Intermolecular bonds

Question 1

What’s the strongest type of intermolecular bond?

Answer 1

Hydrogen bonds.

Question 2

Explain what instantaneous dipole-induced dipole bonds are.

Answer 2

• At any particular moment, the electrons in an atom are likely to be more to one side than the other…
• …causing an atom to have a temporary dipole.
• This dipole can induce another temporary dipole in the opposite direction.
• The two dipoles are then attracted to each other.

Question 3

Explain what gives rise to permanent dipole-permanent dipole intermolecular bonds.

Answer 3

The δ+ and δ- charges on polar molecules cause weak electrostatic forces of attraction between molecules.

Question 4

Explain the differences in the boiling points of these molecules:
Pentane = 36.1 °C
2-methylbutane = 27.7 °C
2,2-dimethylpropane = 9.50 °C
[3 marks]

Answer 4

• The boiling point of a substance depends on the energy needed to overcome the intermolecular forces between the molecules.
• Pentane is the most linear molecule so it has the greatest surface contact, and so has the strongest instantaneous dipole-induced dipole bonds. This gives it the highest boiling point.
• The surface contact of 2-methylbutane is less than that of pentane and that of 2,2-dimethylpropane is smaller still, meaning that these substances have weaker instantaneous dipole-induced dipole bonds and consequently lower boiling points.

Question 5

What intermolecular bonds are present in methane?

[1 mark]

Answer 5

Instantaneous dipole-induced dipole bonds only.

Question 6

How does the shape of organic molecules affect the intermolecular bonds between them?

Answer 6

• The longer the carbon chain, the stronger the instantaneous dipole-induced dipole bonds - because there’s more molecular surface contact and more electrons to interact.
• Branched-chain alkanes can’t pack closely together and their molecular surface contact is small compared to straight chain alkanes of similar molecular mass. So fewer instantaneous dipole-induced dipole bonds can form.