Here’s a general chemistry concept that frequently appears on the DAT. We’re talking about intermolecular forces, not to be confused with intRAmolecular forces, which bind the atoms within a single particle (i.e. covalent bonds, metallic bonds, ionic bonds).
IntERmolecular forces are the interactions that occur between neighboring particles and have a large effect on a compound’s physical properties such as the melting point, boiling point, viscosity, etc. Here are the four intermolecular forces you should know in order of DECREASING strength:
- Ion-Dipole: the interaction between an ion and an oppositely charged dipole. (example: the positive cation of NaCl will be surrounded be water’s oxygens which have a negative dipole).
- Hydrogen Bonding: requires a hydrogen to be covalently bound to F, O, or N. The large contrast in electronegativities between the hydrogen and these other F, O, N atoms creates large dipoles. Partially-positive hydrogens will interact with partially-negative F,O, or Ns of neighboring molecules.
- Dipole-Dipole: These can be basically thought of as weaker hydrogen bonds that do not contain F,O, or N as the electronegative atoms. The concept is the same as above except on a smaller scale since the dipoles will not be as large. Just remember, there will be a partially-positive atom that interacts with a partially-negative atom of a neighboring molecule.
- London Dispersion: All molecules have these. It’s the very brief attraction between neighboring molecules due to the random movement of electrons. At any one snippet of time, the electrons on an atom may be bunched on one side making that side partially-negative while the electron-deficient side is partially-positive. These partially-positive/negative atomic domains interact with the domains of the atoms of neighboring molecules, but on a much smaller scale that the dipole-dipole interactions. However, these forces occur in such large numbers that their summation can’t be ignored. The higher the molecular weight, the stronger the London dispersion forces.
Now, it’s time to apply what we know about IM forces.
Larger intermolecular forces equate to:
- A higher melting point, boiling point, viscosity, surface tension.
- A lower vapor pressure.
Think of an increase in IM forces as an increase in the stickiness between molecules. The stickier they are, the harder they will be to separate (melt/boil), the more tension they will have between them (viscous) and the less likely they will be to escape into the gas phase (vapor pressure).