Chemistry • Year 11 • Module 3 • Lesson 4
Combustion Reactions
Build HSC Band 5–6 extended-response technique: evaluate combustion products, their hazards, and fuel choices using real Australian data and scenarios.
1. Data-based extended response — Karratha LNG facility and incomplete combustion risk (Band 5–6)
8 marks Band 5–6
Scenario. The Woodside Karratha LNG facility on the Pilbara coast of Western Australia is Australia’s largest LNG (liquefied natural gas) export facility. Methane (CH₄) is the primary component. The facility operates high-temperature flares to combust waste methane and prevent its direct release to the atmosphere. An engineering team is evaluating two flare operating modes:
- Mode A — High-flow flare: high gas velocity through the burner tip, adequate air entrainment, near-complete combustion.
- Mode B — Low-flow flare: low gas velocity, reduced air entrainment, signs of smoky yellow flame and occasional soot deposits.
The table below shows measured flare exhaust data for Mode A and Mode B during a recent trial:
| Parameter | Mode A (high-flow) | Mode B (low-flow) |
|---|---|---|
| CO concentration in exhaust (ppm) | 85 | 6 400 |
| CO₂ concentration in exhaust (%) | 9.4 | 4.1 |
| Flame colour | Blue/clear | Yellow/smoky |
| Visible soot emission | None | Yes (dark plume) |
| Combustion efficiency (%) | 98.7 | 61.3 |
| Methane released unburned (% of flow) | <0.1 | 3.9 |
Table 1. Flare exhaust gas composition during trial. CO safe short-term limit (15 min): 200 ppm. Data modelled after real LNG flare performance studies.
Q1. Analyse and evaluate the combustion chemistry of Mode A and Mode B, and recommend which mode should be adopted for routine flare operation. In your response you must:
- Write a balanced equation for the complete combustion of methane and a balanced equation for one possible incomplete combustion of methane producing CO.
- Analyse the data in Table 1 to distinguish which mode represents complete combustion and which represents incomplete combustion, citing at least three pieces of evidence.
- Explain the specific physiological mechanism by which CO causes harm to workers, and use the data to quantify the hazard posed by Mode B.
- Evaluate both modes against two environmental criteria (air quality and climate impact) to reach a justified recommendation.
2. Stimulus-based extended response — Australia’s 2019–20 Black Summer bushfire smoke (Band 5–6)
7 marks Band 5–6
Stimulus. During the 2019–20 Black Summer bushfires in south-eastern Australia, the NSW Environment Protection Authority monitored air quality in Sydney and surrounding areas. At the peak of the fire season (January 2020), PM₂.₅ (fine particulate matter, diameter <2.5 μm) reached up to 700 μg m⁻³ in Sydney — more than 23 times the 24-hour safe standard of 25 μg m⁻³. Simultaneously, CO monitors recorded episodic spikes above 30 ppm at stations downwind of smouldering zones in the Blue Mountains and Snowy Mountains regions. The active flame fronts, however, produced lower CO readings even though they were visually more dramatic.
A media commentator wrote: “The most dangerous part of a bushfire is the visible flame front — if you can’t see it burning, you’re safe.”
Q2. Evaluate the media commentator’s claim using the chemistry of combustion reactions and the stimulus data. In your response you must:
- Use combustion chemistry to explain why the visible flame front produces less CO and fine particulate than the smouldering zone, referring to oxygen availability and the type of combustion occurring in each zone.
- Identify the specific combustion products responsible for the two measured hazards (CO and PM₂.₅) and explain the chemical origin of each.
- Evaluate the claim: identify which part (if any) is defensible and which parts are chemically incorrect, using data from the stimulus.
- Reformulate the commentator’s claim into a chemically accurate statement.
Q1 — Sample Band 6 response (8 marks), annotated
Equations:
Complete combustion of methane: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g). Check: 1C, 4H, 4O each side. [1 mark — correctly balanced complete combustion equation]
Incomplete combustion of methane producing CO: 2CH₄(g) + 3O₂(g) → 2CO(g) + 4H₂O(g). Check: 2C, 8H, 6O each side. [1 mark — correctly balanced incomplete combustion equation]
Analysis — Mode A vs Mode B:
Mode A represents complete combustion: (i) CO concentration is only 85 ppm (well below the 200 ppm safe limit), consistent with nearly all carbon oxidised to CO₂; (ii) CO₂ concentration is 9.4%, higher than Mode B, indicating more complete carbon oxidation; (iii) flame is blue/clear — characteristic of complete combustion; (iv) no soot emission and 98.7% combustion efficiency. Mode B represents incomplete combustion: CO at 6 400 ppm is 32 times the safe limit; CO₂% is lower (4.1%) as carbon is being diverted to CO and soot; the yellow/smoky flame and visible dark plume confirm solid carbon soot formation. [2 marks — 1 mark for identifying Mode A as complete with ≥3 pieces of evidence; 1 mark for Mode B as incomplete with evidence]
CO physiological mechanism and hazard quantification:
CO binds to haemoglobin with approximately 200 times the affinity of O₂. Even small amounts of CO occupy haemoglobin binding sites, preventing O₂ transport to cells. This causes tissue hypoxia, leading to headache, loss of consciousness, and death at high exposures. Mode B produces 6 400 ppm CO — 32 times the safe short-term limit of 200 ppm. At concentrations above 1 000 ppm, unconsciousness can occur within one hour; Mode B’s 6 400 ppm exceeds this by more than 6-fold, presenting an acute lethal risk to personnel near the flare. [2 marks — 1 mark for mechanism (haemoglobin affinity ×200, O₂ transport blocked); 1 mark for quantified hazard comparison (6 400 vs 200 ppm, 32× over limit)]
Environmental evaluation and recommendation:
Air quality: Mode B emits 6 400 ppm CO and visible soot — carbon soot particles are air pollutants that contaminate the local atmosphere and pose a health hazard. Mode A emits only 85 ppm CO and no soot, far better for local air quality and worker safety. Climate impact: Mode B releases 3.9% of methane flow unburned into the atmosphere. Methane is a fossil fuel whose combustion releases CO₂ (lesson key term: fossil fuels combust to release CO₂, contributing to climate change); direct release of unburned methane as a fossil-fuel compound also contributes to environmental harm. Mode A releases <0.1% unburned methane, meaning almost all the methane is converted to CO₂ and H₂O rather than released directly — making Mode A vastly superior. Recommendation: Mode A should be adopted for routine flare operation. It satisfies both environmental criteria (air quality and climate impact) and eliminates the acute occupational CO hazard. If minimum gas flow through the flare drops below the threshold needed for Mode A, the gas should be routed back into the process rather than flared under Mode B conditions. [2 marks — 1 mark for evaluating each environmental criterion (air quality, climate) with justification; 1 mark for a clear justified recommendation that integrates both criteria]
Marking criteria (8 marks total):
- 1 mark — Correctly balanced equation for complete combustion of CH₄ with state symbols.
