AQA · A-Level · Chemistry

    Course version: 7405 · Version from Sept 2016

    Fractional distillation of crude oil: Alkanes are saturated hydrocarbons.

    Two words, each doing work.

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

    • Point 1 of 2
    • 2 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Two words, each doing work. Hydrocarbon means the molecule contains only carbon and hydrogen. Saturated means every carbon to carbon bond is a single bond, so the molecule holds as many hydrogen atoms as it possibly can. The general formula of an open-chain alkane is CₙH₂ₙ₊₂, while cycloalkanes, having closed the ring, are CₙH₂ₙ. Every carbon has four σ bonds arranged tetrahedrally at 109.5°. Because carbon and hydrogen have almost the same electronegativity the bonds are essentially non-polar, and the only forces between molecules are London forces. That explains why alkanes are so unreactive, since there is no δ⁺ carbon for a nucleophile to attack, and why boiling point rises steadily with chain length.

    What earns the marks

    • Hydrocarbon meaning a compound of carbon and hydrogen only.
    • Saturated meaning that all the carbon to carbon bonds are single bonds.
    • The general formula CₙH₂ₙ₊₂ for an open-chain alkane.
    • The tetrahedral shape at each carbon with a bond angle of 109.5°.

    Common mistakes and how to put them right

    • Defining saturated as containing no double bonds without saying between carbon atoms.

    • Applying CₙH₂ₙ₊₂ to a cycloalkane, which is CₙH₂ₙ.

    • Giving the bond angle in an alkane as 120° or as 90°.

    • Describing alkanes as polar because C–H bonds have slight polarity, when the molecules are effectively non-polar.

    Exam tips

    • Define both words separately if the question offers two marks for the term.
    • Check whether the alkane is a ring before applying a general formula.
    • Link low reactivity to non-polar bonds and to strong C–C and C–H bonds, not just to being stable.

    Try the lesson

    The start of the Fractional distillation of crude oil: Alkanes are saturated hydrocarbons. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which statement describes an alkane?

    Listen

    Two words, each doing work. Hydrocarbon means the molecule contains only carbon and hydrogen. Saturated means every carbon to carbon bond is a single bond, so the molecule holds as many hydrogen atoms as it possibly can. The general formula of an open-chain alkane is CₙH₂ₙ₊₂, while cycloalkanes, having closed the ring, are CₙH₂ₙ. Every carbon has four σ bonds arranged tetrahedrally at 109.5°. Because carbon and hydrogen have almost the same electronegativity the bonds are essentially non-polar, and the only forces between molecules are London forces. That explains why alkanes are so unreactive, since there is no δ⁺ carbon for a nucleophile to attack, and why boiling point rises steadily with chain length. A strong answer earns marks for these points. hydrocarbon meaning a compound of carbon and hydrogen only. saturated meaning that all the carbon to carbon bonds are single bonds. the general formula CₙH₂ₙ₊₂ for an open-chain alkane. the tetrahedral shape at each carbon with a bond angle of 109.5°. Exam tip: Define both words separately if the question offers two marks for the term.

    See it

    A top-down flowchart starting from a single node labelled Alkane, which branches into six nodes: Hydrocarbon: carbon and hydrogen only; Saturated: all C–C bonds single; Open-chain: CₙH₂ₙ₊₂; Cycloalkane: CₙH₂ₙ; Tetrahedral at each carbon, 109.5°; and Essentially non-polar bonds. The non-polar bonds node then leads to two nodes: Only London forces between molecules, and Low reactivity: no δ⁺ carbon for a nucleophile. The London forces node leads to a final node: Boiling point rises with chain length.
    It shows how the definition of an alkane branches into its key features and how non-polar bonds lead to London forces, low reactivity and rising boiling point.

    Key words

    Say what each word means before you turn the card.

    hydrocarbon

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

    saturated

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

    See every definition
    hydrocarbon
    A molecule that contains only carbon and hydrogen.
    saturated
    Every carbon to carbon bond is a single bond, so the molecule holds as many hydrogen atoms as it possibly can.

    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 starting from a single node labelled Alkane, which branches into six nodes: Hydrocarbon: carbon and hydrogen only; Saturated: all C–C bonds single; Open-chain: CₙH₂ₙ₊₂; Cycloalkane: CₙH₂ₙ; Tetrahedral at each carbon, 109.5°; and Essentially non-polar bonds. The non-polar bonds node then leads to two nodes: Only London forces between molecules, and Low reactivity: no δ⁺ carbon for a nucleophile. The London forces node leads to a final node: Boiling point rises with chain length.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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