Chemistry • Year 12 • Module 7 • Lesson 5

Hydrocarbon Reactions

Build HSC Band 5–6 extended-response technique by evaluating hydrocarbon reaction data, analysing Australian industrial contexts, and critiquing scientific claims.

Master · Extended Response

1. Extended response — combustion emission trade-offs in Australian energy (Band 5–6)

8 marks Band 5–6

Stimulus.

Australia generates a significant fraction of its electricity from coal combustion. Coal is a complex hydrocarbon-rich solid. In a modern power station, pulverised coal is burned in forced air at temperatures above 1400°C; in a domestic wood heater or an older industrial boiler burning at lower oxygen availability, the same fuel produces a visibly different flame and a different exhaust profile. The table below shows typical exhaust composition measurements (per tonne of carbon in fuel) for three combustion scenarios.

Combustion scenario	CO₂ (kg)	CO (kg)	Particulate carbon — soot (kg)	Flame colour
A: Modern coal power station (excess O₂)	3667	2	<0.1	Blue-white
B: Domestic wood heater (moderate O₂)	3400	110	8	Yellow-orange
C: Smouldering fire, low O₂	2900	520	45	Smoky orange

Indicative values adapted from CSIRO and NGER combustion emission factors (2022).

Q1. Analyse and evaluate the three combustion scenarios using the lesson’s model of complete and incomplete combustion. In your response you must:

Define complete combustion and incomplete combustion and link each to the oxygen supply condition.
Use the data in the table to classify Scenarios A, B, and C on a complete–incomplete continuum and justify your classification with specific values.
Explain, at the molecular/bond level, why CO and soot are produced instead of CO₂ in Scenarios B and C.
Compare the health and environmental consequences of the three scenarios using at least two distinct criteria.
Reach a justified, evidence-based judgement about which scenario poses the greatest short-term public health risk and which poses the greatest long-term climate risk.

Stuck? Plan: define both types of combustion (1 mark) → classify the three scenarios with data evidence (1–2 marks) → molecular explanation (1–2 marks) → two impact criteria (2 marks) → dual judgement (1 mark).

2. Source critique — a chemistry textbook claim (Band 5–6)

7 marks Band 5–6

"The bromine test is a universal test for any hydrocarbon. When any hydrocarbon is bubbled through orange bromine water, the bromine water decolourises because the hydrocarbon reacts with bromine. An alkane such as methane will decolourise bromine water just as quickly as an alkene such as ethene, since both are hydrocarbons that react with halogens. Therefore, the bromine water test cannot be used to distinguish between alkanes and alkenes."

— Adapted from a Year 12 Chemistry revision guide (composite claim, not attributed to a specific source).

Q2. Evaluate this claim. Identify the elements that are scientifically correct, identify each specific error, explain the correct chemistry, and reformulate the final sentence into a scientifically defensible statement.

Stuck? Bromine water decolourises by addition across C=C. Does methane have a C=C? Can methane do substitution with Br₂ rapidly at room temperature without UV? What does that tell you about the claim?

3. Evaluate this student’s response (Band 5)

5 marks Band 5

Exam question (original):

"Explain why the reaction of propene with bromine is classified as an addition reaction rather than a substitution reaction. Include a balanced equation."

Student’s response:

"The reaction is addition because bromine is added to propene. Propene reacts with bromine to give a new molecule CH₃CHBrCHBr. There is no substitution because no hydrogen atoms are removed. The balanced equation is: CH₃CH=CH₂ + Br₂ → CH₃CH=CHBrBr. This is an addition reaction."

Q3. The student’s response scores 2 out of 4 in a trial marking. Identify two errors or omissions and explain what a Band 5 response would include instead. 5 marks (2 marks for error identification/correction; 3 marks for demonstrating what a stronger response looks like)

Stuck? Look at the equation the student wrote. Is the product correct (are both Br atoms on adjacent single-bonded carbons)? Does the student explain the C=C bond role? Does the response distinguish addition from substitution at the bond level?

