AQA A-Level Chemistry: Intermolecular Forces and Their Effects on Physical Properties — mark scheme explained

The question

Explain the types of intermolecular forces present in water (H 2 O) and how they influence its boiling point. [Paraphrased for study — not reproduced from any exam paper.]

Mark scheme, decoded

What each mark is really for — in plain English — and the wording trap that loses it.

• S1

  Step 1: Identify the molecular structure of water.

• S2

  Water has a bent shape with two hydrogen atoms bonded to an oxygen atom.

• S3

  Step 2: Determine the electronegativity difference between the atoms.

• S4

  Oxygen is more electronegative than hydrogen, leading to a permanent dipole moment in the molecule.

• S5

  Step 3: Identify the types of intermolecular forces.

• S6

  Water molecules experience both permanent dipole-dipole forces and hydrogen bonding.

• S7

  Step 4: Explain how these forces influence the boiling point.

• S8

  The strong hydrogen bonds between water molecules require a lot of energy to break, resulting in a high boiling point of 100°C.

Model answer

Worked through, with each step tagged to the mark it earns.

1. S1

   Step 1: Identify the molecular structure of water.

2. S2

   Water has a bent shape with two hydrogen atoms bonded to an oxygen atom.

3. S3

   Step 2: Determine the electronegativity difference between the atoms.

4. S4

   Oxygen is more electronegative than hydrogen, leading to a permanent dipole moment in the molecule.

5. S5

   Step 3: Identify the types of intermolecular forces.

6. S6

   Water molecules experience both permanent dipole-dipole forces and hydrogen bonding.

7. S7

   Step 4: Explain how these forces influence the boiling point.

8. S8

   The strong hydrogen bonds between water molecules require a lot of energy to break, resulting in a high boiling point of 100°C.

9. Final answer: Water (H 2 O) has both permanent dipole-dipole forces and hydrogen bonding. The strong hydrogen bonds between water molecules require a significant amount of energy to overcome, leading to a high boiling point of 100°C.

Common mistakes

• Misidentifying the types of intermolecular forces in a molecule. — Review the definitions and characteristics of permanent dipole-dipole forces, van der Waals forces, and hydrogen bonding. Practice identifying these forces in different molecules.
• Failing to consider the strength of intermolecular forces when comparing boiling points. — Always compare the strengths of intermolecular forces, especially hydrogen bonding, when explaining differences in physical properties like boiling points.
• Not fully explaining how intermolecular forces influence physical properties. — Practice providing detailed and specific explanations, using examples to illustrate how intermolecular forces affect melting and boiling points.
• Confusing hydrogen bonding with other types of dipole-dipole interactions. — Review the criteria for hydrogen bonding (hydrogen bonded to a highly electronegative atom) and practice identifying it in different molecules.
• Failing to explain the low density of ice correctly. — Practice explaining how hydrogen bonds cause water molecules to arrange in a hexagonal lattice, taking up more space and resulting in lower density.
• Not considering the size and surface area of molecules when comparing van der Waals forces. — Always consider both the number of electrons and the size/surface area of molecules when comparing the strength of van der Waals forces.

Where the marks go

• Full worked solution (all marking points)4 marks

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