6.4 Hydrogen bonding

Question 1

Definition: A hydrogen bond is

Answer 1

an attraction between a lone pair of electrons on an
electronegative atom in one molecule and a hydrogen atom in another molecule attached to
an electronegative atom

Question 2

Essential criteria for hydrogen bonds to form:

Answer 2

1. a HYDROGEN ATOM attached to a high electronegative atom F, O or N
2. an unpaired pair of electrons on the electronegative atom

Question 3

rules for the

Diagram showing hydrogen bonding:

Answer 3

What details are required…

Hydrogen bond = dashed line

Dipoles MUST be shown

Lone pairs MUST be drawn

Hydrogen bond comes from lone pair on
electronegative atom to H (that is attached to
another electronegative atom (H-O….H hydrogen
bond is 180o)

You MUST label the hydrogen bond.

Question 4

The anomalous properties of water (H2O):

Answer 4

• solid is less dense than liquid
• relativley high mp nd bp

Question 5

why does water

have relativley high mp and bp and ice floats?

Answer 5

• hydrogen bonds are additional to the london forces
• more energy is needeed to overcome these hydrogen bonds
• when ice melts the rigid lattice breaks
• when water boils the hydrogen bonds break completely

The structure of water as ICE is very OPEN. This means that solid water (ice) is less dense than
the liquid (water)
Hydrogen bonds hold water molecules apart in this open structure.
The water molecules are therefore further apart in ice (than water)
So, solid ice is less dense than liquid water and floats.

Question 6

intermolecular forces in order of strength strongest to weekest

Answer 6

strongest:
- Hydrogen
- permanent
- london/induced
weakest: