AQA · A-Level · Chemistry

    Course version: 7405 · Version from Sept 2016

    Structure, bonding and reactivity: Alkenes are unsaturated hydrocarbons.

    Alkenes contain only carbon and hydrogen, so they are hydrocarbons, and they contain at least one carbon–carbon double bond, so they are unsaturated.

    Checked against the AQA 7405 specification (opens in a new tab) on .

    • Point 1 of 2
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Alkenes contain only carbon and hydrogen, so they are hydrocarbons, and they contain at least one carbon–carbon double bond, so they are unsaturated. Unsaturated has a precise meaning: the molecule can add extra atoms across that double bond without losing anything, unlike a saturated alkane, which can only substitute. A single double bond gives the general formula CₙH₂ₙ, two hydrogens fewer than the alkane with the same number of carbons. The practical consequence is the test for unsaturation: shaking with bromine water at room temperature turns it from orange to colourless as bromine adds across the double bond, while an alkane leaves it orange. Say colourless, not clear, because the alkane mixture is also clear.

    What earns the marks

    • Defining unsaturated as containing a carbon–carbon double bond.
    • Saying an unsaturated molecule can undergo addition reactions.
    • The general formula CₙH₂ₙ for an alkene with one double bond.
    • The bromine water observation, orange to colourless, at room temperature.

    Common mistakes and how to put them right

    • Writing that bromine water "goes clear", when the required word is colourless.

    • Defining unsaturated as containing any double bond, when it must be carbon to carbon.

    • Giving the general formula as CₙH₂ₙ₊₂, which is the alkane formula.

    • Saying "decolourises bromine water" without stating the starting colour.

    Exam tips

    • Give both colours in any observation answer, before and after.
    • Count the hydrogens against CₙH₂ₙ to check a structure you have drawn is really an alkene.
    • Remember hydrocarbon means carbon and hydrogen only, so a molecule containing oxygen is not an alkene.

    Try the lesson

    The start of the Structure, bonding and reactivity: Alkenes are unsaturated hydrocarbons. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which statement best describes why alkenes are classified as unsaturated hydrocarbons?

    Listen

    Alkenes contain only carbon and hydrogen, so they are hydrocarbons, and they contain at least one carbon–carbon double bond, so they are unsaturated. Unsaturated has a precise meaning: the molecule can add extra atoms across that double bond without losing anything, unlike a saturated alkane, which can only substitute. A single double bond gives the general formula CₙH₂ₙ, two hydrogens fewer than the alkane with the same number of carbons. The practical consequence is the test for unsaturation: shaking with bromine water at room temperature turns it from orange to colourless as bromine adds across the double bond, while an alkane leaves it orange. Say colourless, not clear, because the alkane mixture is also clear. A strong answer earns marks for these points. defining unsaturated as containing a carbon–carbon double bond. saying an unsaturated molecule can undergo addition reactions. the general formula CₙH₂ₙ for an alkene with one double bond. the bromine water observation, orange to colourless, at room temperature. Exam tip: Give both colours in any observation answer, before and after.

    See it

    A top-down flowchart. The first box reads 'Alkene: hydrocarbon of carbon and hydrogen only' and points down to a second box, 'Contains at least one carbon–carbon double bond'. From this second box two arrows lead down. One goes to 'Unsaturated: can add extra atoms across the double bond', the other to 'General formula CnH2n with one double bond'. From the 'Unsaturated' box an arrow leads down to 'Test: shake with bromine water at room temperature'. From the test box two arrows lead down: one to 'Alkene: orange to colourless' and one to 'Alkane: stays orange'.
    It shows how the carbon–carbon double bond makes an alkene unsaturated, gives its general formula, and leads to the bromine water test result compared with an alkane.

    Key words

    Say what each word means before you turn the card.

    unsaturation

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    carbon–carbon double bond

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    general formula

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    test with bromine water

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    See every definition
    unsaturation
    Containing a carbon–carbon double bond, so the molecule can add extra atoms across that double bond without losing anything.
    carbon–carbon double bond
    The feature that makes a hydrocarbon unsaturated, allowing it to add extra atoms without losing anything.
    general formula
    For an alkene with one double bond, CₙH₂ₙ, which is two hydrogens fewer than the alkane with the same number of carbons.
    test with bromine water
    Shaking with bromine water at room temperature turns it from orange to colourless as bromine adds across the double bond, while an alkane leaves it orange.

    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 reads 'Alkene: hydrocarbon of carbon and hydrogen only' and points down to a second box, 'Contains at least one carbon–carbon double bond'. From this second box two arrows lead down. One goes to 'Unsaturated: can add extra atoms across the double bond', the other to 'General formula CnH2n with one double bond'. From the 'Unsaturated' box an arrow leads down to 'Test: shake with bromine water at room temperature'. From the test box two arrows lead down: one to 'Alkene: orange to colourless' and one to 'Alkane: stays orange'.

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