AQA A-Level Chemistry AS Paper 2, June 2025: Question 14

1 mark · Easy difficulty · Multiple Choice

Identify the correct statement regarding the properties and structure of sulfur dioxide.

Practise this question

Question

Question 14 asks: 'Sulfur dioxide can be found in the flue gases from the combustion of some fuels. Which statement about sulfur dioxide is correct?' followed by four multiple-choice options: A 'Sulfur dioxide molecules have a linear shape', B 'Sulfur dioxide is a basic oxide', C 'Sulfur dioxide reacts with calcium oxide to form CaSO4 as the main product', and D 'Sulfur dioxide molecules contain polar bonds'.
Question text

14 Sulfur dioxide can be found in the flue gases from the combustion of some fuels.

Which statement about sulfur dioxide is correct?

[1 mark]

A Sulfur dioxide molecules have a linear shape.

B Sulfur dioxide is a basic oxide.

C Sulfur dioxide reacts with calcium oxide to form CaSO4 as the main product.

D Sulfur dioxide molecules contain polar bonds.

Mark scheme

Show the mark scheme Mark scheme table row for question 14 indicating that the correct option is D ('Sulfur dioxide molecules contain polar bonds.') for 1 mark (AO1).

14 D 1 (AO1) Sulfur dioxide molecules contain polar bonds.

How to answer it

Properties of Sulfur Dioxide in Flue Gases

📌 What this question tests

This question assesses your synoptic understanding of Periodicity and Bonding (electronegativity, bond polarity, and VSEPR molecular shapes) alongside environmental and inorganic chemistry from Alkanes/Group 2 (acidic nature of SO₂ and flue gas desulfurisation).

Question 14 (Multiple Choice)

1 Mark • Assessment Objective 1 (AO1)

✅ Correct Answer: D

Sulfur dioxide molecules contain polar bonds.

Oxygen is significantly more electronegative than sulfur (Paulings: O ≈ 3.5, S ≈ 2.5). The bonding electron density is pulled towards the oxygen atoms, creating polar bonds: Sδ+=Oδ- .

Award 1 mark for option D.

💡 Key Knowledge Breakdown

  • Electronegativity: A difference in electronegativity between two covalently bonded non-metal atoms always leads to a permanent bond dipole.
  • Acidic Oxides: Non-metal oxides (like SO₂ and SO₃) are acidic, reacting with water to form sulfurous acid (H₂SO₃).
  • Flue Gas Desulfurisation: SO₂ reacts with basic oxides/carbonates (such as CaO or CaCO₃) to produce calcium sulfite:
    CaO + SO₂ → CaSO₃
  • VSEPR Shape: In SO₂, sulfur has 2 bonding regions (double bonds) and 1 lone pair (3 electron-dense areas), giving a bent / V-shaped geometry with a bond angle of ≈ 119°.

📐 Systematic Evaluation of Options

  • A is incorrect: The central sulfur atom has one lone pair of electrons and two double bonds to oxygen. 3 regions of electron density arrange in a trigonal planar geometry, so the molecular shape is bent/non-linear, not linear like CO₂.
  • B is incorrect: SO₂ is an acidic oxide (typical for non-metals across Period 3), not a basic oxide.
  • C is incorrect: The direct reaction of CaO with SO₂ yields CaSO₃ (calcium sulfite), not CaSO₄ (calcium sulfate). CaSO₄ is only formed upon subsequent oxidation.
  • D is correct: Because oxygen is more electronegative than sulfur, each S=O bond is polar.

❌ Common Errors & Traps

  • Confusing SO₂ with CO₂: Students assume SO₂ is linear like CO₂. However, sulfur has 6 valence electrons (forming two double bonds and leaving 1 lone pair), whereas carbon has no lone pairs left in CO₂.
  • The CaSO₄ Trap in Option C: Remembering that gypsum/calcium sulfate is the eventual commercial product of flue-gas desulfurisation plants leads students to wrongly believe the immediate product of CaO + SO₂ is CaSO₄. The direct neutralisation product is CaSO₃ .
  • Polar Bond vs Polar Molecule: Option D states the molecule contains polar bonds. Even though SO₂ is also a polar molecule (dipoles do not cancel because it is bent), the statement only requires bond polarity, which is indisputably true.

🧠 Exam Technique: Elimination Strategy

Multiple-choice questions with qualitative statements are best tackled by rapid falsification:

  1. Check acid/base character: Non-metal oxide = acidic → eliminate B immediately.
  2. Check valence electron count: Sulfur is in Group 6; 4 electrons used for bonds + 2 non-bonding electrons = 1 lone pair. Lone pairs repel and force a bent shape → eliminate A.
  3. Check reaction stoichiometry: CaO + SO₂ only has three oxygens on the left side, so the primary product must be CaSO₃ without an external oxidising agent → eliminate C.
  4. Confirm D: Different non-metal atoms joined covalently have different electronegativities → polar bonds confirmed.

Topics

Physical Chemistry · Inorganic Chemistry · Organic Chemistry · 3.1.3 Bonding · 3.2.4 Properties of Period 3 Elements and Their Oxides · 3.3.2 Alkanes

Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.