AQA GCSE Chemistry Chemistry Paper 2 (Higher), November 2021: Question 6

✅ Correct Answer

Test: Add bromine water [1 mark]

Result: (Changes from) orange / brown to colourless [1 mark]

💡 Key Knowledge

• Alkenes and cycloalkenes contain a reactive C=C double bond (they are unsaturated).
• Bromine adds across the double bond via an addition reaction, converting orange bromine water into a colourless dibromo-compound.

❌ Common Errors & Misconceptions

• Writing that the solution turns "clear" instead of "colourless". Bromine water is already clear (transparent); it loses its colour to become colourless. Examiners strictly do not accept "clear".
• Forgetting to state the initial colour (best practice is always to write: "orange to colourless").

🧠 Exam Technique

Even though cycloalkenes are unfamiliar ring molecules, the question states they "react in a similar way to alkenes". Always apply standard alkene reactions to unfamiliar unsaturated hydrocarbons!

✅ Correct Answer

General formula: C_nH_2n-2 [1 mark]

🧠 Step-by-Step Deduction

• When n = 4 → H = 6 → (2 × 4) - 2 = 6
• When n = 5 → H = 8 → (2 × 5) - 2 = 8
• When n = 6 → H = 10 → (2 × 6) - 2 = 10
• Rule: Multiply the number of carbons (n) by 2 and subtract 2 → C_nH_2n-2

❌ Common Trap

Do not confuse this with normal straight-chain alkenes (C_nH_2n) or cycloalkanes (C_nH_2n). Forming a ring removes 2 hydrogens; having a double bond removes another 2 hydrogens, giving C_nH_2n-2.

✅ Correct Answer

Complete the displayed structure as 1,2-dichlorocyclohexane:

Each carbon in the ring forms 4 single covalent bonds: the original C=C double bond breaks into a C-C single bond, and one Cl atom binds to each of those two carbons.

💡 Addition Mechanism Rules

• In an addition reaction, the double bond opens up into a single bond.
• The diatomic halogen molecule (Cl₂) splits so that one halogen atom attaches to each carbon of the former double bond.
• Every carbon atom in the product must have exactly four covalent bonds.

❌ Common Errors

• Leaving the double bond intact (a 5-bonded carbon is an immediate mark deduction!).
• Attaching both Cl atoms to the same carbon atom (allowed only 1 partial mark by the mark scheme).
• Missing out hydrogen atoms or drawing bonds that do not clearly connect to the atoms.

🧠 Exam Technique

Count your bonds around every carbon atom when you finish drawing. If any carbon has 3 or 5 lines attached to it, check your work again!

❌ Common Calculation Traps

• Forgetting there are 2 chlorines: Using 35.5 instead of 71 in the numerator gives 23.2% (incorrect).
• Arithmetic slips in M_r: If you calculate an incorrect M_r, you can still gain "error carried forward" (ECF) marks for step 2 and step 3 as long as your working is clearly shown.
• Rounding errors: Never round intermediate values too early. Round only your final answer (e.g. 46.4% to 3 significant figures).

🧠 Top Tip for Full Marks

Always write out the explicit fraction (71 / 153) × 100 . Even if you press a wrong button on your calculator, examiners award the second method mark for seeing the correct ratio written down.