Edexcel A-Level Chemistry AS Paper 1, June 2016: Question 6

5 marks · Medium difficulty · Open Response

Explain the differences in bond angles between ammonia and water, and compare the O-H and S-H bond lengths.

Practise this question

Question

The question contains two parts. Part (a) shows diagrams of ammonia (showing an H-N-H bond angle of 107 degrees) and water (showing an H-O-H bond angle of 104.5 degrees) and asks to explain why the bond angle in water is less than in ammonia, worth 2 marks. Part (b) shows diagrams of hydrogen sulfide (S-H bond length 133.6 pm) and water (O-H bond length 95.7 pm) and asks to explain why the O-H and S-H bond lengths are different, worth 3 marks.
Question text

6 (a) The diagram shows bond angles in ammonia and water.

N O

H 107° H H 104.5° H

H

Explain why the bond angle in water is less than the bond angle in ammonia.

(2)

(b) Explain why the O–H and S–H bond lengths are different.

S O

133.6 pm 95.7 pm

H H H H

(3)

(Total for Question 6 = 5 marks)

Mark scheme

Show the mark scheme The mark scheme for question 6(a) awards 2 marks: one for stating oxygen has more lone pairs than nitrogen (two versus one), and one for stating that lone pair-lone pair or lone pair-bonding pair repulsion is greater. The mark scheme for question 6(b) awards 3 marks: one for sulfur being a larger atom with more electron shells, one for greater inner shell shielding, and one for reduced nuclear attraction for the bonding electrons resulting in a longer bond length.

How to answer it

Shapes of Molecules and Covalent Bond Lengths

What this question tests

This question assesses your understanding of Electron Pair Repulsion Theory (VSEPR)—specifically how lone pairs affect bond angles—alongside periodic trends in atomic radius, shielding, and how these factors determine covalent bond length.

Part (a): Explaining Bond Angles in Water vs. Ammonia

Question 6 (a) — 2 Marks

✅ Correct Answer

  • Water (H₂O) has two lone pairs on the central oxygen atom, whereas ammonia (NH₃) only has one lone pair on the central nitrogen atom.
  • Lone pair–lone pair repulsion is greater than lone pair–bonding pair repulsion, resulting in greater compression of the bond angle in water (104.5°) compared to ammonia (107°).

💡 Key Knowledge

  • Repulsion hierarchy: Lone pair–lone pair > Lone pair–bonding pair > Bonding pair–bonding pair.
  • Each lone pair reduces the bond angle by approximately 2.5°.

🧠 Exam Technique

  • Always explicitly compare both molecules: state how many lone pairs each species has.
  • Use precise terminology: refer to repulsion between electron pairs rather than vague phrasing.

❌ Common Errors

  • Saying atoms are "pushed together" instead of explaining that electron pairs repel.
  • Forgetting to mention the difference in the number of lone pairs between the two molecules.
Mark Breakdown: (1) Mark for identifying that oxygen has two lone pairs while nitrogen has one (or water has one more lone pair). (2) Mark for stating that repulsion from the oxygen lone pairs is greater, reducing the bond angle.

Part (b): Explaining O–H and S–H Bond Length Differences

Question 6 (b) — 3 Marks

✅ Correct Answer

  • Sulfur is a larger atom than oxygen / has a larger atomic radius / has more electron shells.
  • Sulfur has greater inner-shell shielding compared to oxygen.
  • This results in a weaker nuclear attraction for the shared bonding electrons, producing a longer bond length (133.6 pm vs. 95.7 pm).

💡 Key Knowledge

  • Down Group 16, atomic radius increases due to the addition of principal quantum shells.
  • Inner shells shield the outer valence electrons and bonding pairs from the attractive pull of the positive nucleus.

🧠 Exam Technique

  • Structure your answer causally: Size/Shells ➔ Shielding ➔ Attraction ➔ Bond Length.
  • Treat each marking point as independent—make sure you explicitly write out each step of the chain.

❌ Common Errors

  • Referring to "ionic radius" or "ions" (these are covalent molecules, not ions).
  • Mentioning "nuclear charge" alone without linking it to shielding or distance (sulfur actually has a higher nuclear charge than oxygen, but distance and shielding override it).
  • Discussing electronegativity instead of atomic size and shielding.
Mark Breakdown: (1) Sulfur is larger / has more shells. (2) Sulfur has greater shielding. (3) Reduced nuclear attraction for bonding electrons leads to a longer bond length.

Topics

Physical Chemistry · Topic 2: Bonding and Structure

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, June 2016. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.