Intermolecular Forces Debriefing

In each of the following pairs, predict which substance will have the stronger intermolecular forces:

PF₃	PF₅	Are either of these molecules polar? Molecules with fairly similar molecular weights will have London forces of similar strengths. Good! PF₃ will have dipole-dipole forces (since it is a polar molecule) as well as London forces. (These two molecules will have London forces similar in strength since they have fairly similar molecular weights.)

CO₂	OCS	Are either of these molecules polar? Molecules with fairly similar molecular weights will have London forces of similar strengths. Good! CO₂ is a nonpolar molecule and will only exhibit London forces. OCS is a polar molecule and will exhibit both dipole-dipole forces and London forces (which will be similar in strength to the London forces in CO₂).

CH₃OH	H₂CO	Will either of these molecules have the capability of making hydrogen bonds? Remember in order to form hydrogen bonds, a molecule must have a hydrogen bonded to a fairly electronegative atom. Good! Methanol contains a hydrogen bonded to an oxygen atom and lone electron pairs on the oxygen atom. This allows hydrogen bonds to form. Hydrogen bonds are an especially strong type of dipole-dipole forces.

How will the boiling points of propanol and ethyl methyl ether (structures shown below) compare?


propanol	ethyl methyl ether

Good! Propanol will be able to form hydrogen bonds because it contains a hydrogen bonded to an oxygen atom AND there are lone electron pairs on the oxygen atom. The boiling points of propanol and ethyl methyl ether are 97.2ºC and 7.4ºC respectively - what a difference hydrogen bonding makes!

These two molecules have similar London forces since they have the same molecular weight. Both of these molecules are polar molecules and will thus have dipole-dipole forces. Will one have stronger dipole-dipole forces? (HINT: Can either of these molecules participate in hydrogen bonding?)

The hydrocarbon C₂₅H₅₂ is a solid at room temperature and its boiling point is greater than 400ºC. Which has stronger intermolecular forces, C₂₅H₅₂ or H₂O?

Good! A higher boiling point and that C₂₅H₅₂ is a solid at room temperature while H₂0 is a liquid at room temperature indicates that C₂₅H₅₂ has stronger intermolecular forces than H₂O. This case illustrates that with large molecules London forces can be stronger than some of the strongest dipole-dipole forces (the hydrogen bonds in water).

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A higher boiling point indicates that more energy is required to overcome the intermolecular forces present in C₂₅H₅₂. If more energy is required, does this mean the intermolecular forces are stronger or weaker than those in H₂O?