A-Level · Chemistry · AQA · Mark scheme decoded
AQA A-Level Chemistry: Intermolecular Forces and Their Effects on Physical Properties — mark scheme explained
The short answer
In A-Level Chemistry, understanding the different types of intermolecular forces is crucial for explaining various physical properties of substances, such as melting and boiling points. This section focuses on three main types of intermolecular forces: permanent dipole-dipole forces, induced dipole-dipole (van der Waals or dispersion) forces, and hydrogen bonding.
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
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For explanation questions, marks are typically awarded for clear and concise descriptions that demonstrate a deep understanding of the topic. For comparison questions, marks are given for identifying similarities and differences and relating them to the strength of intermolecular forces. For prediction questions, marks are awarded for applying concepts correctly to unfamiliar molecules.
What the command words demand
- Explain
- Provide a detailed description of the concept, including relevant principles and relationships.
- Compare
- Identify similarities and differences between two or more substances or concepts.
- Predict
- Use your understanding of intermolecular forces to predict physical properties of unfamiliar molecules.
- Identify
- Recognize and name the types of intermolecular forces present in a given molecule.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the problem, provide detailed explanations, and check your work.
- Identify the molecular structure of water.0 marksWater has a bent shape with two hydrogen atoms bonded to an oxygen atom.
- Determine the electronegativity difference between the atoms.1 markOxygen is more electronegative than hydrogen, leading to a permanent dipole moment in the molecule.
- Identify the types of intermolecular forces.1 markWater molecules experience both permanent dipole-dipole forces and hydrogen bonding.
- Explain how these forces influence the boiling point.2 marksThe strong hydrogen bonds between water molecules require a lot of energy to break, resulting in a high boiling point of 100°C.
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.
Work through every step correctly and you earn all 4 marks.
Another worked example
Compare the boiling points of methane (CH 4 ) and butane (C 4 H 10 ) and explain why they differ.
- Identify the molecular structures of methane and butane.0 marksMethane is a simple alkane with one carbon atom, while butane has four carbon atoms.
- Determine the types of intermolecular forces present in each molecule.1 markBoth methane and butane experience van der Waals (dispersion) forces.
- Compare the strength of these forces.2 marksButane has more electrons and a larger surface area, leading to stronger van der Waals forces compared to methane.
- Explain how this affects their boiling points.1 markMethane has a low boiling point (-161.5°C) due to weak van der Waals forces, while butane has a higher boiling point (-0.5°C) because of stronger van der Waals forces.
Final answer: Methane (CH 4 ) and butane (C 4 H 10 ) both experience van der Waals forces, but butane has more electrons and a larger surface area, leading to stronger van der Waals forces. This results in methane having a low boiling point (-161.5°C) and butane having a higher boiling point (-0.5°C).
Work through every step correctly and you earn all 4 marks.
Common mistakes
Misidentifying the types of intermolecular forces in a molecule.
Why it happens: Students may not fully understand the criteria for each type of intermolecular force, leading to incorrect identification.
Fix: 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.
Why it happens: Students may focus only on the presence of a particular type of force without considering its relative strength.
Fix: 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.
Why it happens: Students may provide incomplete or vague explanations that do not clearly link the type of force to the observed property.
Fix: 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.
Why it happens: Students may not fully understand the specific conditions required for hydrogen bonding, leading to confusion with other dipole-dipole forces.
Fix: 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.
Why it happens: Students may not fully understand the role of hydrogen bonding in the hexagonal lattice structure of ice.
Fix: 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.
Why it happens: Students may focus only on the number of electrons without considering the molecular size and surface area.
Fix: 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
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Intermolecular Forces | Identify the intermolecular forces in water and explain their effect on boiling point. | 4 |
| Compare Boiling Points | Compare methane and butane boiling points and explain the difference using intermolecular forces. | 4 |
| Explain Density Difference | Explain how hydrogen bonding creates an open lattice in ice, lowering its density below water. | 3 |
| Compare Boiling Points | Explain a boiling point difference by comparing the intermolecular forces present in each molecule. | 4 |
| Total across these question types | 15 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.