Pearson Edexcel IAL Chemistry 5.5 Test for C=C double bonds

Practise using bromine water and acidified manganate(VII) observations to decide whether molecules contain a C=C double bond.

Exam points

state bromine water changing orange or yellow-brown to colourless for a C=C bond
interpret no decolourisation as evidence that a product is saturated or has no C=C bond
compare alkane and alkene observations with acidified potassium manganate(VII)

[Maximum number: 2]

This question is about fuels and polymers. Used coffee grounds have been suggested as a carbon-neutral fuel to replace some fossil fuels.

Long chain alkanes are converted into smaller, more useful molecules including alkenes.

Give a test for alkenes, including the positive result.

bromine water / bromine solution / $\mathrm{Br}_{2}(\mathrm{aq})$ (1)

- yellow / orange / (red-)brown to colourless

(1)

MP2 is dependent on a correct reagent for MP1

Allow bromine / liquid bromine Ignore heat

Do not award "in UV light"

Do not award iodine

Allow decolourises or "turns colourless"

Accept:

$\mathrm{KMnO}_{4}$ with acid / $\mathbf{H}^{+}$ scores 1 mark purple to colourless scores 1 mark (allow decolourises)

Allow

$\mathrm{KMnO}_{4}$ with alkali / $\mathbf{O H}^{-}$ scores 1 mark purple → green scores 1 mark

(2)

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5.5—The qualitative test for a C=C double bond using bromine or bromine water question 1