Edexcel A-Level Chemistry AS Paper 2, November 2021: Question 5

✅ Correct Answer

2CH₄ + 3O₂ → 2CO + 4H₂O

(Multiples of this equation are also fully accepted).

💡 Key Knowledge

• Incomplete combustion occurs when there is a limited supply of oxygen.
• Instead of forming carbon dioxide (CO₂), carbon monoxide (CO) and water (H₂O) are produced.

🧠 Exam Technique

Balance the carbon and hydrogen atoms first before attempting to balance the oxygen atoms. Remember to check if your coefficients can be simplified, though correct multiples still gain full credit.

❌ Common Errors

Students frequently mix up incomplete and complete combustion products, writing CO₂ instead of CO, or accidentally including carbon (soot) when the question explicitly restricts products to carbon monoxide and water only.

✅ Correct Answer

• Sulfur oxides: Sulfur impurities/compounds present in the fossil fuel react with oxygen from the air.
• Nitrogen oxides: Nitrogen in the air reacts with oxygen (at high temperatures inside the engine/furnace).

💡 Key Knowledge

• Fossil fuels are biological in origin and often contain sulfur-containing contaminants.
• Atmospheric nitrogen (N₂) is extremely unreactive under normal conditions, but the high thermal energy of combustion provides the activation energy needed for nitrogen to react with oxygen.

🧠 Exam Technique

Be precise about the source of each reactant. Examiners penalise answers that confuse fuel-bound sulfur with atmospheric nitrogen. Mentioning "oxygen from the air" is essential.

❌ Common Errors

A major misconception is stating that nitrogen oxides come from nitrogen compounds impurities within the fuel. While fuel nitrogen exists, the primary source tested at AS-Level is fixation of atmospheric N₂.

✅ Correct Answer

• Although sulfur dioxide is removed, carbon dioxide is produced in the first reaction ( SO₂(g) + CaCO₃(s) → CaSO₃(s) + CO₂(g) ).
• Carbon dioxide is a greenhouse gas that contributes to global warming and must also be managed.

💡 Key Knowledge

• Flue gas desulfurisation (FGD) solves acid rain caused by SO₂, but trades it for carbon emissions.
• Carbon dioxide is a key driver of climate change, meaning technological fixes often create alternative environmental dilemmas.

🧠 Exam Technique

Explicitly link your explanation back to the provided equations. Quote or reference CO₂ production as proof that the process introduces a different environmental pollutant.

❌ Common Errors

Vague answers such as "it creates pollution" or "it uses up energy" fail to score. You must specifically identify that carbon dioxide gas is released and link it to the greenhouse effect/global warming.

✅ Correct Answer

Sulfur (Group 6) forms a central atom bonded to two oxygen atoms. Typically represented with:

• One double covalent bond (sharing 4 electrons total) and one dative covalent / coordinate bond (sharing 2 electrons from sulfur), OR two double bonds with a lone pair on sulfur.
• A lone pair clearly shown on the central sulfur atom.
• Outer shell electrons correctly paired around all atoms.

💡 Key Knowledge

Sulfur can expand its octet because it is in Period 3 and has access to low-lying vacant 3d orbitals. SO₂ has a bent/V-shaped molecular geometry due to the repulsion of the lone pair on the sulfur atom.

🧠 Exam Technique

Count your electrons carefully: Sulfur has 6 outer electrons, and each oxygen contributes 6. Ensure inner shells are omitted if the question asks for outer electrons only, and clearly display any lone pairs on the central atom.

❌ Common Errors

A frequent zero-mark error is drawing 3 electron pairs in a single bond or creating invalid S-O triple bonds. Do not draw 3 electron S-O bonds.