6.4 - Hydrogen Bonding

Question 1

Define hydrogen bond

Answer 1

Type of permanent dipole- dipole interactions found between molecules containing:
- an electronegative atom with a lone pair of electrons Eg. Oxygen, nitrogen or fluorine
- a hydrogen atom attached to an electronegative atom

Question 2

What are the anomalous properties of water?

Answer 2

• solid ice is less dense than the liquid water
• solids = denser than their liquids as the particles in solids are more closely packed together than in their liquid state
• in ice the water molecules are packed together in a 3D hydrogen- bonded network in a rigid lattice
• each oxygen atom is surrounded by hydrogen atoms
• this way of packing the molecules in a solid and the relatively long bond lengths ot the hydrogen bond means that the water molecules are slightly further apart than in the liquid form
• thus ice has a lower density than liquid water
• the hydrogen bond extends outwards
• holding water molecules slightly apart and forming an open tetrahedral lattice full of holes
• the bond angle of an hydrogen atom which is involved in an hydrogen bond is 180°
• the holes in an open lattice structure decrease the density of water on freezing
• when ice melts the ice lattice collapses and the molecules move close together
• thus liquid water is denser than solid ice

Question 3

What are 2 other anomalous properties of water?

Answer 3

• relatively high surface tension and viscosity
• allows insects to walk on pond surfaces

Question 4

Why does water have a melting/boiling point higher than expected?

Answer 4

Hydrogen bonds are stronger than other intermolecular forces so extra strength is required to overcome the forces