Fractional distillation of crude oil gives far more long-chain fractions than anyone wants and too little petrol. Cracking fixes the mismatch: heat the large alkanes over a catalyst and they split into smaller alkanes for fuel and alkenes for making plastics.
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.
Build alkanes of different chain lengths in the simulator and compare their formulae with CnH2n+2. The long ones are the molecules that cracking breaks up.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What type of reaction is cracking?
Which conditions are used for catalytic cracking?
Which products are formed when a long-chain alkane is cracked?
Why is cracking carried out?
Atoms are conserved: add up the carbons and the hydrogens on each side. The missing product is usually an alkene, sometimes hydrogen.
| Alkane cracked | Products | Check (C, H) |
|---|---|---|
| C10H22 | C8H18 + C2H4 | 10, 22 = 8 + 2, 18 + 4 |
| C12H26 | C8H18 + 2C2H4 | 12, 26 = 8 + 4, 18 + 8 |
| C14H30 | C8H18 + C2H4 + C4H8 | 14, 30 = 8 + 2 + 4, 18 + 4 + 8 |
| C2H6 | C2H4 + H2 | 2, 6 = 2, 4 + 2 |
Worked example. C16H34 → C10H22 + 2C2H4 + X. Carbon: 16 − 10 − 4 = 2. Hydrogen: 34 − 22 − 8 = 4. So X is C2H4, and the equation is C16H34 → C10H22 + 3C2H4.
Compare what crude oil gives with what the market wants.
| Fraction | Supply from crude oil | Demand | Result |
|---|---|---|---|
| refinery gas, gasoline (petrol) | low | high | shortage |
| naphtha, kerosene | medium | medium | roughly matched |
| gas oil (diesel), fuel oil | high | lower | surplus: crack 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.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
C12H26 → C6H14 + C2H4 + X. What is X?
Ethane is cracked to give ethene and one other product. What is it?
Which test shows that the products of cracking contain an alkene?
Which statement about the products of cracking decane, C10H22, is correct?
This skill is now recorded on your profile. Keep the chain going.