Year 9 Science · Unit 2 · Lesson 15
Challenge Worksheet
Learning Goals
1. Name the type of catalyst used in catalytic cracking, and give one advantage of catalytic cracking over thermal cracking.
2. One molecule of hexadecane, C₁₆H₃₄, is cracked into one molecule of octane, two molecules of ethene and one other acyclic (non-ring) hydrocarbon molecule. Write the balanced equation, and name the family the other molecule belongs to.
3. An alkane CₙH₂ₙ₊₂ cracks into two acyclic (non-ring) hydrocarbon molecules, and one of them is an alkane. Use carbon and hydrogen counts to show that the other molecule fits the alkene general formula CₘH₂ₘ. Hint: try it for C₁₀H₂₂ → C₇H₁₆ + ?, then explain the general result.
Find the mistakes: a student wrote this answer
“The energy transition is simple: we just replace coal and oil with solar panels and wind turbines, and the problem is solved. Once we stop burning fossil fuels, we won’t need crude oil any more. Planes and ships can simply switch to batteries charged by solar power.”
4. Identify three errors or oversimplifications in the student’s answer. For each, explain the science that shows it is wrong.
Read the data
| Energy store | Energy per kg (MJ) | Energy per litre (MJ) |
|---|---|---|
| Jet fuel (kerosene fraction) | 43 | 35 |
| Liquid hydrogen, at −253 °C | 120 | 8.5 |
| Lithium-ion battery cells | 0.9 | 2.5 |
Source: typical published values, rounded. Fuel values are the energy released on burning; battery values are for good modern cells (about 250 Wh per kg).
5. A short-haul jet takes off carrying 15 tonnes of jet fuel. Its maximum take-off mass, with fuel, passengers and cargo, is 78 tonnes. Ignoring differences in engine efficiency, calculate (a) the mass of battery cells that would store the same energy, and (b) how many litres of liquid hydrogen would hold the same energy as 1 litre of jet fuel.
(a) t
(b) L
6. Use your answers to explain why replacing coal in power stations is described as “easy” but replacing jet fuel as “hard”. What would liquid hydrogen fix, and what would it not fix?
7. Two ideas are proposed for using less crude oil: hydrogen made with renewable electricity, and plant-based plastics such as PLA from sugarcane or corn starch. Evaluate how far each one can reduce Australia’s dependence on crude oil.