Organic chemistry · GCSE Chemistry
Alkenes
GCSE Chemistry revision on alkenes: unsaturated hydrocarbons, CnH2n, the C=C functional group, the bromine-water test, and addition reactions with hydrogen, steam, bromine and other halogens.
Alkenes are unsaturated: they contain C=C. Formula CnH2n. Bromine water orange to colourless is the test. The double bond opens in addition reactions — hydrogen, steam or a halogen add across it.
The important bits
What you need to know
- 1
Alkenes are a homologous series of unsaturated hydrocarbons with the general formula CnH2n. Unsaturated means they contain a carbon–carbon double bond, C=C, which is the functional group.
- 2
The first members are ethene C₂H₄ and propene C₃H₆. There is no stable “methene”: a double bond needs two carbons. Displayed formulae must show the C=C clearly.
- 3
Alkenes are more reactive than alkanes because the double bond can open. That is why they undergo addition reactions and why they are useful feedstocks for polymers.
- 4
Bromine water is orange-brown. With an alkene it turns colourless because Br₂ adds across C=C to make a colourless dibromoalkane. Alkanes do not decolourise bromine water in the same conditions.
- 5
Hydrogenation: alkene + H₂ → alkane, using a nickel catalyst. Ethene + hydrogen → ethane. This is an addition reaction and is used to harden vegetable oils in food chemistry contexts.
- 6
Hydration: alkene + steam → alcohol, with a phosphoric acid catalyst at high temperature and pressure. Ethene + H₂O → ethanol. This is the industrial route to ethanol from crude-oil ethene.
- 7
Halogens add across C=C: ethene + Br₂ → 1,2-dibromoethane. The same pattern works for chlorine and iodine. The product is saturated; the double bond has gone.
- 8
Addition polymerisation starts from alkenes: the C=C in each monomer opens and monomers join into a long chain. That is the next topic, but the reason it works is this double bond.
Quotations worth analysing
Short evidence. Real method.
“Alkenes are unsaturated hydrocarbons with the general formula CnH2n and contain a C=C double bond.”
Unsaturated, CnH2n and C=C are the three phrases that score. Do not call the double bond “two extra hydrogens” — it means two fewer hydrogens than the alkane.
“Bromine water turns from orange to colourless in the presence of an alkene.”
Name the colour change in both directions. “It goes clear” is weaker than “colourless”. Alkanes leave the bromine water orange.
“C₂H₄ + H₂O → C₂H₅OH (steam, phosphoric acid catalyst)”
This is addition of water across C=C, not fermentation. Conditions matter: steam, high temperature and pressure, acid catalyst.
Go deeper
The bromine-water test is an addition reaction you can see
Shake a few drops of bromine water with the unknown. If C=C is present, Br–Br adds across the double bond and the orange colour disappears because Br₂ has been used up. The organic product is colourless 1,2-dibromoethane from ethene, or the matching dibromo compound from a larger alkene. Alkanes have no C=C, so they do not add bromine under these conditions and the mixture stays orange. UV light can slowly substitute alkanes, which is not the GCSE test — keep the test as a quick colour change for unsaturation. Write “orange to colourless”, not “clear”, and say that an addition reaction has happened. That sentence is worth two marks on many papers.
Go deeper
Addition means two reactants, one product
The double bond opens, two extra atoms or groups attach to those two carbons, and nothing else is eliminated. Hydrogen gives an alkane. Steam gives an alcohol. A halogen gives a dihaloalkane. Contrast that with combustion, which is not an addition, and with cracking, which breaks a large molecule into smaller ones. Displayed formulae make the story visible: draw C=C, then draw the same two carbons with a single bond and the new atoms on each side. Students lose marks by leaving the double bond in the product. If the product still has C=C, you have not finished the addition. Nickel for hydrogen, phosphoric acid for steam: name the catalyst when the question asks for conditions.
Go deeper
Ethene is the industrial hinge between oil and other organics
Cracking of long alkanes produces ethene. From ethene you can make ethanol by hydration, poly(ethene) by addition polymerisation, and a range of other chemicals by further addition. That is why a mark scheme that looks like “organic” is often also a crude-oil story. Fermentation also makes ethanol, but from sugar, with yeast, at about 30–40 °C, in anaerobic conditions — different feedstock, different conditions, lower concentration. If a question asks how ethanol is made from crude oil, they want steam plus ethene, not yeast. If they ask how you tell ethene from ethane, they want bromine water, not a boiling-point table, unless they give you data.
See the idea in action
A colourless gas decolourises bromine water. Propose a structure and write the addition with hydrogen. The test shows C=C, so the gas can be ethene, C₂H₄. Displayed: two carbons joined by a double bond, two hydrogens on each carbon. Hydrogenation: C₂H₄ + H₂ → C₂H₆, nickel catalyst. The product is ethane, which would not decolourise bromine water. Hydration of the same ethene: C₂H₄ + H₂O → C₂H₅OH, steam and phosphoric acid.
Exam technique
Turn knowledge into marks
Define alkenes with unsaturated, CnH2n and C=C. For tests, write orange to colourless and name addition. In displayed products, the double bond must have become a single bond with extra atoms attached. Name nickel or phosphoric acid when conditions are asked.
Common mistakes
Do not give these marks away
- 01
Using CnH2n+2 for alkenes, or drawing a double bond still in the addition product.
- 02
Writing that bromine water “goes clear” without orange to colourless, or saying alkanes decolourise it.
- 03
Confusing hydration of ethene (steam, acid catalyst) with fermentation of sugar by yeast.
What is observed when bromine water is shaken with ethene?
ANo change; the mixture stays orange
BThe bromine water turns from orange to colourless
CA white precipitate forms
DThe mixture turns limewater cloudy
Show the answer
The bromine water turns from orange to colourless. Ethene is unsaturated. Br₂ adds across the C=C double bond, so the orange bromine is used up and the solution becomes colourless.
Quick questions
If this is the bit you searched
What is the general formula of alkenes GCSE?
CnH2n. They are unsaturated hydrocarbons containing a carbon–carbon double bond. Ethene is C₂H₄ and propene is C₃H₆.
How do you test for an alkene?
Add bromine water. It turns from orange to colourless if a C=C double bond is present. Alkanes do not give that colour change under the same conditions.
What is an addition reaction of an alkene?
A reaction in which atoms add across the C=C double bond and a single product forms, for example hydrogen making an alkane, steam making an alcohol, or bromine making a dibromoalkane.
How is ethanol made from ethene?
Hydration: ethene reacts with steam to form ethanol, using a phosphoric acid catalyst at high temperature and pressure. C₂H₄ + H₂O → C₂H₅OH.