Edexcel IGCSE Chemistry · Spec 4.23-4.28
Alkenes & the Bromine Water Test
The alkenes, their reactions, and the bromine water test for unsaturation.
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Alkenes & the Bromine Water Test
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Explained
Alkenes, and the test that tells them from alkanes
Alkenes are hydrocarbons containing a carbon to carbon double bond. Their general formula is C n H 2n, so ethene is C2H4, propene is C3H6 and butene is C4H8.
Compare that with the alkanes, C n H 2n plus 2. For the same number of carbons an alkene holds two fewer hydrogens, because the double bond uses up two bonding positions. The alkene is therefore described as unsaturated, meaning it could hold more hydrogen, while an alkane is saturated and could not.
Why the double bond matters
One of the two bonds in a carbon to carbon double bond is weaker than the other and breaks relatively easily. That single fact makes alkenes far more reactive than alkanes, and it explains every reaction in this topic.
Alkenes undergo addition reactions. The double bond opens and another molecule adds across it, forming a single bond and leaving nothing behind. Hydrogen adds to give an alkane, steam adds to give an alcohol, and bromine adds to give a dibromo compound.
The bromine water test
Add bromine water to the sample and shake. Bromine water is orange.
With an alkene it turns colourless, immediately, because the bromine adds across the double bond. With an alkane it stays orange, because there is no double bond for the bromine to add to.
Two points of wording matter. Say colourless, not clear, because both liquids are clear to look through and only one loses its colour. And say decolourised rather than discoloured.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme sets out both halves of this comparison in a single question, three marks available for each substance.
For ethane it credits that the reaction is a substitution, that the mixture does not decolourise, stays orange, or decolourises very slowly, that ultraviolet radiation or light is needed, and that the products are bromoethane and hydrogen bromide.
For ethene it credits that the reaction is an addition, that the bromine water decolourises immediately or turns colourless, and that the product is dibromoethane.
Notice that alkanes do react with bromine, but only in ultraviolet light and by substitution rather than addition. The test works because of the speed and the conditions, not because one reacts and the other never does.
A separate mark scheme on the same paper series answers a question with the single line alkanes not alkenes, which is a reminder that the two words differ by one letter and are read carefully by the examiner even when they are written carelessly.
Where alkenes come from and what they are for
Alkenes are made by cracking, which breaks long chain alkanes from crude oil into shorter molecules. Cracking always produces at least one alkene, because there are not enough hydrogens to go round once the chain has been split.
Their reactivity is what makes them useful. They polymerise, so ethene becomes poly(ethene) and propene becomes poly(propene). They react with steam to make alcohols, which is how industrial ethanol is produced. And they are the starting point for a large part of the chemical industry, which is why the demand for short chain molecules exceeds what crude oil supplies directly.
Spec 4.23-4.28
What you need to know
- Describe the alkenes and their general formula
- Describe their addition reactions
- Use bromine water to test for a carbon-carbon double bond
Active recall
Quick check
Answer each question before opening the answer.
What is the general formula of the alkenes and what makes them unsaturated?
CnH2n; they contain a carbon–carbon double bond (C=C).
What is the result of the bromine water test with an alkene?
Bromine water is decolourised (orange to colourless); with an alkane it stays orange.
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