AQA A-Level Chemistry AS Paper 2, June 2025: Question 6

✅ Correct Answer

Add + H₂O to the reactant side:

Cyclohexene + H₂O → Cyclohexanol

❌ Common Errors

• Writing + H₃PO₄ or + H⁺ on the reactant side. Phosphoric acid is a catalyst, not a stoichiometric reagent.
• Writing H₃PO₄ or H⁺ on both sides of the equation.

✅ Correct Arrows (3 Marks)

• Arrow 1 (M1): From the C=C double bond to the H⁺ ion.
• Arrow 2 (M2): From the lone pair on the oxygen atom of :OH₂ to the positively charged carbon atom (carbocation) on the ring.
• Arrow 3 (M3): From one of the O–H single bonds onto the O⁺ atom of the protonated intermediate.

🧠 Exam Technique: Curly Arrow Rules

• Start precisely: Arrows must begin either from the centre of a bond or directly from a lone pair of electrons.
• Point precisely: The arrowhead must point directly at the atom receiving electrons.
• Never draw arrows pointing away from a positive charge; electrons move toward positive or electron-deficient centres!

📐 Step-by-Step Calculation

Step 1: Calculate total mass of solution

Mass = Volume × Density = 6.0 cm³ × 1.69 g cm⁻³ = 10.14 g

Step 2: Calculate mass of pure H₃PO₄ (85% by mass)

Mass of pure acid = 10.14 g × 0.85 = 8.619 g

Step 3: Calculate M_r of H₃PO₄

M_r = (3 × 1.0) + 31.0 + (4 × 16.0) = 98.0 g mol⁻¹

Step 4: Calculate moles

Moles = 8.619 / 98.0 = 0.087948... mol

Final Answer: 0.088 mol (or 0.0879 mol, at least 2 sig figs)

❌ Common Errors & Examiner Traps

• Forgetting the 85% purity: Dividing 10.14 directly by 98.0 gives 0.103 mol (loses M2, scores only 1 mark).
• Incorrect M_r: Ensure correct atomic masses from the Periodic Table (H = 1.0, P = 31.0, O = 16.0 → 98.0). If M_r is wrong, error carried forward (ECF) allows 1 mark.
• Premature rounding: Keep intermediate values in your calculator memory to avoid rounding errors in the final answer.

✅ Correct Answer

• M1: To ensure the outer jacket of the condenser fills completely with water (prevents air bubbles / air pockets from forming).
• M2: To keep the condenser cooler / ensure more efficient cooling so that vapour condenses effectively.

❌ Common Misconceptions

• Saying "to condense water vapour" – the liquid being purified is cyclohexanol, not water!
• Only stating "it cools the vapour" without explaining that filling from the bottom prevents airlocks and fills the entire jacket.

✅ Correct Answer

To produce smaller bubbles / prevent the formation of large bubbles (or promote smooth and even boiling).

❌ Avoid Vague Wording

• Do not just write "to stop bumping" – explain the physical action: producing small bubbles / preventing vigorous uneven boiling.
• Do not say "to lower the boiling point" or "to speed up the reaction" (anti-bumping granules are chemically inert).

✅ Correct Answer

The organic mixture / cyclohexanol is flammable.

❌ Insufficient Answers

• "To control the temperature better" is ignored by the mark scheme because the primary laboratory safety hazard is flammability near a naked flame.

✅ Correct Answers

Why dangerous (M1): The system is completely closed / sealed (which leads to a hazardous build-up of pressure inside glassware).

How to solve (M2): Provide a vent / opening on or near the collection flask (or do not seal the delivery adapter into the receiving flask).

🧠 Exam Technique: Apparatus Spotting

Always examine joints carefully in distillation diagrams:

• A sealed joint at the collection vessel forms a rigid, airtight system. As gases are heated, pressure rises: pV = nRT .
• Heating a closed system can cause catastrophic apparatus explosion!

✅ Correct Answer

Elimination (or acid-catalysed elimination)

❌ Common Errors

• Dehydration: This describes the reaction type (loss of water), not the formal IUPAC reaction mechanism name. The mark scheme explicitly states "ignore dehydration".
• Do not write "electrophilic elimination" or "nucleophilic elimination". Simply write elimination.

✅ Balanced Chemical Equation

C₆H₁₁OH + 8½ O₂ → 6 CO₂ + 6 H₂O

Or using whole integer coefficients:

2 C₆H₁₁OH + 17 O₂ → 12 CO₂ + 12 H₂O

🧠 Balancing Tip: Don't forget the Oxygen in the Alcohol!

• 6 Carbons → 6 CO₂ (needs 12 'O' atoms)
• 12 Hydrogens (11 + 1) → 6 H₂O (needs 6 'O' atoms)
• Total O needed on right = 12 + 6 = 18 'O' atoms
• There is already 1 'O' in C₆H₁₁OH, so you only need 17 more 'O' atoms from oxygen gas → 8.5 O₂ (or 17/2 O₂ ).

💡 Carbon-Neutral Meaning (M1)

An activity/fuel that results in no net emissions of carbon dioxide to the atmosphere/air (the CO₂ released during burning equals the CO₂ absorbed during growth of the plant material).

✅ Why This Mixture is NOT Carbon-Neutral (M2)

Any one valid point:

• It contains diesel, which is a fossil fuel / non-renewable and releases locked-up ancient carbon as net CO₂.
• Fossil fuels are burned / energy is consumed during the transportation, harvesting, or industrial processing of the biomass.