Cracking and Refinery Demand
Explain why refineries crack hydrocarbons, describe the conditions, and use atom conservation to deduce products.
On this page
A refinery can separate the molecules in crude oil, but separation cannot turn a large molecule into a smaller one. Cracking changes molecules to help match the products of a refinery to what people need.
Separation or chemical change?
Fractional distillation separates a mixture into fractions with different boiling ranges. The molecules retain their chemical identities. Cracking breaks hydrocarbon molecules into smaller molecules: bonds break and new bonds form, so it is a chemical change.
Conditions and products
For the catalytic cracking studied here, vaporised hydrocarbons are heated to a high temperature and passed over a catalyst such as aluminium oxide, Al₂O₃, or silicon dioxide, SiO₂. The catalyst speeds up the reaction; it does not supply carbon or hydrogen to the products.
Cracking a larger alkane can produce a smaller alkane and an alkene:
Hydrogen may also be produced. This separate model equation shows how an alkane can form an alkene and hydrogen:
Swipe or scroll sideways to inspect the complete overview.
These are examples, not a promise that every cracking reaction makes exactly two products in the same proportions. Real cracking produces a mixture. Each valid equation must conserve every type of atom.
Why a refinery needs cracking
Crude oil does not necessarily supply fractions in the proportions customers demand. If heavier fractions are in surplus while smaller fuel molecules and chemical feedstocks are in short supply, more fractional distillation cannot solve that mismatch. It only separates what is already present.
Cracking converts some surplus larger molecules into smaller molecules. Alkenes such as ethene are useful feedstocks for making plastics and other chemicals; smaller alkanes are useful fuels. The needed product is not always a fuel.
Work out a missing product
Guided practice 1
Cracking products (logic)
Guided equation
Use atom conservation to find the missing product
Hints
Hint 1: subtract atoms
Hint 2: series check
View solution step by step
Conserve carbon
Method
Subtract the two carbon atoms in ethene from the ten in decane.Reason
Cracking rearranges atoms; it does not create or destroy them.Working
10-2 = 8 carbon atoms remain.Conserve hydrogen
Method
Subtract the four hydrogen atoms in ethene from the 22 in decane.Reason
The missing product must contain all remaining hydrogen atoms.Working
22-4 = 18, giving C₈H₁₈.Check and write the equation
Method
Confirm C₈H₁₈ is an alkane and write the products.Reason
For n = 8, 2n + 2 = 18 and both sides now have identical atom totals.Working
C₁₀H₂₂ → C₈H₁₈ + C₂H₄.
Try without prompts
Mind stretcher 1: Match a refinery supply to demandExtension
The table is an illustrative model, not measured refinery data. Supply and demand refer to the same total mass.
| Fraction group | Supply / % | Demand / % |
|---|---|---|
| Smaller-molecule fractions | 30 | 50 |
| Medium-molecule fractions | 30 | 30 |
| Larger-molecule fractions | 40 | 20 |
Identify the surplus and shortage. Explain why another round of fractional distillation cannot correct them, and how cracking can help. Separately, deduce X in C₄H₁₀ → C₄H₈ + X. Would X decolourise bromine water in the usual test?
Show answer
The larger-molecule group has a surplus of twenty percentage points; the smaller-molecule group has a shortage of twenty. Distillation separates existing molecules without shortening them. Cracking can turn some larger molecules into smaller fuel molecules and alkenes. It need not convert all surplus material into exactly the desired products, so the table alone does not determine a cracking yield.
X is H₂: subtracting four C and eight H atoms leaves no C and two H. Hydrogen contains no C = C and does not decolourise bromine water in this test. Not every cracking product is an alkene.
Mind stretcher 2: Distinguish cracking from fractional distillationExtension
A learner says: “Cracking separates crude oil into fractions.” Replace this with an accurate sentence for cracking and one for fractional distillation.
Show answer
Cracking changes hydrocarbon molecules into smaller molecules using heat and, in the catalytic process studied here, a catalyst. Fractional distillation separates crude oil into fractions by boiling behaviour while preserving the molecules’ identities.
Bring the ideas together
Use molecule size and demand to explain why cracking is useful. Use atom conservation to check an equation. Heat and a catalyst help the reaction happen; they do not change the requirement for equal atom totals.
Practise and check
Use the Organic Chemistry topic check to practise and check your understanding.
Syllabus and review details
- SEC G3 Pure Chemistry 2027 · 2027
Content structure and subject content, PDF pages 9–24
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