Hydrogen Bonding Intermolecular Forces Compounds  Elmhurst College
Other Molecules Boiling Point DNA  Chemistry Department
Density of Ice Proteins    Virtual ChemBook

Hydrogen Bonding

and the Boiling Point of Water

Boiling Point Definition:

In a liquid the molecules are packed closely together with many random movements possible as molecules slip past each other. As a liquid is heated, the temperature is increased. As the temperature increases, the kinetic energy increases which causes increasing molecular motion (vibrations and molecules slipping pas each other). Eventually the molecular motion becomes so intense that the forces of attraction between the molecules is disrupted to to the extent the molecules break free of the liquid and become a gas. At the temperature of the boiling point, the liquid turns into a gas. The molecules are not in contact each other in the gaseous state.

Water Liquid to Water Gas:

This animation shows how water molecules are able to break the forces of attraction i.e. the hydrogen bonds to each other and escape as the gas molecule. This is what is happening inside the gas bubble as it is rising to the surface to break and release the water gas molecules.

Animated GIF "Boiling" - Courtesy of General Chemistry Help - Purdue University

Polarity and Boiling Point:

The polarity of the molecules determines the forces of attraction between the molecules in the liquid state. Polar molecules are attracted by the opposite charge effect (the positive end of one molecule is attracted to the negative end of another molecule. Molecules have different degrees of polarity as determined by the functional group present.

Principle: The greater the forces of attraction the higher the boiling point or the greater the polarity the higher the boiling point.

In the case of water, hydrogen bonding, which is a special case of polar dipole forces exerts a very strong effect to keep the molecules in a liquid state until a fairly high temperature is reached.

This is shown in the graphic on the left for a similar set of molecules in Group VI of the periodic table. If water behaved as a normal polar molecule it would have boiled at about - 100 C (shown in red). The other molecules are slightly polar and show the increase in boiling point with molecular weight which is normal.

Instead, water boils at +100 C, which is very abnormal. The major reason for this abnormal behavior is the strong attractions afforded by the hydrogen bonds. It takes a lot more kinetic energy in an increased temperature to break the hydrogen bonds to free the water molecules as the gas.

More normal behavior is seen in dimethyl ether (CH3)2O which has no hydrogen bonds possible.