Intermolecular forces, electronegativity and bond properties

AS Level Chemistry · Chemical bonding. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Electronegativity is the power of an atom to pull the shared electrons of a covalent bond towards itself. If two bonded atoms differ in electronegativity, the bond is polar: one end is δ− and the other is δ+. A molecule has an overall dipole only if its bond dipoles do not cancel, so the shape matters.

The forces between molecules are called van der Waals' forces. All molecules have instantaneous dipole–induced dipole (id-id) forces. Polar molecules also have permanent dipole–permanent dipole (pd-pd) forces.

A hydrogen bond is the strongest type. It forms between an H atom bonded to N or O and a lone pair on an N or O atom of a nearby molecule. All these forces are much weaker than ionic, covalent or metallic bonds.

Rules to remember

Common mistake

Students say that covalent bonds break when a molecular substance boils. Only the forces between the molecules are overcome; the O–H bonds inside each water molecule stay intact.

Worked example

Ethane (Mr 30), fluoromethane (Mr 34) and methanol (Mr 32) have similar numbers of electrons. Put them in order of increasing boiling point.

  1. Similar numbers of electrons means the id-id forces are of similar strength in all three.
  2. Ethane, CH3CH3, is non-polar, so it has id-id forces only.
  3. Fluoromethane, CH3F, has a polar C–F bond, so it also has pd-pd forces, but it has no H bonded to N or O.
  4. Methanol, CH3OH, has an O–H group, so it forms hydrogen bonds, the strongest of the three.

Answer: ethane < fluoromethane < methanol

Practice questions

Try each one, then open the answer.

1. What is the maximum number of hydrogen bonds that one water molecule can form with neighbouring water molecules, as in ice?

  1. A
    1
  2. B
    2
  3. C
    3
  4. D
    4
Show answer

Answer: D. Each water molecule has two H atoms (each can bond to a lone pair on a neighbour) and two lone pairs (each can accept an H from a neighbour), giving four hydrogen bonds arranged tetrahedrally in ice.

2. Which compound forms hydrogen bonds between its own molecules?

  1. A
    CH3OCH3
  2. B
    CH3Cl
  3. C
    CH3CHO
  4. D
    CH3CH2OH
Show answer

Answer: D. Hydrogen bonding needs a hydrogen atom bonded directly to N or O. Only ethanol has an O–H group; the ether and aldehyde contain oxygen but no O–H, so they cannot hydrogen bond with themselves.

3. Propanone (Mr 58) boils at 56 °C but butane (Mr 58) boils at −1 °C. What is the main reason?

  1. A
    propanone forms hydrogen bonds between its molecules
  2. B
    propanone molecules have a permanent dipole, so permanent dipole–permanent dipole forces act in addition to id-id forces
  3. C
    butane has weaker covalent bonds
  4. D
    propanone has more electrons than butane
Show answer

Answer: B. The polar C=O group gives propanone a permanent dipole. Propanone has no O–H or N–H, so it cannot hydrogen bond with itself; both molecules have the same number of electrons, so id-id forces are similar.

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