Year 9 Science · Unit 2 · Lesson 14
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Learning Goals
1. Name the products of complete and of incomplete combustion of a hydrocarbon, and give the flame colour you would expect for each.
2. Balance the equation for the complete combustion of ethane: C₂H₆ + O₂ → CO₂ + H₂O. Balance C, then H, then O. Hint: if you need half an O₂, double every number in the equation.
3. In a poorly ventilated heater, butane, C₄H₁₀, forms carbon monoxide and H₂O only. Write this reaction as an equation with state symbols, and balance it. Every substance is a gas at flame temperature.
Real-world context
In a garage or a closed room, a petrol generator or a faulty gas heater can make enough carbon monoxide to kill within minutes. Carbon monoxide has no colour and no smell, so it cannot be noticed without an alarm.
4. Explain why an engine or heater with a limited supply of oxygen makes CO instead of CO₂. Refer to the number of oxygen atoms each carbon atom ends up with.
Investigation: a candle under a jar
Students lit a tea-light candle, lowered an upturned glass jar over it, and timed how long the flame lasted. They used the same candle each time and let fresh air into the jar between trials.
| Jar volume (mL) | Trial 1 (s) | Trial 2 (s) | Average (s) |
|---|---|---|---|
| 250 | 8 | 9 | 8.5 |
| 500 | 16 | 15 | 15.5 |
| 750 | 23 | 25 | 24.0 |
| 1000 | 31 | 32 |
Source: sample class results.
5. Calculate the average for the 1000 mL jar and write it in Table 1.
6. Describe the pattern between jar volume and burning time. Use combustion to explain it, and predict the burning time under a 2000 mL jar.
7. In every trial, just before the flame went out it turned more yellow and gave off a thin black smoke. Explain this observation.
8. A traveller plans to run a portable LPG (propane, C₃H₈) heater inside a closed caravan overnight. Use two balanced equations, one for propane burning in plenty of oxygen and one for propane burning to carbon monoxide, to explain why this plan is dangerous.