 2: Intermolecular Forces
Next player in the Bond is Right is HYDROGEN BONDING. First of all, this isn’t
REALLY a bond. It’s an intermolecular force. Don’t get bitten by that one later. It’s called that
unfortunately because the hydrogen must be bonded to an N, F, Cl, or O atom AND it has to be
attracted to another high EN atom to have hydrogen bonding occur. So if you see a C-H bond, like
in methane, don’t bite on it! It’s a trap!!
H2O
These compounds have high boiling points, high viscosity, and low evaporation rates.
And lastly, we have ION-ION INTERACTIONS. If the ∆EN is about 2-3, you
have a huge electronegativity difference and most likely strong ionic character. Good examples are
any of the salts…metals (cations of Groups Ia, IIa, or IIIa) with nonmetals (anions of Groups VIa or
VIIa) in a polar substance...like water.
NaCl
These salts are solids and have extremely high melting points. They are rarely in liquid form and
therefore don’t really have an evaporation rate. But, as a professor I once knew said, “can you
imagine how long someone would have to wait around to watch salt evaporate?!”