AQA GCSE Chemistry Chemistry Paper 2 (Foundation), June 2025: Question 2

✅ Correct Answers

• 02.1: Tick Propene [1 mark]
• 02.2: Tick 9 [1 mark]
• 02.3: Tick Making a polymer [1 mark]

💡 Key Knowledge

• Alkene prefixes: 1 carbon = meth- (no alkene possible), 2 = eth- (C₂H₄), 3 = prop- (C₃H₆), 4 = but- (C₄H₈).
• General formula: Alkenes have the formula C_nH_2n.
• Polymerisation: Alkenes contain a reactive C=C double bond, allowing them to open up and join together to form addition polymers (e.g. poly(propene)).

❌ Common Errors

• Miscounting atoms: Students often choose 3 (only counting carbons) or 6 (only counting hydrogens). The question asks for total atoms: 3 + 6 = 9.
• Wrong naming: Confusing propene (3 carbons) with butene (4 carbons).

✅ Correct Answers & Diagram Details

• Scale mark: Linear scale on y-axis numbered at equal intervals: 40, 60, 80, 100 (0 and 20 are already given).
• Plotting mark: A vertical bar for carbon drawn to exactly 86% (tolerance: ±½ small square).
• Label mark: The bar must be clearly labelled Carbon on the x-axis underneath.

🧠 Graph Drawing Technique

• Determine small-square value: If 20% takes 10 small squares, each small square is worth 2%.
• To find 86%: Count 3 small squares above the 80 line (80 + 2 + 2 + 2 = 86%).
• Always use a sharp pencil and a ruler to draw bar edges neatly. Keep bar widths identical.

❌ Common Errors

• Uneven spacing of numbers on the y-axis (e.g. jumping randomly from 20 to 50).
• Drawing the bar at 85% instead of 86% due to misreading the grid scale.
• Forgetting to write the label "Carbon" below the plotted bar.

✅ Correct Answers

• Test: Add bromine water [1 mark]
• Result: Turns colourless (or decolourises / turns from orange to colourless) [1 mark]

💡 Why This Happens

Alkenes are unsaturated hydrocarbons containing a C=C double bond. In an addition reaction, the double bond breaks open and bromine atoms bond to the carbon atoms, removing the orange bromine from the solution and forming a colourless dibromoalkane.

❌ Critical Trap: "Clear" vs "Colourless"

• Never write "clear": Examiners explicitly state "ignore clear". A solution can be clear and orange!
• You must write "colourless" or "decolourises".
• Alkanes do not react, so with alkanes bromine water stays orange/brown.

📐 Balancing Step-by-Step (02.6)

Reaction: C₁₀H₂₂ → C₃H₆ + C?H₁₆

1. Count total carbons on left side = 10
2. Count known carbons on right side (in C₃H₆) = 3
3. Subtract to find missing carbons: 10 − 3 = 7
4. Check hydrogens: 6 + 16 = 22 (balances perfectly!).

✅ Part 02.7 Answer

"Smaller alkanes from cracking are useful as fuels." [1 mark]

💡 Why Do We Crack Long Alkanes?

• Fractional distillation produces more long-chain hydrocarbons than market demand requires.
• Long hydrocarbons have high boiling points and are not very flammable, making poor fuels.
• Cracking breaks them into:
  • Shorter alkanes: high demand as clean-burning fuels (petrol/diesel).
  • Alkenes: raw material for plastics and polymers.

✅ Correct Answers

• 02.8: Tick The molecules collide more frequently at 750 °C. [1 mark]
• 02.9: The rate of reaction is higher / faster / increases [1 mark]
  (or allow: at 450 °C the rate is lower/slower)

💡 Collision Theory Recap

• Higher temperature → particles gain kinetic energy and move faster.
• They collide more frequently (more collisions per second).
• A greater proportion of colliding molecules have energy greater than or equal to the activation energy (Eₐ), leading to more successful collisions and a higher rate.

🧠 Exam Technique

• Always mention the word frequently or "rate of collisions" rather than just "more collisions".
• In comparative questions (750 °C vs 450 °C), state clearly which temperature you are describing: "At 750 °C the rate is higher."