AQA · GCSE · Chemistry

    Course version: 8462 · Version from Sept 2018

    The products of cracking include alkanes and another type of hydrocarbon…

    Full specification wording

    Cracking and alkenes: The products of cracking include alkanes and another type of hydrocarbon called alkenes.

    Cracking does not give a single product; it gives a mixture of smaller hydrocarbons.

    Checked against the AQA 8462 specification (opens in a new tab) on .

    • Point 4 of 11
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Cracking does not give a single product; it gives a mixture of smaller hydrocarbons. These products belong to two homologous series: alkanes and alkenes. Alkanes have only single carbon–carbon bonds and are saturated; alkenes contain at least one carbon–carbon double bond and are unsaturated. For example, cracking decane can produce octane, an alkane, and ethene, an alkene. The alkane products are useful as fuels, while the alkene products are reactive starting materials for making polymers such as poly(ethene). Because a mixture forms, the products must be separated, often by fractional distillation. Recognising the two product families helps you predict formulae and explain why cracking is valuable to industry.

    What earns the marks

    • Cracking produces smaller hydrocarbon molecules, not a single pure substance.
    • The products include alkanes, which are saturated hydrocarbons with only single carbon–carbon bonds.
    • The products also include alkenes, which are unsaturated hydrocarbons containing a carbon–carbon double bond.
    • Alkane products are useful as fuels, while alkene products are used to make polymers and other chemicals.
    • The mixture of products is separated, for example by fractional distillation.

    Common mistakes and how to put them right

    • Saying cracking produces only alkenes.

      Put it right by stating that alkanes are also produced.

    • Confusing the terms saturated and unsaturated.

      Put it right by linking saturated to single bonds in alkanes and unsaturated to a double bond in alkenes.

    • Writing alkane and alkene formulae incorrectly.

      Put it right by using displayed or structural formulae to show the double bond in alkenes.

    Exam tips

    • When naming products, state clearly which is the alkane and which is the alkene.
    • Use the general formulae CₙH₂ₙ₊₂ for alkanes and CₙH₂ₙ for alkenes to check a suggested product.
    • Link each product family to a use: alkanes as fuels, alkenes as polymer feedstocks.

    Try the lesson

    The start of the Cracking and alkenes: The products of cracking include alkanes and another type of hydrocarbon called alkenes. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Cracking produces a mixture of hydrocarbons. Which two types of hydrocarbon are always named as the products of cracking?

    Listen

    Cracking does not give a single product; it gives a mixture of smaller hydrocarbons. These products belong to two homologous series: alkanes and alkenes. Alkanes have only single carbon–carbon bonds and are saturated; alkenes contain at least one carbon–carbon double bond and are unsaturated. For example, cracking decane can produce octane, an alkane, and ethene, an alkene. The alkane products are useful as fuels, while the alkene products are reactive starting materials for making polymers such as poly(ethene). Because a mixture forms, the products must be separated, often by fractional distillation. Recognising the two product families helps you predict formulae and explain why cracking is valuable to industry. A strong answer earns marks for these points. Cracking produces smaller hydrocarbon molecules, not a single pure substance. The products include alkanes, which are saturated hydrocarbons with only single carbon–carbon bonds. The products also include alkenes, which are unsaturated hydrocarbons containing a carbon–carbon double bond. Alkane products are useful as fuels, while alkene products are used to make polymers and other chemicals. A common slip: Saying cracking produces only alkenes. Instead, stating that alkanes are also produced. Exam tip: When naming products, state clearly which is the alkane and which is the alkene.

    See it

    A top-down flowchart. At the top, a node labelled Cracking leads down to a node labelled Mixture of smaller hydrocarbons. From this mixture node, two branches go to Alkanes and Alkenes. The Alkanes branch leads to a node reading Saturated: only single carbon–carbon bonds and to a node reading Useful as fuels. The Alkenes branch leads to a node reading Unsaturated: contain a carbon–carbon double bond and to a node reading Reactive starting materials for making polymers. A further arrow from the mixture node leads to a node reading Products separated, often by fractional distillation.
    It shows that cracking produces a mixture of alkanes and alkenes, how the two families differ in bonding and saturation, their uses, and that the mixture is separated.

    Key words

    Say what each word means before you turn the card.

    Alkanes

    Try answering aloud or in your head before turning the card.

    Alkenes

    Try answering aloud or in your head before turning the card.

    Saturated

    Try answering aloud or in your head before turning the card.

    Unsaturated

    Try answering aloud or in your head before turning the card.

    See every definition
    Alkanes
    Hydrocarbons that have only single carbon–carbon bonds and are saturated, and are useful as fuels.
    Alkenes
    Hydrocarbons that contain at least one carbon–carbon double bond and are unsaturated, and are reactive starting materials for making polymers.
    Saturated
    Describes alkanes, which have only single carbon–carbon bonds.
    Unsaturated
    Describes alkenes, which contain a carbon–carbon double bond.

    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. At the top, a node labelled Cracking leads down to a node labelled Mixture of smaller hydrocarbons. From this mixture node, two branches go to Alkanes and Alkenes. The Alkanes branch leads to a node reading Saturated: only single carbon–carbon bonds and to a node reading Useful as fuels. The Alkenes branch leads to a node reading Unsaturated: contain a carbon–carbon double bond and to a node reading Reactive starting materials for making polymers. A further arrow from the mixture node leads to a node reading Products separated, often by fractional distillation.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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