The MCAT will test you on intermolecular forces. Take note, this is an important section on the MCAT because it can be used to predict trends in things like boiling and melting points. So make sure that you have this down!
If you’re ready to start studying, check out our free video lesson on MCAT Intermolecular Forces, and keep reading for a quick review of intermolecular forces.
MCAT Intermolecular Forces Review
Let’s take a look at three kinds of intermolecular forces:
1. London Dispersion Forces
London dispersion forces involve what are called temporarily induced dipoles.
A dipole can be temporarily induced when we have a long, straight alcane with all the electrons on one side of the molecule, which will repel the electrons of the next molecule.
By inducing a dipole, we’ve made this a quasi-polar compound. This will slightly increase the boiling point.
London dispersion forces are size dependent. The longer this chain is (i.e. the more electrons that exist in the compound), the easier it is to polarize the compound, making London dispersion forces stronger.
2. Dipole-Dipole Interactions
Dipole-dipole interactions occur between polar molecules.
An example of a polar molecule would be CH3Cl, or chloromethane. Chloromethane is a carbon with three hydrogens and a chlorine attached to it.
The carbon-hydrogen bonds are essentially non-polar, but the carbon-chlorine bond is polar. This results in a slightly positive charge on the carbon and a slightly negative charge on the chlorine, because it is more electronegative and will suck up those electrons to hold on to it.
This results in a much stronger attraction between the molecules in their solid and the liquid forms than in their vapor form. In the vapor, there is still this attraction if they come close to each other, but since vapor molecules tend to be so far apart, this force isn’t as strong. The result of this dipole-dipole interaction though, is that molecules want stay as the liquid for as long as possible, because there are intermolecular forces holding them together.
3. Hydrogen Bonding
Hydrogen bonding occurs when hydrogen is bonded to:
All of these bondings will significantly increase the boiling point when compared to something that doesn’t have a hydrogen bonding.
Which of these three forces is the strongest?
- Hydrogen bonding is far and away the strongest.
- London dispersion forces, which are just induced dipoles and not permanent dipoles, are the weakest.
- The dipole-dipole interactions are in between the two.
However, if the molecule chain starts to get very long, all of a sudden the hydrogen bonding is a much smaller portion of the total molecule and becomes less significant than the London dispersion forces.
This doesn’t mean that hydrogen bonding is weaker, just that the molecule has become so big that the hydrogen bonding can’t overcome the rest of the molecule.
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