6.4 - Hydrogen bonding

Question 1

What is meant by a hydrogen bond?

Answer 1

A strong dipole-dipole interaction between a delta+ hydrogen atom of -NH, -OH, or HF and a lone pair of electrons on a delta- atom containing N, O , or F on a different molecule

Question 2

How to draw hydrogen bonds?

Answer 2

Dashed line from hydrogen atom to lone pair

Question 3

Why is solid ice less dense than liquid water?

Answer 3

• Hydrogen bonds hold water molecules apart in an open lattice structure
• The water molecules in ice are further apart than in water
• So solid ice is less dense than liquid ice and floats

Question 4

Why does water have a high melting and boiling point?

Answer 4

• Water has hydrogen bonds as well as London forces
• Hydrogen bonds are strong and have high bond enthalpies
• This means that a large amount of energy is required to break the hydrogen bonds as well as the London forces in water
• As a result, water has high melting and boiling points