Carbon Chains
Alkenes and their double bond: why the carbon-carbon double bond makes alkenes unsaturated and reactive, their addition reactions with hydrogen, water and halogens, and the bromine water test that turns from orange to colourless.
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The double bond that reacts
Alkenes are hydrocarbons with a special feature: a carbon-carbon double bond. That double bond is a spare pair of bonds waiting to open up, which makes alkenes far more reactive than compounds with only single bonds. This module works through the structure of alkenes, their addition reactions, and the bromine water test that spots a double bond in seconds.
Alkene words to know
Learn these before you open a double bond.
Single bonds against a double bond
The double bond is what makes an alkene stand out.
Spot the bond, prove it with bromine
In the exam, spot an alkene by its double bond and prove it with bromine water. An alkene is unsaturated: its carbon-carbon double bond can open so atoms add across it in an addition reaction with hydrogen, water or a halogen. The sure test is bromine water: an alkene wipes the orange colour out of it and leaves it clear, while a saturated compound leaves the orange alone. Name the double bond, name the test, and give the colour change exactly.
Match the term
- alkene
- unsaturated
- addition reaction
- bromine water
- a hydrocarbon with a double bond
- having a double bond that can open
- atoms adding across the double bond
- the test for a double bond
Match the reaction
- an alkene plus hydrogen
- an alkene plus steam
- an alkene plus bromine water
- the general formula of an alkene
- makes a saturated compound
- makes an alcohol
- takes the orange colour away
- has twice as many hydrogens as carbons
What does bromine water show?
What colour change shows an alkene when you add bromine water to it?
- Orange to colourless
- Colourless to orange
- Blue to red
- No change happens
True about alkenes
Select the TWO true statements about alkenes.
- An alkene has a carbon-carbon double bond
- Alkenes decolourise bromine water
- An alkene is saturated with only single bonds
- The double bond makes alkenes less reactive
Count the hydrogens
Ethene has 2 carbon atoms. In an alkene the number of hydrogen atoms is twice the number of carbon atoms. Multiply 2 by 2 to find the number of hydrogen atoms in ethene. What is the answer?
Order the bromine test
Put the steps of testing for an alkene with bromine water in order, earliest first.
- Add bromine water to the sample
- Shake the tube to mix them
- Watch the colour of the bromine water
- If it goes colourless, a double bond is present
Complete the rules
A hydrocarbon with a carbon-carbon double bond is an _____. Because it has a double bond it is _____. It reacts by atoms adding across the bond in an _____ reaction. It turns bromine water from orange to _____.
Spot the alkene facts
Tap the TWO true statements about alkenes.
- they have a carbon-carbon double bond
- they decolourise bromine water
- they are saturated hydrocarbons
- they cannot take part in addition reactions
Test the sample
Read each case and choose the best answer.
- You add bromine water to a sample and it turns from orange to colourless. What does the sample contain?
- An alkene reacts with hydrogen so that atoms add across its double bond. What kind of reaction is this?
- Why is an alkene more reactive than a saturated compound?
Explain alkenes
Explain what an alkene is and how you test for one.
- Explain the alkene double bond and what unsaturated means
- Explain addition reactions with hydrogen, water or a halogen
- Explain the bromine water test and the exact colour change