Understanding Hydrogen Bonding

Hydrogen Bonding

• Stands as the strongest form of intermolecular bonding, occurring between molecules.
• Distinguished as a type of permanent dipole–permanent dipole interaction.
• Requires a species with a highly electronegative atom (F, O, N) possessing an available lone pair of electrons.
• Also requires a species containing an -OH or -NH group.
• Involves hydrogen covalently bonded to an electronegative atom (O or N), leading to a highly polarized bond.
• Hydrogen (H) becomes partially positive (δ+), enabling it to form a bond with the lone pair of an O or N atom in another molecule.
• The angle between the -OH/-NH group and the hydrogen bond must be 180°.
• The number of hydrogen bonds is determined by the number of hydrogen atoms attached to O or N, and the number of lone pairs on O or N.
• Ammonia (NH3) can form a maximum of one hydrogen bond due to one lone pair on nitrogen (N).
• Water (H2O) can form a maximum of two hydrogen bonds due to two lone pairs on oxygen (O).

Properties of Water

• Hydrogen bonding gives water unique characteristics like high melting and boiling points.
• Also gives it high surface tension and anomalous density of ice compared to water.

High Melting & Boiling Points

• Attributed to the strong intermolecular forces of hydrogen bonding between water molecules.
• In ice (solid H2O) and water (liquid H2O), molecules are tightly held together by hydrogen bonds.
• Overcoming these requires significant energy to break apart the water molecules for melting or boiling.

Enthalpy of Vaporisation

• Enthalpy changes increase from H2S to H2Te due to the increased number of electrons in Group 16 elements, leading to stronger instantaneous dipole-induced dipole forces.
• Water has a much higher enthalpy change of vaporisation than expected because of its hydrogen bonds.

High Surface Tension

• Defined as the ability of a liquid surface to resist external forces.
• Water molecules at the surface bond to other water molecules through hydrogen bonds
• This causes the surface to compress and tighten.

Density

• Solids are generally denser than liquids because their particles are more closely packed.
• Ice (solid water), water molecules form a 3D hydrogen-bonded network in a rigid lattice.
• Each oxygen atom is surrounded by hydrogen atoms, which leads to water molecules being slightly further apart than in liquid form.
• Thus, ice has a lower density than liquid water.