Organic chemistry · GCSE Chemistry
Cracking
GCSE Chemistry revision on cracking long-chain alkanes into shorter alkanes and alkenes: thermal and catalytic conditions, example equations, and why industry matches supply to demand for fuels and polymers.
Cracking is thermal decomposition of large alkanes. You always get smaller molecules, and at least one product is an alkene (C=C). That alkene makes polymers; the shorter alkane is a more useful fuel.
The important bits
What you need to know
- 1
Cracking converts long-chain hydrocarbons into shorter, more useful molecules. It is a thermal decomposition: breaking large molecules by heating, not by fractional distillation.
- 2
Catalytic cracking: vaporised hydrocarbons are passed over a hot catalyst (often a zeolite) at moderate temperature. Steam (thermal) cracking uses higher temperature, typically with steam, and no zeolite.
- 3
Products are a shorter alkane and at least one alkene, because there is not enough hydrogen to saturate every fragment. Count atoms: they must balance.
- 4
Example: C₁₀H₂₂ → C₈H₁₈ + C₂H₄, or C₁₀H₂₂ → C₂H₄ + C₃H₆ + C₅H₁₂. Many valid combinations exist; every carbon and hydrogen on the left must appear on the right.
- 5
Shorter alkanes are in higher demand as fuels (petrol) because they are less viscous and more flammable. Distillation alone does not make enough of them.
- 6
Alkenes are used as monomers for addition polymers and as feedstock to make alcohols and other chemicals. Cracking is how an oil refinery supplies the organic chemical industry.
- 7
You can tell a cracking product mixture contains an alkene with bromine water: orange to colourless. The original long alkane would not decolourise bromine water.
- 8
Cracking is a chemical change: new substances form, C–C bonds break, and displayed formulae of products are not the same as the reactant. Distillation only sorts existing molecules.
Quotations worth analysing
Short evidence. Real method.
“Cracking is the thermal decomposition of long-chain hydrocarbons into shorter alkanes and alkenes.”
Thermal decomposition, shorter, alkane and alkene: those words score. Do not call it distillation or “breaking oil into petrol” without naming the alkene.
“C₁₀H₂₂ → C₈H₁₈ + C₂H₄”
Atoms balance: 10 C and 22 H on both sides. One product is saturated (C₈H₁₈), one is unsaturated (C₂H₄). Other splits are allowed if the atoms still balance.
“There is a higher demand for shorter-chain hydrocarbons as fuels, and alkenes are needed to make polymers.”
Give both reasons: fuel demand and polymer feedstock. “To make petrol” alone often misses the alkene mark.
Go deeper
Balance the carbon, then the hydrogen will tell you an alkene appeared
A long alkane has formula CnH2n+2. If you split it into two alkanes, you would need extra hydrogen that does not exist. So at least one product must be an alkene (CnH2n) or a mixture that includes alkenes. Check: C₁₂H₂₆ → C₈H₁₈ + C₄H₈ works (12 C, 26 H). C₁₂H₂₆ → C₈H₁₈ + C₄H₁₀ does not, because the right-hand side has 28 H. That atom-count is the quickest way to spot a wrong equation in a multiple-choice. In displayed formulae, one product must show C=C. If every product is drawn with only single bonds, the hydrogen count will be too high.
Go deeper
Why bother: the market, not the molecules’ feelings
Fractional distillation of a typical crude oil gives more long-chain fractions than society wants to burn in cars, and not enough petrol-range molecules. Long chains are also less useful as fuels because they are viscous and hard to ignite. Cracking turns surplus heavy fractions into petrol-sized alkanes. At the same time, modern life wants ethene and propene for poly(ethene), poly(propene) and other chemicals. Those alkenes are not sitting in the oil as a large separate fraction; cracking manufactures them. A question that asks “why is cracking necessary” wants supply and demand plus a named use of the alkene, not “to make the molecules happier”.
Go deeper
Conditions: catalyst versus steam, both still cracking
Catalytic cracking uses a zeolite catalyst and a high but not extreme temperature, which saves energy compared with steam cracking and favours branched hydrocarbons useful in petrol. Steam cracking uses very high temperature and steam to produce a high yield of small alkenes such as ethene. GCSE will accept “high temperature and a catalyst” or “high temperature and steam”. Do not write that the catalyst is used up, and do not write that cracking is reversible equilibrium like Haber — you are decomposing molecules, then separating products. Bromine water on the product mixture going colourless is the cheap laboratory evidence that alkenes formed.
See the idea in action
C₁₆H₃₄ is cracked to octane and an alkene. Octane is C₈H₁₈, so the other product must contain 8 carbons and 16 hydrogens: C₈H₁₆, an alkene (CnH2n). Equation: C₁₆H₃₄ → C₈H₁₈ + C₈H₁₆. Check: 16 C and 34 H on both sides. C₈H₁₆ would decolourise bromine water; C₁₆H₃₄ would not. The octane is a more useful petrol-range fuel; the alkene can be polymerised.
Exam technique
Turn knowledge into marks
Call cracking thermal decomposition, not distillation. Write an equation that balances and includes an alkene. Give two reasons: more useful short fuels, and alkenes for polymers. Mention high temperature and a catalyst or steam if conditions are asked.
Common mistakes
Do not give these marks away
- 01
Writing a cracking equation with two alkanes and no alkene, so the hydrogen atoms do not balance.
- 02
Calling cracking fractional distillation, or saying it only “purifies” crude oil.
- 03
Forgetting a use for the alkene, or claiming the catalyst is used up.
Which equation could represent cracking of decane?
AC₁₀H₂₂ → C₈H₁₈ + C₂H₆
BC₁₀H₂₂ → C₈H₁₈ + C₂H₄
CC₁₀H₂₂ → 10C + 11H₂
DC₁₀H₂₂ → C₁₀H₂₀ + H₂ only, with no smaller fuel
Show the answer
C₁₀H₂₂ → C₈H₁₈ + C₂H₄. Atoms balance and one product is an alkene. C₈H₁₈ + C₂H₆ has 24 hydrogens, two too many, so that split is not possible without extra hydrogen.
Quick questions
If this is the bit you searched
What is cracking in GCSE Chemistry?
Thermal decomposition of long-chain hydrocarbons into smaller, more useful molecules. Products are shorter alkanes and alkenes.
Why is cracking necessary?
Distillation produces more long-chain hydrocarbons than are needed, while demand is high for shorter fuels such as petrol and for alkenes used to make polymers.
What conditions are used for cracking?
High temperature with a catalyst (catalytic cracking, often a zeolite) or high temperature with steam (thermal / steam cracking).
How can you tell that cracking has made an alkene?
The product mixture decolourises bromine water (orange to colourless). The original alkane does not, because it is saturated.