AQA · GCSE · Combined Science

    Course version: 8464 · Version from Sept 2018

    Cracking and alkenes: Alkenes are more reactive than alkanes and react with bromine water, which is used as a test for alkenes.

    Alkenes contain a carbon–carbon double bond, which makes them more reactive than alkanes.

    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Alkenes contain a carbon–carbon double bond, which makes them more reactive than alkanes. A simple test uses bromine water, an orange solution. When an alkene is shaken with bromine water, the orange colour disappears and the solution becomes colourless. This decolourisation happens because the alkene reacts with bromine in an addition reaction, breaking the double bond. Alkanes do not react in this way under the same conditions, so the orange colour remains. The test is therefore used to distinguish alkenes from alkanes. You should be able to describe the test, state the colour change from orange to colourless, and explain that it is the double bond that causes the reaction. Remember that the test is qualitative: it shows the presence of an alkene but does not identify which alkene.

    What earns the marks

    • Alkenes are more reactive than alkanes because they contain a carbon–carbon double bond.
    • Bromine water is an orange solution used to test for the presence of an alkene.
    • When an alkene is added to bromine water and shaken, the orange colour is lost and the solution becomes colourless.
    • The reaction is an addition reaction in which bromine adds across the carbon–carbon double bond.
    • Alkanes do not decolourise bromine water under the same conditions, so the orange colour remains.
    • The test is qualitative and shows that an alkene is present, but it does not identify the specific alkene.

    Common mistakes and how to put them right

    • Saying bromine water turns clear rather than colourless.

      Put it right: The correct observation is that the orange colour disappears and the solution becomes colourless.

    • Thinking alkanes also decolourise bromine water.

      Put it right: Alkanes do not react with bromine water in this test, so the orange colour persists.

    • Confusing the test with a test for alkanes.

      Put it right: Bromine water is used to test for alkenes, not alkanes.

    Exam tips

    • State the starting colour and the final colour precisely: orange to colourless.
    • Link the colour change to the reaction of bromine with the carbon–carbon double bond.
    • If asked to compare, say that alkanes do not decolourise bromine water, so the test distinguishes the two families.

    Try the lesson

    The start of the Cracking and alkenes: Alkenes are more reactive than alkanes and react with bromine water, which is used as a test for alkenes. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Bromine water is added to a sample of an unknown hydrocarbon and the orange colour disappears. What does this result show about the hydrocarbon?

    Listen

    Alkenes contain a carbon–carbon double bond, which makes them more reactive than alkanes. A simple test uses bromine water, an orange solution. When an alkene is shaken with bromine water, the orange colour disappears and the solution becomes colourless. This decolourisation happens because the alkene reacts with bromine in an addition reaction, breaking the double bond. Alkanes do not react in this way under the same conditions, so the orange colour remains. The test is therefore used to distinguish alkenes from alkanes. You should be able to describe the test, state the colour change from orange to colourless, and explain that it is the double bond that causes the reaction. Remember that the test is qualitative: it shows the presence of an alkene but does not identify which alkene. A strong answer earns marks for these points. Alkenes are more reactive than alkanes because they contain a carbon–carbon double bond. Bromine water is an orange solution used to test for the presence of an alkene. When an alkene is added to bromine water and shaken, the orange colour is lost and the solution becomes colourless. The reaction is an addition reaction in which bromine adds across the carbon–carbon double bond. A common slip: Saying bromine water turns clear rather than colourless. Instead, the correct observation is that the orange colour disappears and the solution becomes colourless. Exam tip: State the starting colour and the final colour precisely: orange to colourless.

    See it

    A top-down flowchart. The first box says "Alkene or alkane" and an arrow leads to a second box, "Add bromine water (orange) and shake". From this box two arrows branch out. One arrow goes to "Alkene: contains carbon–carbon double bond", which leads to "Bromine adds across the double bond (addition reaction)", which leads to "Orange colour disappears, solution becomes colourless", which leads to "Shows an alkene is present (qualitative)". The other arrow from the second box goes to "Alkane: no reaction under same conditions", which leads to "Orange colour remains", which leads to "Distinguishes alkane from alkene".
    It shows how shaking a substance with orange bromine water distinguishes an alkene, which decolourises it through an addition reaction at the double bond, from an alkane, which leaves the orange colour unchanged.

    Key words

    Say what each word means before you turn the card.

    Bromine water

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    Addition reaction

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    Decolourisation

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    Qualitative test

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    See every definition
    Bromine water
    An orange solution used as a test for the presence of an alkene.
    Addition reaction
    A reaction in which bromine adds across the carbon–carbon double bond, breaking the double bond.
    Decolourisation
    The loss of the orange colour when an alkene is shaken with bromine water, leaving the solution colourless.
    Qualitative test
    A test that shows the presence of an alkene but does not identify which alkene.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. The first box says "Alkene or alkane" and an arrow leads to a second box, "Add bromine water (orange) and shake". From this box two arrows branch out. One arrow goes to "Alkene: contains carbon–carbon double bond", which leads to "Bromine adds across the double bond (addition reaction)", which leads to "Orange colour disappears, solution becomes colourless", which leads to "Shows an alkene is present (qualitative)". The other arrow from the second box goes to "Alkane: no reaction under same conditions", which leads to "Orange colour remains", which leads to "Distinguishes alkane from alkene".

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