Module 2 - Hydrogen Bonding

Question 1

Define hydrogen bond.

Answer 1

A hydrogen bond is an intermolecular attraction which acts between a lone pair of electrons on a N, O or F atom in one molecule and a hydrogen atom in a different molecule. (However hydrogen bonding only occurs when a hydrogen atom is directly bonded to a N, O or F atom).

Question 2

Hydrogen bond: Summary

Answer 2

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

• an electronegative atom with a lone pair of electrons, e.g. oxygen, nitrogen, fluorine.
• a hydrogen atom attached to an electronegative atom, e.g. H-O, H-N, H-F.

Question 3

Drawing hydrogen bonds:

Answer 3

• The hydrogen bond is shown by a dashed line with annotation.
• The lone pair of electrons on the oxygen in water, and the nitrogen in ammonia, play a key role in a hydrogen bond.
• The shape around the hydrogen atom involved in the hydrogen bond is linear.
• When drawing a diagram to show hydrogen bonding, remember to show lone pairs and atom polarities. Hydrogen is always δ+.

Question 4

Water molecules…

Answer 4

• H2O is polar as each O-H bond have a permanent dipole. (O=δ-, H=δ+).
• The two dipoles act in different directions but do not oppose each other.
• Overall, the oxygen end of the molecule has a δ- charge, and the hydrogen end of the molecule has a δ+ charge.

Question 5

Water density: Why is ice less dense than liquid water?

Answer 5

• As a solid, H2O is less dense than liquid H2O because the hydrogen bonds cause the water molecules in ice to form an open lattice structure, and the linear H-O-H arrangement hold the H2O molecules apart.
• This results in larger gaps between the water molecules in ice compared to liquid water = less dense.

Question 6

Water boiling point…

Answer 6

• Water has a relatively high boiling point as there are strong hydrogen bonds present.
• This means higher temperatures are needed to gain enough energy to break the hydrogen bonds as hydrogen bonds are the strongest of the intermolecular forces.

Question 7

Which of the following has the highest b.p.?

NH3, PF3, SF6

Answer 7

Identify the types of intermolecular forces:
NH3 = permanent dipole, LDFs, hydrogen bonding.
PF3 = permanent dipole, LDFs.
SF6 = LDFs.
^thus NH3 has highest b.p.