Hydrogen bonding

Question 1

What atoms need to be present for hydrogen bonding to occur?

Answer 1

Hydrogen bonding only occurs when hydrogen is covalently bonded to fluorine, nitrogen or oxygen.

Question 2

Name three substances that undergo hydrogen bonding.

Answer 2

Any of these three:

• Water (H₂O)
• Ammonia (NH₃)
• Hydrogen fluoride (HF)
• Alcohols (end in -OH)
• Amines (end in -NH)

Question 3

Why is ice less dense than water?

Answer 3

• As liquid water cools to form ice, the molecules make more hydrogen bonds and arrange themselves into a regular lattice structure.
• In this regular structure, the H₂O molecules are further apart on average than the molecules in liquid water - so ice is less dense than liquid water.

Question 4

Which of the Group 7 hydrides has the highest boiling point?

Answer 4

HF has the highest boiling point from the group 7 hydrides.

HF > HI > HBr > HCl

Question 5

For each of the following pairs of compounds, state which will have the higher boiling point.
i) Ammonia (NH₃) and methane (CH₄).
ii) Water (H₂O) and hydrogen sulfide (H₂S).
iii) Butane and propan-1-ol.
[3 marks]

Answer 5

i) Ammonia
ii) Water
iii) Propan-1-ol

Question 6

An organic compound used as antifreeze is ethane-1,2-diol. The boiling point of ethane-1,2-diol is 197°C, whereas the boiling point of ethanol is 78°C. Suggest a reason for this difference.
[1 mark]

Answer 6

Ethane-1,2-diol has stronger intermolecular bonds because there are two alcohol groups, twice as many in ethanol. Therefore ethane-1,2-diol can form twice as many hydrogen bonds as ethanol.

Question 7

The weaker the intermolecular bonds in a liquid, the ______ its rate of evaporation will be.

Answer 7

faster

Question 8

How do hydrogen bonds affect the properties of a substance?

Answer 8

Substances that form hydrogen bonds…

• …have high melting points
• …have high boiling points
• …are soluble in water