Year 9 Science · Unit 2 · Lesson 14
Challenge Worksheet
Learning Goals
1. Explain what makes a hydrocarbon flame yellow, and what a blue flame tells you about the oxygen supply.
Find the mistakes: four equations from a student’s book
(a) Complete combustion of methane: CH₄ + O₂ → CO₂ + 2H₂O
(b) Complete combustion of hexane: 2C₆H₁₄ + 19O₂ → 12CO₂ + 14H₂O
(c) Complete combustion of propane: C₃H₈ + 5O → 3CO₂ + 4H₂O
(d) Incomplete combustion of octane: 2C₈H₁₈ + 17O₂ → 16CO₂ + 18H₂O
2. Only one equation is correct. For each wrong one, say what the mistake is and write the correct equation. Show atom counts for at least one.
3. When heptane, C₇H₁₆(l), has almost no air, its only products are soot, C(s), and H₂O(g). Give the balanced equation.
Someone claims…
“Natural gas (methane) is a clean fuel because burning it produces only water and no carbon. It should be promoted as a green energy source to replace coal.”
Read the data
| Fuel | CO₂ made (g) | Energy released (kJ) | CO₂ per MJ of energy (g) |
|---|---|---|---|
| Methane (natural gas) | 2.75 | 55.5 | |
| Octane (petrol) | 3.09 | 47.9 | |
| Carbon (a simple model of coal) | 3.67 | 32.8 |
Source: CO₂ from the equations (C = 12, H = 1, O = 16); energy values from a data book. Real coal holds some hydrogen too, so it gives a little less CO₂ per MJ.
4. Using CH₄ + 2O₂ → CO₂ + 2H₂O, show that 16 g of methane makes 44 g of CO₂, and so 2.75 g of CO₂ for every gram of methane.
5. Complete the last column of Table 1. (1 MJ = 1000 kJ, so CO₂ per MJ = CO₂ made ÷ energy released × 1000.) Round to the nearest gram.
6. Which part of the claim is chemically wrong? Use the balanced equation to show it. Which part does Table 1 support?
7. Evaluate the statement: “Carbon monoxide poisoning would end if every gas appliance in homes were replaced with an electric one.” Give one reason it would help and two sources of CO it would not remove.
8. Evaluate whether natural gas should be promoted as a “green” fuel to replace coal. Use your values from Table 1 and the balanced equation for methane, and name two further pieces of evidence you would need before deciding.