AQA GCSE Chemistry Chemistry Paper 2 (Foundation), November 2020: Question 10

10 marks · Standard Demand difficulty · Extended Answer

Compare the molecular formulae, bromine water test observations, and the structure, bonding, and reactions of alkanes and alkenes.

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Question

Question 10 about hydrocarbons: Part 10.1 asks to draw a line from Hexane and Hexene to their molecular formulae from options C6H8, C6H10, C6H12, C6H14, and C6H16. Part 10.2 asks for the observations when bromine water is added to hexane and to hexene. Part 10.3 displays Figure 8 showing the structural formulae of ethane (C2H6) and ethene (C2H4) and asks to compare ethane with ethene with reference to their structure and bonding, and their reactions (6 marks).
Question text

10 This question is about hydrocarbons.

Hexane and hexene are hydrocarbons containing six carbon atoms in each molecule.

Hexane is an alkane and hexene is an alkene.

10.1 Draw one line from each hydrocarbon to the formula of that hydrocarbon.

[2 marks]

Hydrocarbon Formula

C6H8

Hexane C6H10

C6H12

Hexene C6H14

C6H16

10.2 Bromine water is added to hexane and to hexene.

What would be observed when bromine water is added to hexane and to hexene?

[2 marks]

Hexane

Hexene

10.3 Ethane is an alkane and ethene is an alkene.

Figure 8 shows the displayed structural formulae of ethane and of ethene.

Figure 8

Compare ethane with ethene.

You should refer to:

• their structure and bonding

• their reactions.

[6 marks]

Mark scheme

Show the mark scheme Mark scheme for Question 10: 10.1 shows Hexane linked to C6H14 and Hexene linked to C6H12 (1 mark each). 10.2 awards 1 mark for '(remains) orange' or no colour change for hexane, and 1 mark for '(becomes) colourless' for hexene. 10.3 is a level of response mark scheme for 6 marks comparing structure and bonding (both hydrocarbons, 2 carbon atoms, ethane has 6 H while ethene has 4 H, single C-C bond vs double C=C bond, covalent bonds, small molecules) and reactions (complete/incomplete combustion, bromine water decolourisation by ethene, reactivity, addition reactions with hydrogen, water, halogens, polymerisation).

Question 10

AO /

Question Answers Extra information Mark

Spec. Ref.

10.1 AO2

1 4.7.1.1

4.7.2.1

additional line from a box on the left negates the mark for that box

must be in this order

10.2 AO2

(remains) orange allow no (colour) change 1 4.7.1.4

(becomes) colourless ignore initial colour 1

ignore clear

Question 10 (continued)

AO /

Question Answers Mark

Spec. Ref.

Level 2: Scientifically relevant features are identified; the way(s) in

10.3 which they are similar/different is made clear and (where 4–6

AO1

appropriate) the magnitude of the similarity/difference is noted.

AO2

Level 1: Relevant features are identified and differences noted. 1–3

No relevant content 0 4.7.1.1

4.7.1.3

Indicative content 4.7.1.4

4.7.2.1

Structure and bonding 4.7.2.2

4.9.3.1

• both are hydrocarbons

• both contain two carbon atoms (per molecule)

• ethane contains six hydrogen atoms (per molecule)

• (but) ethene contains four hydrogen atoms (per molecule)

• both have covalent bonds

• ethane contains a single C—C bond

• (but) ethene contains a double bond

• both contain C—H bonds

• both small molecules

Reactions

• both react with oxygen in complete combustion reactions

• to produce water and carbon dioxide

• both react with oxygen in incomplete combustion reactions

• to produce water, carbon monoxide and carbon

• incomplete combustion is more likely with ethene

• ethene decolourises bromine water

• (but) ethane does not decolourise bromine water

• ethene is more reactive (than ethane)

• ethene can react with hydrogen (to produce ethane)

• ethene can react with water (to produce ethanol)

• ethene can react with halogens (to produce halogenoalkanes)

• ethene can undergo addition reactions

• ethene can polymerise (to produce poly(ethene))

ignore physical properties

ignore references to flammability

Total 10

How to answer it

Hydrocarbons: Alkanes vs Alkenes

📋 What this question tests

This question assesses core organic chemistry fundamentals from Topic 7 (Organic Chemistry):

  • Recalling and applying general formulas for alkanes (CnH2n+2) and alkenes (CnH2n).
  • The chemical test to distinguish saturated and unsaturated hydrocarbons using bromine water.
  • Structuring an extended 6-mark comparative response addressing both structure/bonding and chemical reactions of ethane and ethene.

