module 2: 6.4 hydrogen bonding

Question 1

define the term hydrogen bond

Answer 1

a hydrogen bond is a special type of permanent dipole-dipole interaction found between molecules containing:

• an electronegative atom with a lone pair
• a hydrogen atom attached to an electronegative atom

Question 2

why is ice less dense than water

Answer 2

• hydrogen bonds hold water molecules apart in an open lattice structure
• the water molecules in ice are further apart than in water.
• ice is less dense than water and floats

Question 3

why does water have a relatively high melting and boiling point

Answer 3

• hydrogen bonds are extra forces, over and above the London forces
• and appreciable quantity of energy is required to break the hydrogen bonds in water, so water has much higher melting and boiling points than would be expected from just London forces
• when the ice lattice breaks, the rigid arrangement of hydrogen bonds in ice is broken. when water boils, the hydrogen bonds break completely

Question 4

anomalous properties of water

Answer 4

• relatively high surface tension and viscosity
• high melting and boiling points
• ice is less dense than water (due to ice forming an open tetrahedral lattice full of holes) allowing 4 hydrogen bonds to form