Edexcel A-Level Chemistry AS Paper 1, June 2018: Question 7
6 marks · Medium difficulty · Extended Writing
Discuss the reasons for the differences in the boiling temperatures of hydrogen fluoride, water, and methane using given data and electronegativity values.
Practise this questionQuestion
Question text
7 The compounds hydrogen fluoride, water and methane, all have simple molecular
structures, but they have significantly different boiling temperatures.
Discuss the reasons for the differences in the boiling temperatures of the three
compounds, using the data in the table and the Pauling electronegativity values in
the Data Booklet.
Boiling temperature
Compound Number of electrons
/°C
CH4 −161.5 10
H2O 100.0 10
HF 19.5 10
(6)
Mark scheme
Show the mark scheme
Question
Acceptable Answer Additional Guidance Mark
Number
This question assesses a student’s ability to show a Guidance on how the mark scheme should be (6)
coherent and logically structured answer with applied:
linkages and fully-sustained reasoning.
The mark for indicative content should be
Marks are awarded for indicative content and for added to the mark for lines of reasoning.
how the answer is structured and shows lines of
reasoning. For example, an answer with five indicative
marking points, which is partially structured
The following table shows how the marks should be with some linkages and lines of reasoning,
awarded for indicative content. scores 4 marks (3 marks for indicative
content and 1 mark for partial structure and
Number of indicative Number of marks awarded some linkages and lines of reasoning).
marking points seen in for indicative marking
answer points If there are no linkages between points, the
64 same five indicative marking points would
5–4 3 yield an overall score of 3 marks (3 marks for
3–2 2 indicative content and no marks for linkages).
Question
Acceptable Answer Additional Guidance Mark
Number
The following table shows how the marks should be awarded
contd for structure and lines of reasoning.
Number of marks awarded
for structure of answer and In general it would be expected
sustained line of reasoning that 5 or 6 indicative points would get 2
Answer shows a coherent and 2 reasoning marks, and 3 or 4 indicative
logical structure with linkages and points would get 1 mark
fully sustained lines of reasoning for reasoning, and 0, 1 or 2
demonstrated throughout. indicative points would score zero marks
Answer is partially structured with 1 for reasoning.
some linkages and lines of
reasoning. If there is any incorrect chemistry,
Answer has no linkages between 0 deduct mark(s) from the reasoning. If
points and is unstructured.
no reasoning mark(s) awarded do not
deduct mark(s).
Read all of the answer first as IPs can be
Indicative content: found anywhere in the answer
IP1 Electrons
Same number of electrons so similar / the same Allow high electronegativity of F and O
(compared to H)
London forces / van der Waals’ forces / dispersion
Allow HF and H2O (highly) polar and CH4
forces non polar
IP2 Electronegativity
Large electronegativity differences in HF and H2O Allow IP2 for any three of:
and small in CH4 / quoting all electronegativity F=4.0, O=3.5, H=2.1, C=2.5
values of differences / combination of previous Allow IP2 for any two of:
three alternatives covering all three bonds HF=1.9, HO=1.4, HC=0.4
These values may be seen anywhere
IP3 Intermolecular forces in methane
Only (weak) London forces / van der Waals’
Allow no dipole-dipole forces / no hydrogen
forces / dispersion forces in CH4 bonds in CH
Award IP3 if London forces are the only
intermolecular forces mentioned in CH4
IP4 Intermolecular forces in water and hydrogen
fluoride
Hydrogen bonding in both HF and H2O (but not
CH4)
IP5 Relative numbers of hydrogen bonds
May be shown in a diagram
More hydrogen bonds / (average of) twice as
many hydrogen bonds in H2O than in HF
IP6 Energy
More energy needed to break stronger Do not award IP6 for any clear indication of
intermolecular forces / less needed to break covalent bond breaking or ionic bond
weaker intermolecular forces. breaking
How to answer it
Comparing Boiling Temperatures of CH₄, H₂O, and HF
What this question tests
This is a 6-mark extended-response question assessing your ability to link molecular structure, electronegativity, and intermolecular forces to physical properties. You must combine data analysis (number of electrons) with chemical theory (London forces vs. hydrogen bonding) while maintaining a logical, coherent line of reasoning.
Question 7: Discussion of Boiling Temperatures
Full Mark Scheme Breakdown & Model Answer Strategy
💡 Key Knowledge: Indicative Content (IPs)
- IP1 (Electrons): All three molecules have 10 electrons, meaning they have very similar London / van der Waals' forces.
- IP2 (Electronegativity): Large electronegativity differences exist in HF and H₂O, whereas CH₄ is non-polar (or has very small differences).
- IP3 (Methane Forces): Methane only possesses weak London forces between molecules.
- IP4 (Polar Forces): Both HF and H₂O exhibit hydrogen bonding alongside London forces.
- IP5 (Ratio): Water has more hydrogen bonds per molecule (average of 2 per molecule) compared to HF (1 per molecule).
- IP6 (Energy): More energy is required to break stronger intermolecular forces (hydrogen bonds) than weaker ones (London forces).
🧠 Exam Technique: Level of Response
This question uses a levels-of-response marking grid:
- Indicative Content (up to 4 marks): Awarded based on how many of the 6 key points (IP1–IP6) you successfully include.
- Line of Reasoning (up to 2 marks): Awarded for a coherent, logically structured narrative that explicitly links structure to properties without contradictory or incorrect chemistry.
✅ Model Chemistry Points
A top-level response should flow logically:
- State that CH₄ , H₂O , and HF all have 10 electrons, meaning their London forces are initially comparable.
- Explain that despite this, CH₄ has the lowest boiling temperature (-161.5 °C) because it is non-polar and relies only on weak London forces.
- Introduce electronegativity: F and O are highly electronegative compared to H, creating polar bonds in HF and H₂O .
- Explain that this enables hydrogen bonding between HF and H₂O molecules, which is much stronger than London forces and requires more energy to overcome.
- Compare H₂O (100.0 °C) and HF (19.5 °C): Water forms an average of two hydrogen bonds per molecule, whereas HF forms fewer, meaning more energy is needed to boil water.
❌ Common Student Errors
- Covalent bond breaking: Stating that boiling water breaks O-H or H-F covalent bonds. Boiling only overcomes intermolecular forces, not covalent bonds!
- Ignoring electron data: Failing to mention that electron numbers are identical, missing the crucial comparison point that London forces alone cannot explain the vast differences.
- Confusing hydrogen bonding capacity: Stating HF has more hydrogen bonds than water. Water has two lone pairs on O and two hydrogen atoms, allowing an average of two hydrogen bonds per molecule.
- Unstructured bullet points: Writing disconnected facts without logical transitions, which caps the "Line of Reasoning" marks.
Topics
Physical Chemistry · Topic 2: Bonding and Structure
Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.