Forces between molecules

Weak pulls that decide boiling points

Forces between molecules are called intermolecular forces. They are much weaker than the bonds between atoms. They are weakest in gases and strongest in solids.

The stronger these forces, the higher the boiling point of a liquid and the melting point of a solid.

In HCl, chlorine is more electronegative, so the molecule has a partly positive H end and a partly negative Cl end. Opposite ends of neighbouring molecules attract: dipole–dipole forces.

When hydrogen is bonded to F, O or N, the bond is highly polar and the attraction is strong. This special case is hydrogen bonding. It gives water higher melting and boiling points than H₂S and NH₃.

Boiling separates molecules, not atoms, so it is the intermolecular forces that decide the boiling point.

Key terms

Intermolecular forces

Explaining melting and boiling points.

Electronegativity

Deciding which end of a bond is partly negative.

Dipole–dipole force

Attraction between polar molecules.

Hydrogen bonding

Explaining water's high boiling point.

Short questions with model answers

1. Q1. Why is H₂O a liquid while H₂S is a gas? (MCQ iii, p. 50)

   • In H₂O, H is bonded to O
   • Water molecules form hydrogen bonds
   • H₂S: S is not F, O or N

   Water is polar with strong forces (hydrogen bonds) between its molecules.

2. Q2. Why does iodine exist as a solid while chlorine is a gas? (CRQ vi, p. 51)

   • Both are non-polar diatomic molecules
   • I₂ molecules are much larger and heavier

   Stronger forces between the larger I₂ molecules keep iodine solid; Cl₂'s forces are very weak (p. 48).

Common mistakes

• ✗ Boiling water breaks the O–H bonds inside each molecule.

  ✓ Boiling separates whole molecules; only the forces between molecules are overcome.

• ✗ Any molecule with hydrogen forms hydrogen bonds, such as H₂S and CH₄.

  ✓ Hydrogen must be bonded to F, O or N (p. 45).

• ✗ Intermolecular forces are stronger than covalent bonds.

  ✓ They are generally very weak compared with bonds between atoms (p. 44).

MCQs

1. 1. Considering intermolecular forces, which has the highest boiling point?

   1. (a) H₂S
   2. (b) HF
   3. (c) NH₃
   4. (d) H₂O
   Show answer

   (d) MCQ (vi), p. 50: water's hydrogen bonding gives it the highest boiling point of these (p. 46).

2. 2. Why is H₂O a liquid while H₂S is a gas?

   1. (a) Oxygen atoms are smaller than sulphur atoms
   2. (b) Water is polar with strong forces between its molecules
   3. (c) H₂O is lighter than H₂S
   4. (d) Water freezes easily
   Show answer

   (b) MCQ (iii), p. 50: the book's answer is (b), strong forces between polar water molecules.

3. 3. Hydrogen bonding happens when hydrogen is bonded to:

   1. (a) F, O or N
   2. (b) Any metal
   3. (c) Carbon
   4. (d) Sulphur
   Show answer

   (a) p. 45: hydrogen covalently bonded to highly electronegative F, O or N.

4. 4. In which state are intermolecular forces the weakest?

   1. (a) Solid
   2. (b) Liquid
   3. (c) Gas
   4. (d) All the same
   Show answer

   (c) p. 44: weakest among molecules of gases, strongest among molecules of solids.

5. 5. What attraction acts between neighbouring HCl molecules?

   1. (a) Ionic bonds
   2. (b) Metallic bonds
   3. (c) Hydrogen bonds
   4. (d) Dipole–dipole forces
   Show answer

   (d) p. 45: the δ+ H of one HCl molecule attracts the δ− Cl of the next (Fig 3.12).

Quick revision

• Intermolecular forces act between molecules and are much weaker than bonds.
• Polar molecules attract by dipole–dipole forces; H on F, O or N gives hydrogen bonding.
• Stronger forces mean higher melting and boiling points: water is a liquid, H₂S a gas.