11.5Organic Chemistry
Alkenes — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 11 parts Lesson 5 of 10 1 quiz
Lesson summary
11.5 Alkenes — Notes — IGCSE Chemistry notes: Describe the structure and reactivity of alkenes, their manufacture by cracking, the test for them, addition reactions and addition polymerisation.Alkenes — unsaturated hydrocarbons that contain at least one carbon–carbon double bond,…
- Catalytic cracking
- Test for a cracked product
- - Addition reaction
- Ethene + bromine
- Ethene + hydrogen
- Ethene + steam
Section 1 of 11Structure and formulae
Structure and formulae
Alkenes
- Alkenes unsaturated hydrocarbons that contain at least one carbon–carbon double bond, C=C, with covalent bonds.
General formula CnH2n.
| Name | Molecular formula |
|---|---|
| Ethene | C2H4 |
| Propene | C3H6 |
| But-1-ene | C4H8 |
| Pent-1-ene | C5H10 |
Section 2 of 11Why alkenes are reactive
Why alkenes are reactive
The double bond can open up so that incoming atoms form a new single bond with each carbon atom.
Each carbon then forms four single bonds, which makes alkenes much more reactive than alkanes.
Section 3 of 11Manufacture by cracking
1Need
- Need fractional distillation gives far more long-chain hydrocarbons than are needed, so they are cracked into shorter, more useful molecules.
2
Cracking is the thermal decomposition of long-chain alkanes into shorter alkanes, alkenes and hydrogen.
3Catalytic cracking
- Catalytic cracking long-chain alkanes are heated to about 600–700 °C to vaporise them, then passed over a hot powdered catalyst of alumina or silica.
4
Covalent bonds break randomly on the catalyst surface, giving a mixture of shorter alkanes, alkenes and hydrogen.
5
C10H22 → C8H18 + C2H4 gives octane for petrol and ethene.
6Products
- Products alkenes make polymers and ethanol, hydrogen makes ammonia, and kerosene and diesel are often cracked to give petrol.
7
Hydrogen and a higher proportion of alkenes form at higher temperature and pressure.
8Test for a cracked product
- Test for a cracked product a cracked product that decolourises bromine water contains an alkene.
9
Cracking equations must balance carbon and hydrogen atoms.
10Reason
- Reason shorter alkanes (petrol) and alkenes (for polymers) are more useful and in greater demand.
Section 4 of 11Test for alkenes
Method
Shake the hydrocarbon with orange bromine water.
An alkane leaves the solution orange.
An alkene turns the solution colourless, because bromine adds across the C=C bond and is no longer in solution.
Wording
- Wording say colourless, not clear.
Section 5 of 11Addition reactions (Supplement)
- - Addition reaction
- atoms from a small molecule add across the C=C double bond, the C=C bond breaks and only one product forms.
| Reagent | Conditions | Product |
|---|---|---|
| Bromine | Room temperature | Dibromoalkane (saturated), e.g. 1,2-dibromoethane |
| Hydrogen | Nickel catalyst | Alkane, e.g. ethane |
| Steam | Acid catalyst (hydration) | Alcohol, e.g. ethanol |
1Ethene + bromine
- C2H4 + Br2 → C2H4Br2.
2Ethene + hydrogen
- C2H4 + H2 → C2H6.
3Ethene + steam
- C2H4 + H2O → C2H5OH.
4Steam
- Steam the hydration of ethene needs a phosphoric acid catalyst, high temperature and high pressure.
5Propene
- Propene it gives propane with hydrogen and propanol with steam.
6Product type
- Product type bromine gives a saturated dibromo compound, hydrogen an alkane and steam an alcohol.
Section 6 of 11Uses of addition reactions
Hydrogenation
- Hydrogenation adding hydrogen to vegetable oils turns C=C into C–C and raises the melting point to make solid margarine.
Hydration
- Hydration the hydration of ethene is a major industrial route to ethanol, used as a solvent and fuel.
Polymers
- Polymers alkenes are starting materials for addition polymers.
Section 7 of 11Alkanes and alkenes compared
| Test | Alkane (saturated) | Alkene (unsaturated) |
|---|---|---|
| Bromine water | Stays orange | Decolourised |
| Acidified KMnO4 | No change | Purple to colourless |
| Burning | Cleaner flame | Smoky yellow flame (more carbon per hydrogen) |
Section 8 of 11Addition polymerisation
- Alkenes can add to each other to form addition polymers such as poly(ethene), n C2H4 → (C2H4)n.
Section 9 of 11More from the specification
More from the specification
Synthetic organic acids from hydrocarbon fuels
- Synthetic organic acids from hydrocarbon fuels cracking long-chain alkanes extracted from fossil fuels produces chemical feedstocks converted into carboxylic acids.
Electrophilic Addition Across the C=C Bond
- The reactive pi-bond in the carbon-carbon double bond allows alkenes to undergo rapid addition reactions with halogens, hydrogen, and steam.
- (general formula CnH2n)
The Bromine Water Test for Unsaturation
- Shaking an alkene with orange aqueous bromine water causes rapid decolourisation from orange to colourless as 1,2-dibromoalkane forms, distinguishing alkenes from unreactive alkanes.
Catalytic Hydrogenation and Hydration
- Hydrogenating alkenes over a nickel catalyst produces alkanes,
- (used in margarine manufacturing)
- while reacting alkenes with steam at 300 °C over a phosphoric acid catalyst yields industrial alcohols.
Homologous series
- A family of compounds with the same functional group and general formula.
Substitution reaction
- A reaction in which one atom or group is replaced by another.
Ultraviolet radiation
- Radiation that provides the activation energy for alkane–halogen reactions.
Functional group
- The atom or group of atoms that determines the reactions of a series.
Boiling
- Liquid to gas throughout the liquid at the boiling point.
Combustion
- Burning in oxygen
Carboxylic acid
- A compound containing –COOH with a name ending in -oic acid.
Carbon dioxide
- A gas that turns limewater cloudy.
Fossil fuel
- A fuel formed from the remains of living things, such as coal, natural gas and petroleum.
Displayed formula
- A formula showing all atoms and all bonds.
Section 10 of 11Key terms
| Term | Meaning |
|---|---|
| Alkene | an unsaturated hydrocarbon with a C=C double bond. |
| Cracking | thermal decomposition of long-chain alkanes into shorter alkanes, alkenes and hydrogen. |
| Addition reaction | a reaction in which atoms add across a C=C bond to give one product. |
| Unsaturated | containing at least one carbon–carbon double bond. |
| Bromine water | an orange solution that is decolourised by alkenes. |
| Catalyst | a substance that speeds up a reaction and is not used up. |
| Hydrogenation | the addition of hydrogen to C=C using a nickel catalyst. |
| Hydration | the addition of steam to C=C to make an alcohol. |
Section 11 of 11Exam technique
1
Saying alkenes are saturated — they contain a C=C double bond and are unsaturated
2
Saying bromine water turns clear with alkenes — it turns colourless
3
Saying cracking makes only alkanes or is breaking up crude oil — it is thermal decomposition of large alkanes into smaller alkanes, alkenes and hydrogen
4
Saying an addition reaction forms two products or confusing it with substitution — addition forms only one product
5
Saying hydration happens at room temperature — it needs steam, a phosphoric acid catalyst, high temperature and pressure
Marked practiceQuizzes for this lesson
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