The third of the three intermolecular forces…
• Hydrogen Bonding: A strong dipole- dipole attraction between an
electron deficient hydrogen atom of –NH, -OH or –HF on one molecule
and a lone pair of electrons on a highly electronegative N, O or F on a
different molecule. Type of permanent dipole- dipole interaction.
• Shape: Liner shape around the H atom in hydrogen bonding. Bond
angle is 180o
so when drawing hydrogen bond should be straight.
• Diagrams: Include the lone pairs and atom polarities. H is δ+. Label
bond with dashed line of hydrogen bond. Hydrogen bond must hit
lone pairs. LEARN THIS PIC.
• Describe the hydrogen bonding between two water molecules:
– Labelled diagram
– Water is polar. Permanent dipole across O-H bonds.
– Hδ+ on one water molecule attracts lone pair of electrons on highly electronegative O atom
of another water molecule.
• Strength of Hydrogen Bond:
– Ice/ OH has stronger hydrogen bonds than NH/ NH2.
– O has two lone pairs and N has one lone pair so more hydrogen bonds per molecule.
– OR O more electronegative than N so will attract more to make stronger hydrogen bond.
• Density:
– Liquid H2O is denser than solid ice.
– In solid state H2O molecules are held apart by hydrogen bonds in an open lattice.
• Water has two lone pairs on oxygen and two H atoms, so each water molecule can form four
hydrogen bonds.
• Melting and Boiling Point:
– Water has London Forces and hydrogen bonds.
– Hydrogen bonds are additional forces and stronger than other intermolecular forces.
– So more energy is needed to overcome hydrogen bonds.
– So water has higher melting and boiling point.
• When the ice lattice breaks, the arrangement of hydrogen bonds is broken. When water boils, the
hydrogen bonds broken completely.
• Something like H2S has lower boiling point than H2O as no hydrogen bonding.