Hydrogen Bonding. A property of water is that it has strong intermolecular forces as a result of hydrogen bonding and the dipole moments created by the strong electronegative oxygen and the hydrogen. The energy required to break these bonds accounts for the relatively high melting point of water.

• Oil- Only London Dispersion Forces (the weakest intermolecular force)
• Water- London Dispersion, Dipole-Dipole, and Hydrogen Bonding.

Water had the strongest intermolecular forces and evaporated most slowly. The strength of the intermolecular forces in isopropyl alcohol are in between water and acetone, but probably closer to acetone because the water took much longer to evaporate.

Why does rubbing alcohol dissolve in water? For these two substances to mix or for the rubbing alcohol (isopropyl alcohol) they must be alike. And they are due to existing partial charges! The force that allows these two molecules to interact is the dipole-dipole force.

hydrogen bonding

The especially strong intermolecular forces in ethanol are a result of a special class of dipole-dipole forces called hydrogen bonds.

The ammonia molecule is polar because of it’s pyramidal shape. Hydrogen bonds are caused by highly electronegative atoms. They only occur between hydrogen and oxygen, fluorine or nitrogen, and are the strongest intermolecular force.

Unlike Van der Waals’ forces, hydrogen bonding involves a permanent imbalance of charge and therefore results in permanent dipole attractions. Each ethanol molecule has one O—H bond and thus forms one hydrogen bond with adjacent ethanol molecules.

The C-O bonds of methoxymethane (dimethyl ether) (CH3-O-CH3) are polar. The geometry of the molecule is angular, resulting in an overall molecular dipole. Hence the molecule will be subject to dipole-dipole and dipole/induced dipole interactions as well as the stronger dispersion forces.

Methoxymethane is a slightly polar molecule, with a partial negative charge on the oxygen. The boiling point of ethanol is much higher than methoxymethane, as ethanol is able to form hydrogen bonds, whereas methoxymethane has rather weaker dipole-dipole interactions.

Secondly, notice that for the bigger molecules, the alkanes (which don’t have a permanent dipole and only have dispersion forces available to them) have slightly higher boiling points than the polar molecules.

Intermolecular hydrogen bondings are strong and hence require a large amount of energy to break these hydrogen bonds. In a solution of water and ethanol, hydrogen bonding is the strongest intermolecular force between molecules. That is why the boiling point of Methoxymethane is lower than that of ethanol.

Dipole -dipole interactions occur when the partial charges formed within one molecule are attracted to an opposite partial charge in a nearby molecule. Polar molecules align so that the positive end of one molecule interacts with the negative end of another molecule.