Alkenes
Alkenes
- Alkenes have a carbon–carbon double bond (C=C) → unsaturated hydrocarbons.
- The C=C makes them reactive — they do addition reactions (two molecules join into one).
Practice
Alkenes are unsaturated because they contain:
The C=C double bond makes alkenes unsaturated and reactive (addition reactions).
Cracking
- Cracking breaks large alkanes into smaller, more useful alkanes and alkenes, using a high temperature and a catalyst.
- It also makes hydrogen and provides alkenes for plastics.
Practice
Cracking breaks large alkanes into:
Cracking uses heat and a catalyst to make smaller, more useful alkanes and alkenes (plus hydrogen).
Reactions of alkenes
- Test for unsaturation: shake with bromine water (orange) — an alkene turns it colourless; an alkane does not.
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- hydrogen (nickel catalyst) → an alkane.
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- steam (acid catalyst) → an alcohol.
Practice
Shaking an alkene with orange bromine water turns it:
Alkenes decolourise bromine water (orange → colourless); alkanes do not — this is the test for unsaturation.
You've got it
Key idea
- alkenes have a C=C → unsaturated and reactive (addition reactions)
- cracking: big alkanes → smaller alkanes + alkenes (high temp + catalyst)
- bromine water test: alkene turns it orange → colourless