Question 10.1 (Matching Formulas)

Identifying molecular formulas from general formulas [2 marks]

✅ Correct Answer

  • Hexane connects to: C₆H₁₄ [1 mark]
  • Hexene connects to: C₆H₁₂ [1 mark]

💡 Key Knowledge

  • Hex- prefix means 6 carbon atoms (n = 6).
  • Alkane general formula: CnH2n+2
    For n = 6: C₆H(2×6)+2 = C₆H₁₄
  • Alkene general formula: CnH2n
    For n = 6: C₆H(2×6) = C₆H₁₂

❌ Common Errors

  • Drawing multiple lines from one box: If you draw two lines from "Hexane", the mark for that box is automatically cancelled.
  • Confusing the endings: remembering that -ane means fully saturated with more hydrogens than -ene.
Mark Scheme Note: Exactly one line must leave each box on the left. An additional line negates the mark for that box.

Question 10.2 (Bromine Water Test)

Observations for test for unsaturation [2 marks]

✅ Correct Observations

  • Hexane: (remains) orange (or: no change / no reaction) [1 mark]
  • Hexene: (becomes) colourless (or: decolourises) [1 mark]

🧠 Exam Technique: "Clear" vs "Colourless"

Examiners strictly penalise the word clear when describing colour changes.

  • Colourless describes the absence of color (like pure water).
  • Clear means transparent (you can see through it, but it could still be orange!).
  • Always write "turns colourless" or "decolourises".

💡 Chemical Reason

Alkenes contain a reactive carbon-carbon double bond (C=C). Bromine adds across this double bond in an addition reaction to form a colourless dibromoalkane. Alkanes have only single C–C bonds and do not react with bromine water under normal conditions.

Mark Scheme Note: Must be in the correct order (Hexane first, then Hexene). Ignore initial color of bromine water. Ignore the word "clear".

Question 10.3 (Extended 6-Mark Comparison)

Compare ethane with ethene: structure, bonding, and reactions [6 marks]

🧠 How Level-Based Marking Works (6-mark question)

To achieve Level 2 (5–6 marks), you must include points from BOTH bullet points in the prompt:

  • Both Structure & Bonding AND Reactions must be covered clearly.
  • You must mention both similarities and differences.
  • If you only discuss structure, or only discuss reactions, your answer is capped at Level 1 (max 3 marks).

💡 Area 1: Structure & Bonding

  • Similarities:
    • Both are hydrocarbons (contain only carbon and hydrogen).
    • Both have 2 carbon atoms per molecule.
    • Both contain covalent bonds (shared pairs of electrons).
    • Both contain C–H bonds.
    • Both are small, simple covalent molecules.
  • Differences:
    • Ethane has 6 hydrogen atoms; ethene has 4 hydrogen atoms.
    • Ethane has a single C–C bond (saturated).
    • Ethene has a double C=C bond (unsaturated).

💡 Area 2: Reactions

  • Similarities (Combustion):
    • Both undergo complete combustion with oxygen to produce CO₂ and H₂O.
    • Both can undergo incomplete combustion to form CO, carbon (soot), and H₂O.
  • Differences (Reactivity & Addition):
    • Ethene is more reactive than ethane.
    • Ethene decolourises bromine water; ethane does not.
    • Ethene undergoes addition reactions (e.g. with H₂ to make ethane, with steam to make ethanol).
    • Ethene can polymerise to form poly(ethene); ethane cannot.
    • Incomplete combustion is more likely with ethene (burns with a smokier flame).

✅ Model Answer Structure (Full 6/6 Marks)

Feature Ethane (Alkane) Ethene (Alkene)
Structure & Bonding Has 2 carbons and 6 hydrogens (C₂H₆). Contains only single covalent bonds (C–C and C–H). It is a saturated hydrocarbon. Has 2 carbons and 4 hydrogens (C₂H₄). Contains a double covalent bond (C=C) and C–H bonds. It is an unsaturated hydrocarbon.
Combustion Burns completely in oxygen to form CO₂ and H₂O. Also burns to form CO₂ and H₂O, but burns with a smokier flame due to incomplete combustion.
Addition & Polymerisation Unreactive towards addition reactions. Does not react with bromine water (remains orange). Cannot form addition polymers. Much more reactive. Decolourises bromine water (orange to colourless). Can undergo addition reactions with steam/halogens and polymerises to make poly(ethene).

❌ What Examiners Ignored / Pitfalls

  • Do not discuss physical properties: The mark scheme specifically states: "ignore physical properties" (e.g. melting points, boiling points, state at room temperature). These earn zero marks.
  • Do not mention generic "flammability": State the specific products of combustion (CO₂ and H₂O) rather than saying "one catches fire more easily".
Examiner Insight: High-scoring students used balanced parallel sentences: "Ethane has a single C–C bond, whereas ethene has a double C=C bond." Clear paragraphs separated into "Structure" and "Reactions" guarantee easy access to Level 2.

Topics

Chemistry · C7: Organic Chemistry

Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 2 (Foundation), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.