Hydrogen bonding and properties of waterEdexcel International A Level Chemistry: Revision notes

Oxygen is much more electronegative than hydrogen, so each O–H bond in water is polar, with O δ− and H δ+. Each oxygen also has two lone pairs.

A hydrogen bond forms between the δ+ hydrogen of one molecule and a lone pair on the oxygen of a neighbouring molecule (O–H···O).

Each water molecule has two δ+ hydrogens and two lone pairs, so it can form up to four hydrogen bonds. This extensive network of hydrogen bonds explains the unusual properties of water.

Water melts at 273 K and boils at 373 K. These are very high for a molecule with a relative molecular mass of only 18. Methane (relative molecular mass 16) boils at 112 K.

Methane is non-polar, with only weak London forces. In water the intermolecular forces are hydrogen bonds, which are much stronger than London forces for molecules of this size. A lot more energy is needed to separate the molecules.

Only intermolecular forces are overcome on melting or boiling. The covalent O–H bonds are not broken.

The hydrides of Group 16 have these boiling temperatures:

• H₂S 213 K
• H₂Se 232 K
• H₂Te 271 K
• H₂O 373 K

From H₂S to H₂Te the boiling temperature rises because the number of electrons per molecule increases, so London forces get stronger. Water has the fewest electrons, so on London forces alone it should have the lowest boiling temperature, below 213 K.

The actual value is at least 160 K higher. This is because only water has hydrogen bonds. Sulfur, selenium and tellurium are not electronegative enough.

In ice, each water molecule forms four hydrogen bonds, to four neighbours arranged tetrahedrally around it. The resulting giant structure is a regular, open lattice of hexagonal rings with large empty spaces.

The hydrogen bonds hold the molecules apart at fixed distances. The molecules cannot get closer together without distorting the bonds.

In ice the molecules are held in fixed positions and vibrate. In liquid water they move around.

When ice melts, the regular lattice collapses. Some hydrogen bonds break, and the water molecules pack more closely together. Hydrogen bonds still exist in liquid water but they continually break and re-form as the molecules move.

The same mass of water therefore occupies a smaller volume as a liquid, so liquid water is denser than ice. Ice has a density of about 0.92 g cm⁻³ against about 1.00 g cm⁻³ for water at 273 K.

Consequences: ice floats on water, and water expands by about 9% on freezing, which can burst pipes. Most other substances are denser as solids.

Worked example. 100 g of water freezes. Volume of water = 100 ÷ 1.00 = 100 cm³. Volume of ice = 100 ÷ 0.92 = 109 cm³. The volume increases by about 9%.