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When crude oil is separated by fractional distillation, it produces far more of some fractions than we need and far too little of others. In particular, distillation yields a great deal of long-chain hydrocarbon but not nearly enough short-chain fuel such as petrol, which is in huge demand. The solution is cracking — breaking long, less useful hydrocarbon molecules into shorter, more useful ones. Cracking also produces alkenes, a reactive family of hydrocarbons used to make polymers (plastics). This lesson, part of Topic C6 of OCR Gateway Science A, explains why cracking is needed, how it is done, what an alkene is, the test for an alkene, and how alkenes are used to make polymers.
By the end of this lesson you should be able to explain why long-chain hydrocarbons are cracked, describe catalytic and steam cracking, write balanced equations for cracking, describe alkenes and give their general formula, carry out and interpret the bromine-water test, and describe addition polymerisation.
Fractional distillation produces fractions in fixed proportions set by the make-up of the crude oil — and these proportions do not match demand. There is a surplus of long-chain fractions (such as fuel oil) and a shortage of short-chain fractions, especially petrol, which is needed in enormous quantities for cars. Rather than waste the surplus long-chain hydrocarbons, refineries crack them into the shorter molecules that are in demand.
So cracking solves two problems at once: it uses up less-wanted long-chain hydrocarbons, and it makes more of the valuable short-chain hydrocarbons (and the alkenes needed for plastics).
Exam Tip: The reason for cracking is a supply-and-demand mismatch: distillation gives too much long-chain hydrocarbon and too little petrol. Cracking converts the surplus into more useful, shorter molecules.
Cracking is the breaking of long-chain alkanes into shorter, more useful molecules. It is a form of thermal decomposition — a large molecule is broken down by heat. There are two main methods:
| Method | Conditions |
|---|---|
| Catalytic cracking | The long-chain hydrocarbon is heated until it vaporises, then passed over a hot catalyst |
| Steam cracking | The hydrocarbon is vaporised, mixed with steam and heated to a very high temperature |
In both methods, a long-chain alkane is broken into a shorter-chain alkane plus one or more alkenes. The shorter alkane is useful as a fuel (such as petrol); the alkenes are useful for making polymers.
A cracking reaction always produces a shorter alkane and at least one alkene. For example, decane can crack to give octane (a petrol-sized molecule) and ethene (an alkene):
C10H22→C8H18+C2H4
Check it balances: carbons 10=8+2 ✓; hydrogens 22=18+4 ✓.
Dodecane, C12H26, is cracked to make octane, C8H18, and ethene, C2H4. Write the balanced equation, adding more ethene molecules as needed.
Step 1 — write what is known: C12H26→C8H18+C2H4.
Step 2 — check carbon: left 12; right 8+2=10. Two carbons short, so add another C2H4 → C8H18+2C2H4.
Step 3 — check carbon again: right 8+(2×2)=12 ✓. Check hydrogen: right 18+(2×4)=18+8=26 ✓; left 26 ✓.
C12H26→C8H18+2C2H4
Answer: C12H26→C8H18+2C2H4 — balanced for both carbon and hydrogen.
Exam Tip: Cracking products are always a shorter alkane + one or more alkenes. Check the equation balances by counting carbons and hydrogens; you may need more than one alkene molecule to make it balance.
An alkene is a hydrocarbon that contains a carbon–carbon double bond (C=C). This double bond makes alkenes unsaturated — they do not contain the maximum possible number of hydrogen atoms, because two of the bonds are taken up in the double bond. Alkenes have the general formula CnH2n.
The first two alkenes are:
| Name | Formula | n |
|---|---|---|
| Ethene | C2H4 | 2 |
| Propene | C3H6 | 3 |
The C=C double bond is the functional group of the alkenes — the reactive part — which makes alkenes more reactive than alkanes. Here is the displayed formula of ethene, showing its double bond:
Exam Tip: The key word for alkenes is unsaturated (they contain a C=C double bond); their general formula is CnH2n. Contrast with saturated alkanes (CnH2n+2, only single bonds).
The double bond gives a simple chemical test to tell an alkene from an alkane: bromine water.
| Hydrocarbon | Bromine water result |
|---|---|
| Alkene (has C=C) | Orange → colourless (decolourised) |
| Alkane (no C=C) | Stays orange (no change) |
A student has two colourless gases: one is ethane and one is ethene. They bubble each through orange bromine water. Gas X decolourises the bromine water; gas Y leaves it orange. Identify each gas and explain.
Step 1 — recall the test: an alkene (with a C=C double bond) decolourises bromine water; an alkane does not.
Step 2 — gas X decolourises the bromine water, so it must be the alkene → ethene (C2H4).
Step 3 — gas Y leaves the bromine water orange, so it must be the alkane → ethane (C2H6).
Answer: X is ethene (alkene, decolourises bromine water); Y is ethane (alkane, no change).
Exam Tip: Get the colour change the right way round: an alkene turns bromine water from orange to colourless (decolourised). Saying it "turns orange" is wrong — it removes the orange colour.
Alkenes are extremely useful because they can join together to form polymers — very long molecules that make up plastics. In addition polymerisation, many small alkene molecules (monomers) add together to form one very large molecule (the polymer), with no other product.
For example, many ethene monomers join to make poly(ethene) (polythene):
nC2H4→poly(ethene)
The C=C double bond in each ethene molecule "opens up" and joins to the next monomer, forming a long chain of single-bonded carbons. The repeating part of the polymer is its repeat unit:
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