The test for unsaturated compounds
4 marks · NormalA student investigates the reaction between different alkenes and bromine water. The table shows the results.
| Compound | Observation after adding bromine water |
|---|---|
| Ethene | Colourless |
| Propene | Colourless |
| Butane | Orange |
| Cyclohexene | Colourless |
Which of these compounds are alkenes? Give a reason for your choice.
Write a balanced equation for the reaction of ethene with bromine water.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Ethene, propene, and cyclohexene are alkenes because they decolourise bromine water; their mixture becomes colourless.
(b) \ce{CH2=CH2(g) + Br2(aq) -> CH2BrCH2Br(l)}
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Identifies correct set of alkenes: ethene, propene, cyclohexene (all that go colourless).
B2 (a) · 1 mark · States that these compounds decolourise bromine water because of the C=C (double bond).
B3 (b) · 1 mark · Correctly writes reactants and product in a balanced equation (accept state symbols).
B4 (b) · 1 mark · Correct formula for 1,2-dibromoethane as product.
Worked solution
(a) Ethene, propene and cyclohexene are alkenes because when bromine water is added, their solutions become colourless — showing the presence of a double bond.