6.4 Hydrogen bonds

Question 1

What are hydrogen bonds?

Answer 1

a special type of permanent dipole-dipole interaction found between molecules:
- an electronegative atom with a lone pair of electrons
- a hydrogen atom attached directly to one of the most electronegative elements e.g. N, O, F

Question 2

Where does the hydrogen bond act?

Answer 2

between a lone pair of electrons on an electronegative atom in one molecule and a hydrogen atom in a different molecule

• shown by a dashed line

Question 3

Why is water considered the perfect example of hydrogen bonding?

Answer 3

In H2O, the number of lone pairs of oxygen (2) is balanced with the number of hydrogen atoms available (2)

NH3- 3 hydrogen 1 lone pair

Question 4

Anomalous property of water: Density

Answer 4

Ice is less dense than water
- two lone pairs on oxygen and two hydrogen atoms means each water molecule can form 4 hydrogen bonds

• Hydrogen bonds extend outwards, holding water molecules slightly apart an forming an open tetrahedral lattice full of holes
• holes= decreased density so ice floats above water

Question 5

Anomalous property of water: high melting and boiling point

Answer 5

Water has london forces and hydrogen bonds
- takes more energy to break the additional hydrogen bonds
-ice lattice breaking= rigid arrangement of hydrogen breaking
-water boiling= hydrogen bonds breaking completely