The shape of the lesson
By the end of the lesson, learners can
- What cracking does: Build alkanes of different chain lengths in the simulator and compare their formulae with C n H 2n+2.
- Balancing cracking equations: Atoms are conserved: add up the carbons and the hydrogens on each side.
- Why crack at all?: Compare what crude oil gives with what the market wants.
- Cracking: thermal decomposition of large alkanes.
- Conditions: high temperature and a catalyst (silica or alumina).
- Products: smaller alkanes + alkenes (+ sometimes hydrogen).
- Why: match supply to demand; make alkenes for polymers.
One picture every idea returns to
Cracking is the thermal decomposition of large alkane molecules into smaller alkanes and alkenes, and sometimes hydrogen. It needs a high temperature (about 500–600 °C) and a catalyst (such as silica or alumina, often in the form of zeolites). Cracking equations must balance: C10H22 → C8H18 + C2H4. It is done because crude oil contains more long-chain fractions than are needed and not enough short-chain fractions such as petrol, and because the alkenes made are the starting materials for polymers and other chemicals such as ethanol.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Breaking big molecules into useful ones.. Fractional distillation of crude oil gives far more long-chain fractions than anyone wants and too little petrol. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: What cracking does | Build alkanes of different chain lengths in the simulator and compare their formulae with C n H 2n+2. Project tab 1 of the lab (Hydrocarbons lab, alkanes) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Balancing cracking equations | Atoms are conserved: add up the carbons and the hydrogens on each side. | Pairs |
| 21 to 30 min | Teach and check: Why crack at all? | Compare what crude oil gives with what the market wants. | Frayer Model |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 12 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Why crack at all?, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Frayer Model
Pairs fill a four-box grid for the key term: definition, characteristics, examples and non-examples. The non-examples box is where the misconceptions surface, so give it the most time.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Cracking is thermal decomposition, not combustion: no oxygen is involved and nothing burns. | The products always include an alkene (unless the only other product is hydrogen), because there are not enough hydrogen atoms for every product to be saturated. Always check that both carbon and hydrogen balance. |
Cracking: worksheet
6 questions, 12 marks. Written in the style of the papers; no past paper question is reproduced.
State the conditions used to crack long-chain alkanes.
Complete the equation: C14H30 → C10H22 + ......
Hexadecane, C16H34, is cracked to give octane, C8H18, and ethene only. Write the balanced equation.
Explain why oil companies crack some of the gas oil fraction.
A student bubbles the gaseous product of cracking through bromine water. Describe what is seen and explain what it shows.
Explain why cracking is described as thermal decomposition.
Answer key and marking notes
- A high temperature (about 500–600 °C).
- A catalyst such as silica or alumina (zeolite).
- Carbon left: 14 − 10 = 4. Hydrogen left: 30 − 22 = 8.
- Missing product: C4H8 (or 2C2H4).
- Carbon left after octane: 16 − 8 = 8, so 4 C2H4.
- Hydrogen check: 18 + 4 × 4 = 34.
- C16H34 → C8H18 + 4C2H4
- Crude oil contains more long-chain fractions such as gas oil than are needed.
- There is a higher demand for short-chain fractions such as petrol than crude oil supplies.
- Cracking converts the surplus into petrol and also makes alkenes for polymers.
- The bromine water changes from orange to colourless.
- The product contains an alkene: it is unsaturated (contains a C=C double bond).
- Heat (thermal) is used to break the reactant down.
- One compound is broken into two or more simpler substances (decomposition).
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
What is cracking?
- Athe thermal decomposition of large alkane molecules into smaller molecules
- Bthe separation of petroleum into fractions by boiling point
- Cthe joining of small alkene molecules into a long chain
- Dthe complete combustion of an alkane
Which are the products of cracking a long chain alkane?
- Aa shorter alkane and an alkene
- Bcarbon dioxide and water
- Ctwo alcohols
- Da polymer
Decane is cracked: C10H22 → C8H18 + X. What is X?
