Alkenes and alkanes are both hydrocarbons produced during cracking. Describe how bromine water can be used to distinguish between an alkene and an alkane, and explain why the two types of hydrocarbon give different results.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Add a few drops of bromine water to a sample of each hydrocarbon.
1. With the alkene the orange colour of the bromine water disappears, giving a colourless solution.
2. With the alkane the orange colour remains unchanged.
The alkene contains a carbon–carbon double bond, making it unsaturated and highly reactive. The double bond undergoes electrophilic addition with Br₂, forming a dibromide and decolourising the solution. Alkanes lack a double bond, are saturated and do not react with bromine water, so the orange colour persists.
1. With the alkene the orange colour of the bromine water disappears, giving a colourless solution.
2. With the alkane the orange colour remains unchanged.
The alkene contains a carbon–carbon double bond, making it unsaturated and highly reactive. The double bond undergoes electrophilic addition with Br₂, forming a dibromide and decolourising the solution. Alkanes lack a double bond, are saturated and do not react with bromine water, so the orange colour persists.
Examiner tips
- Show the two separate tests and the colour change for each hydrocarbon.
- Explain the role of the C=C double bond in the reaction.
- Use the terms ‘unsaturated’ and ‘electrophilic addition’ as they are rewarded.
- Mention that the alkane remains orange because it is saturated and unreactive.
Common mistakes
- Failing to describe the colour change for both hydrocarbons.
- Confusing the reaction mechanism or omitting the double bond as the reason for reactivity.
- Using vague terms like ‘reactive’ without linking to the double bond.
Mark scheme (5 marks)
- Add bromine water to each hydrocarbon (sample)
- The alkene decolourises / turns the bromine water from orange to colourless
- The alkane does not decolourise the bromine water / bromine water stays orange with the alkane
- Alkenes have a carbon-carbon double bond, making them unsaturated
- The double bond makes alkenes more reactive than alkanes (so they react with bromine water whereas alkanes do not)
Key terms in this question
alkene · alkane · bromine water
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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