HSC Exam Practice

Sample response. A combustion reaction is a reaction between a fuel and oxygen that produces oxides of the elements present in the fuel and releases energy as heat and light [1 mark — definition]. For a hydrocarbon to undergo complete combustion, two conditions are required: (1) an excess (sufficient) supply of oxygen relative to the fuel [1 mark], and (2) ignition energy to initiate the reaction [1 mark]. Accept also: adequate mixing of fuel and oxygen at a high enough temperature.

Marking notes. 1 mark for a definition that includes fuel + oxygen + oxides (or specific products CO₂ + H₂O for hydrocarbons); 1 mark for sufficient/excess oxygen; 1 mark for a second condition (ignition/activation energy or adequate mixing). Do not award the second mark if students only say “oxygen” without “excess/sufficient”.

Sample response. (a) Complete combustion occurs when oxygen is present in excess relative to the fuel; incomplete combustion occurs when oxygen supply is limited or insufficient [2 marks]. (b) Complete combustion produces CO₂(g) and H₂O(g) as the only carbon-containing and hydrogen-containing products; incomplete combustion produces CO(g) and/or solid carbon soot C(s) instead of CO₂, along with H≶O(g) [2 marks].

Marking notes. 1 mark each for oxygen condition of complete combustion (excess/sufficient) and incomplete combustion (limited/insufficient). 1 mark for products of complete combustion (CO₂ + H₂O only). 1 mark for products of incomplete combustion (CO and/or C soot, with H₂O). Students must explicitly distinguish CO from CO₂ for the product marks.

Sample response. Balance C: 3 carbons in C₃H₈ → 3CO₂ [½ mark]. Balance H: 8 hydrogens → 4H₂O [½ mark]. Balance O: right side has 3×2 + 4×1 = 10 O atoms → need 5O₂ [½ mark]. Multiply if needed — already whole numbers.

C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H≶O(g) [1 mark for fully balanced equation; 1 mark for correct state symbols; 1 mark for atom verification check].

Atom check: Left → 3C, 8H, 10O. Right → 3C, 8H, 6+4=10O. Correct.

Sample response. CO is acutely toxic at low concentrations because it binds to haemoglobin with approximately 200 times the affinity of O₂ [1 mark]. Even small concentrations of CO (as low as a few hundred ppm) can occupy most haemoglobin binding sites, preventing O₂ from attaching and being transported to cells throughout the body [1 mark]. This causes tissue hypoxia — cells starve of oxygen despite continued breathing. CO₂ causes harm primarily by displacing O₂ from the air at very high concentrations (>5%), well above levels that would cause immediate physiological effects [1 mark].

Marking notes. 1 mark for ~200× haemoglobin affinity; 1 mark for mechanism (O₂ transport blocked, hypoxia); 1 mark for explicitly contrasting with CO₂ (acts by displacing O₂ from air at much higher concentration). Do not award marks for vague statements like “CO is poisonous” without mechanism.

Sample response. Complete combustion produces a blue flame; incomplete combustion produces a yellow/orange flame [1 mark]. The yellow colour is caused by glowing solid carbon soot particles (C(s)) that are heated to incandescence by the flame [1 mark]. (The same effect is visible in a candle flame.)

Marking notes. 1 mark for correct flame colours (blue = complete; yellow/orange = incomplete). 1 mark for explaining yellow colour as incandescence of glowing carbon soot particles. Do not accept “it is hotter” or similar without reference to soot.

Sample response. The flame front is an actively burning zone with strong upward airflow, which continuously draws in fresh oxygen from the surroundings, maintaining a high oxygen-to-fuel ratio [1 mark]. This supports predominantly complete combustion, producing CO₂ and H≶O with very little CO. The smouldering zone behind the front consists of slowly burning embers covered by ash, which restricts airflow and oxygen supply [1 mark]. With limited O₂, incomplete combustion dominates and carbon in the fuel is only partially oxidised to CO rather than CO₂, causing CO to accumulate in the air around the smouldering material [1 mark].

Marking notes. 1 mark for identifying that the flame front has abundant oxygen (strong airflow). 1 mark for identifying that the smouldering zone has restricted oxygen (covered by ash, no airflow). 1 mark for linking restricted O₂ to incomplete combustion and CO production (partial carbon oxidation).

Sample response (a) — balanced equation for butane combustion.

C: 4 carbons → 4CO₂; H: 10 hydrogens → 5H≶O; O: right side 8+5=13 O atoms → 13/2 O₂. Clear fraction: multiply all by 2.
2C₄H₁₀(g) + 13O₂(g) → 8CO₂(g) + 10H≶O(g). Atom check: 8C, 20H, 26O each side. Correct.

Marking notes (a). 1 mark for a fully balanced equation. 1 mark for correct state symbols. 1 mark for atom check confirming both sides are balanced.

Sample response (b) — comparison and recommendation.

