AQA GCSE Chemistry Chemistry Paper 2 (Foundation), June 2023: Question 2

✅ Correct Answers

02.1 (2 marks - any two reasons):

• The poly(propene) beaker will melt (or has a low melting point / melts at 160 °C). [1 mark]
• The poly(propene) beaker will burn / ignite (or is flammable). [1 mark]

02.2 (1 mark - any one reason):

• Poly(propene) beakers are less easily broken / shatter-proof (tougher / do not shatter / have higher resistance to impact / last longer / do not need replacing as often). [1 mark]

🧠 Exam Technique: "Use Table 1"

• When the question says "Use Table 1", your answer must link directly to the table headings (Melting point, Flammability, Impact resistance).
• In 02.1, a Bunsen flame reaches over 1000 °C. State the consequence: poly(propene) melts at only 160 °C and burns.
• In 02.2, think about practical laboratory lifetime: even though one plastic beaker costs £2.00 vs £1.50 for glass, you save money over time because glass easily shatters and must be replaced repeatedly.

❌ Common Errors

• Vague answers like "it gets hot" or "it is dangerous" score zero. Name the exact property: melts or burns.
• For 02.2, simply quoting "it is tough" without linking it to why this saves money (e.g. fewer replacements needed because it won't shatter).

💡 Key Knowledge

• Thermosoftening polymers: Poly(propene) has weak intermolecular forces between polymer chains that are easily overcome with heat, leading to a low melting point (160 °C).
• Borosilicate glass: Has a high melting point (850 °C) and does not burn, making it ideal for direct heating.

✅ Correct Answer [1 mark]

Tick: Boron trioxide

💡 Key Knowledge: The Two Types of Glass

• Soda-lime glass: Made by heating a mixture of sand (silicon dioxide), sodium carbonate, and limestone (calcium carbonate). Used for window panes and jars.
• Borosilicate glass: Made by heating sand and boron trioxide. It melts at higher temperatures than soda-lime glass, making it ideal for laboratory glassware (e.g. Pyrex) and ovenware.

❌ Common Distractor Traps

• Clay: Used to make ceramics (bricks, pottery, china), not glass.
• Limestone: Used for soda-lime glass, not the key ingredient specific to borosilicate glass.

Mark scheme note: 1 mark for both correct names (carbon & hydrogen); 1 mark for both correct numbers (3 & 6). If neither full mark is gained, allow 1 mark if any one complete column is correct.

🧠 Counting from Displayed Formulas

Propene is drawn as:

    H   CH₃
    |   |
C = C
    |   |
    H   H

• Carbons: 2 carbons in the C=C double bond + 1 carbon in the –CH₃ methyl group = 3 carbons.
• Hydrogens: 3 on the double bond carbons + 3 in the –CH₃ group = 6 hydrogens.
• Formula fits general alkene formula: C_nH_2n where n = 3 → C₃H₆.

❌ Common Miscount Trap

Students often forget to count the atoms packed into the attached functional group ( –CH₃ ). They see 2 carbons in the backbone and write 2 carbons and 3 hydrogens. Always count every single atom!

✅ Correct Answer [1 mark]

Tick the middle box:

    (  H   CH₃ )
    (  |   |   )
——(—C — C——)——
    (  |   |   ) n
    (  H   H   )

🧠 3 Rules for Drawing / Identifying Repeating Units

1. Open the double bond: The C=C double bond turns into a C–C single bond.
2. Keep attached groups identical: The groups attached to each carbon atom do not change position (Carbon 1 has two H's; Carbon 2 has one H and one CH₃).
3. Extend bonds & brackets: The extension bonds must poke through the brackets with an ' n ' subscript.

💡 The Industrial Pathway

• 1. Fractional Distillation: Separates crude oil into fractions containing hydrocarbons of similar boiling points / chain lengths.
• 2. Cracking (Catalytic or Steam): Breaks down long-chain, less useful alkanes into shorter, more useful alkanes and alkenes (such as propene).
• 3. Addition Polymerisation: Uses high pressure and a catalyst to break double bonds and join thousands of small alkene monomers into long polymer chains.

❌ Irrelevant Options (Distractors)

• Fermentation: Biological production of ethanol using yeast and glucose.
• Reverse osmosis: Desalination method used to produce potable drinking water from sea water.

✅ Correct Answer [2 marks]

Tick two boxes:

• ☑️ Alkene [1 mark]
• ☑️ Monomer [1 mark]

🧠 Decoding the Clues in the Question

• "Contains a double carbon–carbon bond" → By definition, a hydrocarbon with a C=C double bond is an alkene (names ending in -ene).
• "Many hexene molecules join together to form poly(hexene)" → The individual small repeating starting molecules are called monomers (mono = one, mer = part).