- AC2H4
- BC2H6
- CC2H2
- DC8H16
The diagram shows cracking in the laboratory. Liquid paraffin soaks the mineral wool at X. The vapour passes over the hot solid Y and gas Z is collected. Gas Z decolourises bromine water. Which row is correct?
- AY is a catalyst; Z contains an alkene
- BY is a catalyst; Z is an alkane only
- CY is used up in the reaction; Z contains an alkene
- DY is a drying agent; Z is an alcohol
Why do oil refineries crack some fractions?
- Ademand for short chain fractions such as gasoline is greater than the supply from distillation
- Blong chain alkanes are needed for making polymers
- Ccracking turns alkenes into alkanes for fuels
- Dcracking removes sulfur from petroleum
Dodecane is cracked to give octane and ethene only. Which is the balanced equation?
- AC12H26 → C8H18 + 2C2H4
- BC12H26 → C8H18 + C2H4
- CC12H26 → C8H18 + 2C2H6
- DC12H24 → C8H18 + 2C2H4
Hexadecane, C16H34, is cracked to give one molecule of octane, C8H18, and one molecule of hydrocarbon Q. What is Q, and what type of compound is it?
- AC8H16, an alkene
- BC8H18, an alkane
- CC8H16, an alkane
- DC8H14, an alkene
In the laboratory cracking experiment, the delivery tube must be removed from the water before heating stops. Why?
- Ato stop cold water being sucked back into the hot tube, which could crack it
- Bto let the catalyst cool down faster
- Cto stop the alkene reacting with the water
- Dto allow air into the tube so the products can burn
Quiz key
Cracking breaks large, long chain alkanes into smaller alkanes and alkenes (and sometimes hydrogen) using high temperature and a catalyst; it is a thermal decomposition. Separating by boiling point is fractional distillation. Joining alkenes into long chains is addition polymerisation. Combustion produces carbon dioxide and water, not useful smaller hydrocarbons.
Cracking gives a shorter alkane and at least one alkene, and sometimes hydrogen. There are not enough hydrogen atoms in the starting alkane for every product to be saturated, so an alkene must form. Carbon dioxide and water come from combustion. Alcohols need oxygen, which is not present. A polymer forms when alkenes are joined, the opposite of cracking.
10 − 8 = 2 carbon atoms and 22 − 18 = 4 hydrogen atoms remain, so X is C2H4, ethene. C2H6 would need 24 hydrogens on the left. C2H2 leaves two hydrogens unaccounted for. C8H16 has far too many carbon atoms.
The hot broken pot or aluminium oxide at Y is a catalyst: it speeds up the decomposition and is not used up. Gas Z decolourises bromine water, so it contains an unsaturated alkene such as ethene. An alkane alone would leave bromine water orange. A catalyst is not used up, and Y is not drying anything. No oxygen is present to form an alcohol.
Fractional distillation gives more long chain fractions than are needed and not enough short chain fractions such as gasoline. Cracking converts the surplus into gasoline and also makes alkenes, which are used to make polymers and other chemicals. Polymers are made from alkenes, not from long alkanes. Cracking makes alkenes from alkanes, not the other way round. Removing sulfur is a separate process.
Dodecane is C12H26. Removing C8H18 leaves C4H8, which is two ethene molecules, so the equation is C12H26 → C8H18 + 2C2H4. With only one C2H4, two carbons and four hydrogens are missing. Ethane, C2H6, would need 30 hydrogens on the left. C12H24 is not the formula of dodecane, an alkane.
16 − 8 = 8 carbons and 34 − 18 = 16 hydrogens, so Q is C8H16. This fits CnH2n, so Q is an alkene (octene). C8H18 would need 36 hydrogens in the starting alkane. C8H16 cannot be an alkane because it has fewer than 2n + 2 hydrogens. C8H14 leaves two hydrogens unaccounted for.
When heating stops, the gas in the tube cools and contracts, so the pressure falls and water could be sucked back into the hot tube, cracking the glass. This is called suck back. Removing the delivery tube has nothing to do with cooling the catalyst. Ethene does not react with cold water without a catalyst and high temperature. Letting air in to burn the products would be unsafe and is not the aim.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.