AQA GCSE Chemistry Chemistry Paper 2 (Higher), November 2020: Question 3

10 marks · Standard Demand difficulty · Extended Answer

Identify formulas of hexane and hexene, state the observations when bromine water is added to each, and compare ethane with ethene in terms of structure, bonding, and reactions.

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Question

Question text

03 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.

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

[2 marks]

Hydrocarbon Formula

C6H8

Hexane C6H10

C6H12

Hexene C6H14

C6H16

03.2 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

03.3 Ethane is an alkane and ethene is an alkene.

Figure 1 shows the displayed structural formulae of ethane and of ethene.

Figure 1

Compare ethane with ethene.

You should refer to:

• their structure and bonding

• their reactions.

[6 marks]

Mark scheme

Show the mark scheme

Question 3

AO /

Question Answers Extra information Mark

Spec. Ref.

03.1 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

03.2 AO2

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

(becomes) colourless ignore initial colour 1

ignore clear

Question 3 (continued)

AO/

Question Answers Mark

Spec. Ref

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

03.3 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

Alkanes and Alkenes: Structure, Formulae & Reactions

📋 WHAT THIS QUESTION TESTS

This question assesses core understanding of Organic Chemistry (AQA Topic 7):

Recalling and applying general formulae for homologous series (alkanes and alkenes).
Knowing the standard chemical test used to distinguish between saturated and unsaturated hydrocarbons (the bromine water test).
Structuring an extended 6-mark comparative response covering covalent bonding, molecular structure, combustion, and addition reactions.

QUESTION 03.1 (2 MARKS)

Matching Hydrocarbons to Molecular Formulae

Identifying molecular formulae using general formulae

✅ Correct Matches

Hexane ➔ C₆H₁₄ [1 mark]
Hexene ➔ C₆H₁₂ [1 mark]

📐 General Formula Breakdown

Hex- prefix means n = 6 carbons.
Alkane formula: C_nH_2n+2
For n = 6: H = (2 × 6) + 2 = 14 ➔ C₆H₁₄
Alkene formula: C_nH_2n
For n = 6: H = 2 × 6 = 12 ➔ C₆H₁₂

❌ Common Errors

Drawing multiple lines: Drawing two lines from one box negates the mark for that box!
Confusing the endings: mistaking -ane (alkane, saturated) with -ene (alkene, unsaturated).

Mark Scheme Note: Exactly one line must leave each box on the left. An additional line from any box cancels the mark.

QUESTION 03.2 (2 MARKS)

The Bromine Water Test

Observations with saturated vs unsaturated hydrocarbons

✅ Correct Observations

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

💡 Chemical Reasoning

Hexene (alkene) has a reactive carbon-carbon double bond (C=C). Bromine adds across this double bond in an addition reaction, consuming the orange Br₂ molecules to form a colourless dibromoalkane.

Hexane (alkane) has only single bonds (C–C) and does not react with bromine water under normal conditions.

❌ Examiner Pitfalls

Writing "clear" instead of "colourless": Bromine water is already clear (see-through); it changes from orange to colourless. Writing "clear" gets 0 marks!
Swapping the observations around (Hexane does not react; Hexene does).

Mark Scheme Note: Must be in the correct order (Hexane first, then Hexene). Ignore mention of the initial colour; reject "clear".

QUESTION 03.3 (6 MARKS)

Compare Ethane with Ethene

Extended response comparing structure, bonding, and reactions

🧠 How to Score Level 2 (5–6 Marks)

To reach Level 2, you must compare both structure/bonding AND reactions, explicitly stating how they are similar and how they are different.

Level 2 (4–6 marks): Scientifically relevant points from both areas covered. Similarities and differences are clear.
Level 1 (1–3 marks): Relevant facts given, but may only address one theme (e.g. only structure) or lack explicit comparisons.

✅ Model Answer Structure (Indicative Content)

Feature	Ethane (C₂H₆ - Alkane)	Ethene (C₂H₄ - Alkene)
Structure & Formula	Has 2 carbons and 6 hydrogens. Saturated molecule.	Has 2 carbons and 4 hydrogens. Unsaturated molecule.
Bonding	Contains single covalent C–C bond and six C–H bonds.	Contains a double covalent C=C bond and four C–H bonds.
Combustion	Undergoes complete combustion with O₂ to give CO₂ and H₂O. Clean flame.	Undergoes combustion to give CO₂ and H₂O, but burns with a sootier/smoky flame (more prone to incomplete combustion).
Reactivity & Addition	Unreactive compared to ethene; does not decolourise bromine water.	More reactive; decolourises bromine water from orange to colourless.
Other Reactions	Does not undergo addition reactions or polymerisation.	Undergoes addition reactions (with H₂ to make ethane, steam to make ethanol, halogens) and addition polymerisation to make poly(ethene).

💡 Easy Comparison Points to Secure Marks

Similarities: Both are hydrocarbons, both are small covalent molecules, both contain 2 carbon atoms per molecule, both form CO₂ and H₂O in complete combustion.
Differences: Ethane has single C–C bond vs Ethene's double C=C bond; Ethane has 6 H atoms vs Ethene's 4 H atoms; Ethene reacts with bromine water, ethane does not.

❌ What examiners ignored / penalized

Physical properties: Boiling points, states of matter, and viscosity were explicitly ignored by the mark scheme. Stick strictly to bonding/structure and chemical reactions!
Vague statements like "ethene is flammable" (both are flammable).
Forgetting to mention the specific products of addition reactions (e.g. adding water produces ethanol).

Topics

Chemistry · C7: Organic Chemistry

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