- 1 mark — Correctly balanced equation for incomplete combustion of CH₄ producing CO.
- 1 mark — Identifies Mode A as complete combustion using ≥3 data points from Table 1.
- 1 mark — Identifies Mode B as incomplete combustion using ≥3 data points from Table 1.
- 1 mark — Explains CO mechanism (haemoglobin affinity ∼200× O₂; O₂ transport blocked).
- 1 mark — Quantifies Mode B hazard relative to the 200 ppm limit (6 400 ppm = 32× limit) and links to physiological risk.
- 1 mark — Evaluates air quality impact of both modes with evidence.
- 1 mark — Evaluates climate impact (Mode B releases unburned methane into the atmosphere; Mode A converts nearly all fuel to CO₂ + H₂O) and makes a justified, integrated recommendation.
Q2 — Sample Band 6 response (7 marks), annotated
Combustion chemistry of the two zones:
The visible flame front involves intense, open burning with high airflow, providing excess oxygen to the fuel. This produces predominantly complete combustion: CₓH₫ + O₂ → CO₂ + H₂O. CO₂ and water are the main products; very little CO or soot is produced. Behind the flame front, smouldering logs burn under ash with restricted airflow. Low oxygen availability causes incomplete combustion, where carbon is only partially oxidised: the products include CO (toxic gas) and solid carbon soot particles (PM₂.₅) rather than CO₂. [2 marks — 1 mark for each zone correctly described with oxygen condition and combustion type]
Chemical origin of the two hazards:
CO is produced when there is insufficient oxygen to oxidise carbon fully from the +2 oxidation state (CO) to the +4 state (CO₂). PM₂.₅ (fine particulate) consists primarily of solid carbon soot particles (C(s)) produced when oxygen is so limited that carbon is not even oxidised to CO; fine organic compounds and ash fragments also contribute. Both hazards are products of incomplete combustion, not complete combustion. [1 mark — correct chemical origin of both CO and PM₂.₅ with reference to oxygen limitation]
Evaluation of the claim:
Defensible element: The flame front is visually more dramatic and poses a real hazard from radiant heat; it is the more immediately visible danger to someone approaching. Incorrect elements: (i) The stimulus shows that CO monitors recorded spikes above 30 ppm downwind of smouldering zones, which — while lower than the 200 ppm occupational limit — represents persistent exposure in communities. More critically, smouldering can produce thousands of ppm CO in enclosed valleys. (ii) PM₂.₅ reaching 700 μg m⁻³ (23× the safe standard) was measured in Sydney during the smouldering-dominated smoke events, causing widespread respiratory harm. This is chemical evidence that the “invisible” smouldering zone is responsible for greater toxic chemical hazard than the visible flame front. The claim is therefore chemically incorrect in its core assertion. [2 marks — 1 mark for identifying the defensible element; 1 mark for refuting the claim with stimulus data (CO ppm, PM₂.₅ levels)]
Reformulated claim:
“The most hazardous zone of a bushfire in terms of toxic gas and particle exposure is the smouldering zone, not the visible flame front. The flame front’s excess oxygen drives predominantly complete combustion, producing CO₂ and H₂O; the smouldering zone’s restricted oxygen drives incomplete combustion, producing toxic CO and fine soot particles (PM₂.₅) that spread downwind and persist for days or weeks after the flames are extinguished.” [2 marks — 1 mark for reformulation that correctly contrasts the two zones using combustion chemistry; 1 mark for including both CO and PM₂.₅ as products of the more dangerous zone and noting their persistence]
Marking criteria (7 marks total):
- 1 mark — Explains flame front as complete combustion (excess O₂, products CO₂ + H₂O).
- 1 mark — Explains smouldering zone as incomplete combustion (limited O₂, products CO + soot C).
- 1 mark — Correctly identifies chemical origin of both CO (partial oxidation of C) and PM₂.₅ (solid C soot) as products of incomplete combustion.
- 1 mark — Identifies the one defensible element of the claim (flame front is a real heat/radiant hazard).
- 1 mark — Refutes the claim using stimulus data: CO spikes downwind of smouldering zones; PM₂.₅ at 700 μg m⁻³ (23× the safe standard).
- 1 mark — Reformulated claim correctly contrasts the two zones using combustion chemistry.
- 1 mark — Reformulation includes persistence of smouldering hazard (days/weeks) and both CO + PM₂.₅ as the defining products.