Energy output: Butane (49.5 kJ g⁻¹) releases more than twice the energy per gram compared to methanol (22.7 kJ g⁻¹); for the same stove fuel canister mass, butane will heat food or boil water significantly faster and with less fuel [1 mark]. Safety in enclosed space: Both fuels produce CO if O₂ is restricted (Table 2.1). However, butane is a gas stored under pressure — if the canister valve leaks it can accumulate as a flammable gas, creating explosion risk in a tent. Methanol is a liquid but is highly toxic by skin absorption and ingestion, so handling hazards exist, and it also produces CO if O₂ is restricted [1 mark]. Recommendation: Neither fuel is safe to use in an enclosed tent due to CO production from incomplete combustion and/or gas accumulation. If forced to choose on energy grounds alone, butane provides more than twice the energy per gram. However, both should only be used with adequate ventilation; the CO risk applies to both fuels equally [1 mark]. Accept a reasoned recommendation for either fuel provided it refers to data and the CO risk.

Sample response (a) — trend description (3 marks). In the unventilated room, CO concentration increases steadily over time, rising from 0 ppm at t = 0 to approximately 420 ppm at t = 60 minutes [1 mark]. The increase is approximately linear (slightly curved upward) [1 mark]. The CO concentration crosses the 200 ppm safe limit at approximately t = 40 minutes [1 mark]. Accept any reading between 35 and 45 minutes based on graph interpolation.

Sample response (b) — explanation (3 marks). In the unventilated room, the heater combusts methane continuously. As combustion proceeds, O₂ in the sealed room is consumed and its concentration falls [1 mark]. When O₂ supply becomes restricted, incomplete combustion of methane occurs (2CH₄ + 3O₂ → 2CO + 4H≶O), producing CO that accumulates in the room air [1 mark]. In the ventilated room, fresh air continuously replaces the oxygen consumed, maintaining a high O₂:fuel ratio that supports complete combustion (CH₄ + 2O≶ → CO≶ + 2H≶O). No CO accumulates because the combustion remains complete [1 mark].

Sample response (c) — media claim error (2 marks). The error is that CO is colourless AND odourless — it cannot be smelled [1 mark]. This is why CO poisoning is so dangerous: victims receive no sensory warning that it is accumulating to toxic levels. The correct statement is that CO exposure cannot be detected by smell; only a CO monitor can reliably detect CO accumulation before it reaches dangerous concentrations [1 mark].

Sample response. The claim is partially defensible but fundamentally incorrect in key respects. What is defensible: Complete combustion of hydrocarbons does produce only CO≶ and H≶O: CH₄(g) + 2O₂(g) → CO≶(g) + 2H≶O(g). Under ideal conditions with excess oxygen, this is a relatively clean conversion of chemical energy to heat. CO≶ and water are indeed non-toxic at normal atmospheric concentrations, and the energy release is efficient for heating applications [1 mark]. What is incorrect: First, combustion is only “clean” if it is complete. Whenever oxygen is limited — in poorly ventilated rooms, smouldering bushfire zones, or undersupplied industrial flares — incomplete combustion occurs: 2CH₄(g) + 3O≶(g) → 2CO(g) + 4H≶O(g). CO is acutely toxic because it binds haemoglobin with ∼200 times the affinity of O≶, blocking oxygen transport to tissues and causing hypoxia and death [2 marks]. Second, even complete combustion produces CO≶ in large quantities. While CO≶ is non-toxic at normal concentrations, the burning of fossil fuels (coal, oil, natural gas) releases CO≶ from carbon that was locked away for millions of years, contributing significantly to the enhanced greenhouse effect and climate change [1 mark]. Third, solid carbon soot produced by incomplete combustion is a fine particulate (PM≶.₅) that penetrates deep into lung tissue, causing chronic respiratory disease and contributing to air quality problems [1 mark]. Australian context: During the 2019–20 Black Summer bushfires, smouldering zones in the Blue Mountains and Snowy Mountains produced CO spikes and PM≶.₅ levels exceeding 23 times the 24-hour safe standard in Sydney. This is a direct real-world example of combustion — the same type of reaction — producing products that are far from “harmless” [1 mark]. Additionally, the Karratha LNG facility in Western Australia must carefully manage its flare combustion conditions to ensure complete rather than incomplete combustion, precisely because CO and soot production under low-flow conditions creates occupational hazards [1 mark]. Conclusion: Combustion is only “clean and efficient” under the specific conditions of complete combustion. Under restricted oxygen, it produces toxic CO and soot. Even complete combustion releases CO≶ at scale, contributing to climate change. The claim overstates the case: the chemistry of combustion is both useful and potentially hazardous, depending entirely on oxygen availability and environmental context [1 mark].

Marking criteria (8 marks total):

• 1 mark — Identifies the defensible element of the claim (complete combustion with excess O≶ does produce only CO≶ + H≶O; these are non-toxic at normal concentrations).
• 1 mark — Provides a correctly balanced equation for complete combustion of a named hydrocarbon.
• 1 mark — Identifies that incomplete combustion produces CO and/or soot (not CO≶) when O≶ is restricted, with a balanced equation.
• 1 mark — Explains CO toxicity mechanism (haemoglobin affinity ×200; O≶ transport blocked; hypoxia).
• 1 mark — Identifies soot (fine particulate / PM≶.₅) as a hazardous product of incomplete combustion affecting respiratory health.
• 1 mark — Identifies that even complete combustion’s CO≶ product contributes to climate change at scale (fossil fuel context).
• 1 mark — Uses at least one specific Australian real-world context (bushfires / LNG flaring / gas heaters).
• 1 mark — Reaches an explicit evaluative judgement that identifies which part of the claim is defensible and which is incorrect, with a reformulated accurate statement.