Answers — Do not peek before attempting

Q1 — Sample Band 6 response (8 marks), marking criteria

Marking criteria:

1 mark — Correctly defines complete combustion (excess O₂; products CO₂ and H₂O only) and incomplete combustion (limited O₂; products include CO and/or solid carbon/soot).
2 marks — Classifies all three scenarios on the continuum with specific data justification: Scenario A = closest to complete (CO 2 kg, soot <0.1 kg, blue-white flame); Scenario C = most incomplete (CO 520 kg, soot 45 kg, smoky flame); Scenario B = intermediate. Award 1 mark for partial correct classification.
2 marks — Molecular/bond-level explanation: in limited-oxygen conditions, insufficient O₂ is available to oxidise every carbon atom fully; carbon atoms that can only bond to one oxygen form CO (partial oxidation) rather than CO₂ (full oxidation), and unoxidised carbon atoms form soot particles. Award 1 mark for partial explanation.
2 marks — Compares health and environmental consequences across at least two criteria: (a) short-term health: CO toxicity (binds haemoglobin) and particulate matter (respiratory harm) are highest in Scenario C; (b) long-term climate: CO₂ is highest in Scenario A (greenhouse gas with century-scale atmospheric lifetime); CO in Scenarios B and C also contributes indirectly. Award 1 mark for only one criterion addressed.
1 mark — Reaches a dual, justified judgement: Scenario C poses the greatest short-term public health risk (highest CO and soot); Scenario A poses the greatest long-term climate risk (highest CO₂ per tonne of carbon burned). Accept other judgements if data-supported.

Q2 — Source critique (7 marks), marking criteria

What is correct in the source: Bromine water is orange; hydrocarbons do react with halogens; alkenes do decolourise bromine water rapidly by addition. [1 mark for identifying the correct element]

Error 1 — "universal test for any hydrocarbon": The bromine water test is specifically a test for unsaturation (C=C bonds). Saturated hydrocarbons (alkanes) do not react rapidly with bromine water under normal room-light conditions because they have no C=C bond. [1 mark identify; 1 mark correct]

Error 2 — "alkane decolourises just as quickly as alkene": This is false. Alkanes can undergo substitution with Br₂ only under UV light (not room light), and this reaction is much slower than the rapid addition of Br₂ across a C=C double bond in an alkene. Under normal bromine water test conditions (room light, aqueous Br₂), alkanes do NOT decolourise bromine water. [1 mark identify; 1 mark correct]

Reformulated final sentence: “The bromine water test can distinguish between alkanes and alkenes: alkenes rapidly decolourise bromine water at room temperature because Br₂ adds across the C=C double bond; alkanes do not decolourise bromine water under normal conditions (no UV light, room temperature) because they have no double bond and cannot undergo addition.” [1 mark for defensible reformulation]

Q3 — Student response evaluation (5 marks)

Error 1 — Incorrect equation (product still contains C=C): The student wrote CH₃CH=CHBrBr, which incorrectly retains the double bond. In addition, the second Br is shown on the same carbon. The correct product is CH₃CHBrCH₂Br (1,2-dibromopropane), where one Br adds to each carbon of the original double bond and the C=C becomes C–C. [1 mark identify + correction]

Error 2 / Omission — No bond-level explanation: The student says “bromine is added” but never explains that this happens because the C=C double bond (the pi bond) in propene is broken and one Br atom bonds to each of the two previously double-bonded carbons. A Band 5 response would state: “The pi bond in the C=C of propene is broken; Br₂ adds across the double bond so that one Br attaches to each carbon, producing a saturated product with only C–C single bonds.” [1 mark identify + correction]

What a Band 5 response would add (3 marks):

Distinguish addition from substitution at the bond level: addition opens a multiple bond; substitution replaces one atom or group on an already single-bonded carbon. [1]
State the correct balanced equation: CH₃CH=CH₂ + Br₂ → CH₃CHBrCH₂Br. [1]
Connect the bromine test observation: orange bromine water decolourises, confirming Br₂ has been consumed by addition across the C=C. [1]