Edexcel A-Level Chemistry AS Paper 2, June 2024: Question 5

✅ Correct Answers

• Condenser: Prevents the loss of volatile reactants/reagents (e.g. butan-1-ol) or products by condensing vapours back into the flask; OR prevents escape of harmful/toxic/flammable vapours. [1 mark]
• Still head: Prevents pressure build-up / prevents risk of explosion by leaving the apparatus open to the atmosphere. [1 mark]

💡 Key Knowledge

The reaction between conc. H₂SO₄ and NaBr/water is vigorously exothermic:

NaBr + H₂SO₄ → HBr + NaHSO₄

Volatile organic liquids (butan-1-ol boiling point = 117 °C, 1-bromobutane = 102 °C) readily vaporise upon heating. The vertical condenser ensures reflux. The side arm of the still head remains unsealed so gas can escape safely.

🧠 Exam Technique

Always state why condensation is useful. Writing just "it condenses gas" is a mere description and scores zero. You must state that it prevents loss of reactants/products.

❌ Common Errors

• Vaguely stating "the still head holds the tap funnel" (this is purely physical support, not its functional chemical/safety purpose).
• Saying the condenser "condenses toxic HBr or SO₂" (these are gases with boiling points well below room temperature and will not condense under cold tap water).

✅ Correct Answers

(a)(ii) Oxidising agent: Concentrated sulfuric acid / conc. H₂SO₄ [1 mark]

(a)(iii) Inorganic product: Bromine / Br₂ OR sulfur dioxide / SO₂ [1 mark]

(a)(iii) Organic product: Butanal / CH₃CH₂CH₂CHO OR butanoic acid / CH₃CH₂CH₂COOH [1 mark]

💡 Key Knowledge

Concentrated H₂SO₄ acts as an oxidising agent:

• Oxidation of bromide: 2Br⁻ + H₂SO₄ + 2H⁺ → Br₂ + SO₂ + 2H₂O
• Oxidation of alcohol: Butan-1-ol (primary alcohol) is oxidised to butanal (aldehyde) or butanoic acid (carboxylic acid).

❌ Common Errors

• Writing just "dilute sulfuric acid" for (a)(ii) — dilute acid is NOT an oxidising agent!
• Writing molecular formulae like C₄H₈O for the organic product — structural or displayed formulae are required for organic species to show functional group identity.
• Listing but-1-ene as the redox product — elimination is an acid-catalysed dehydration, NOT a redox reaction.

🧠 Exam Technique

If you give both a name and a formula, both must be correct. If you write "bromine, Br", you lose the mark because the formula of elemental bromine is Br₂.

✅ Correct Answer

At a lower temperature, fewer molecules/collisions have energy greater than or equal to the activation energy (E ≥ E_a), so the rate of these unwanted redox reactions decreases significantly / reaction slows down. [1 mark]

🧠 Exam Technique

You can answer using either:

1. Energetics: Lower temperature means fewer particles have the activation energy needed to react.
2. Rate: Low temperature dramatically slows down the rate of the unwanted side reaction.

✅ Correct Description / Points Awarded

• Point 1: The still head is placed directly into the neck of the pear-shaped (or round-bottomed) flask. [1 mark]
• Point 2: A thermometer is placed at the top of the still head (with its bulb adjacent to the side arm). [1 mark]
• Point 3: The condenser is attached to the side arm of the still head sloping downwards, leading to a collecting vessel (beaker/flask). [1 mark]

💡 Apparatus Setup Description

Water must always enter the condenser at the lowest point (bottom) and leave from the top to ensure the water jacket fills completely without air pockets.

❌ Common Errors

• Leaving the condenser upright: distillation requires the condenser to be angled downwards.
• Forgetting the thermometer: without a thermometer at the junction of the still head, you cannot monitor the boiling fraction (101 °C – 103 °C).
• Sealing the system completely (e.g. stopper on collecting vessel): simple distillation must remain open to air to avoid pressure build-up.

✅ Correct Answers

(c)(i) Diagram: A drawn beaker showing two distinct liquid layers: [1 mark]

• Top layer: Water / aqueous layer
• Bottom layer: 1-bromobutane

(c)(ii) Removing aqueous layer: Use a teat pipette (or syringe) to draw off the upper aqueous layer (or pipette out the lower layer into another container). [1 mark]

(c)(iii) Apparatus for Steps 6 & 7: Separating funnel (or dropping funnel / tap funnel). [1 mark]

📐 Density Comparison

• Density of water = 1.00 g cm⁻³
• Density of 1-bromobutane = 1.28 g cm⁻³

Since 1.28 > 1.00, 1-bromobutane is denser and sinks to the bottom. The less dense aqueous phase floats on top.

❌ Common Errors

• In (c)(i): Drawing a separating funnel instead of a beaker as specifically requested in the prompt.
• In (c)(ii): Saying "decant the top layer" — decanting does not provide clean separation of two miscible or liquid-liquid layers and loses the mark.
• In (c)(iii): Writing "burette" — burettes are for titrations, not for washing and shaking immiscible solvent layers.

🧠 Exam Technique

Always re-read the prompt: Step 4 states the distillate was "collected in a small beaker, forming two layers". Part (c)(i) asked for a diagram of the beaker, not standard separating funnel glassware!