AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Crude oil, hydrocarbons and alkanes: Alkane molecules can be represented in the following forms:
Alkanes are saturated hydrocarbons whose molecules can be shown in several complementary ways.
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Alkanes are saturated hydrocarbons whose molecules can be shown in several complementary ways. A molecular formula such as C₂H₆ states only the number of each atom. A displayed formula draws every atom and every covalent bond, so it reveals the single C–C and C–H bonds that make the molecule saturated. A structural formula such as CH₃CH₃ condenses the same information onto one line, grouping atoms bonded to each carbon. A skeletal formula omits C and H labels, showing a zig-zag carbon chain. Whichever representation is used, the molecule is unchanged: the same atoms are joined in the same order. Choosing a form depends on the purpose, for example counting atoms, checking bonding or naming the alkane.
What earns the marks
- Recognise that a molecular formula gives the number of each type of atom but not the arrangement or bonding.
- Interpret a displayed formula by counting each drawn line as one covalent bond and each labelled atom as one atom.
- Convert between a displayed formula and a condensed structural formula such as CH₃CH₃ for ethane.
- Explain that all representations of a given alkane describe the same molecule with the same molecular formula.
- Use the general formula CₙH₂ₙ₊₂ to check that a proposed representation is a valid alkane.
- Identify that alkanes are saturated because every carbon is joined by single bonds only.
Common mistakes and how to put them right
Treating a molecular formula as if it showed how atoms are joined.
Put it right: It shows only the numbers of atoms, so a displayed or structural formula is needed to show bonding.
Counting the C–C bond twice when working out the number of bonds in a displayed formula.
Put it right: Count each drawn line once, as one shared pair of electrons.
Writing a condensed formula with the wrong number of hydrogen atoms, for example CH₂CH₃ for ethane.
Put it right: Check against CₙH₂ₙ₊₂, giving CH₃CH₃ for C₂H₆.
Exam tips
- When asked to draw a displayed formula, show every atom and every bond, and check each carbon has four bonds.
- If a question gives one representation and asks for another, first write the molecular formula to fix the numbers of atoms.
- Use the general formula CₙH₂ₙ₊₂ to test whether a suggested formula is an alkane before commenting on its structure.
Try the lesson
The start of the Crude oil, hydrocarbons and alkanes: Alkane molecules can be represented in the following forms: lesson, made from checked teaching notes for this specification point.
Have a go first
Which of the following is the correct molecular formula for methane?
Listen
Alkanes are saturated hydrocarbons whose molecules can be shown in several complementary ways. A molecular formula such as C₂H₆ states only the number of each atom. A displayed formula draws every atom and every covalent bond, so it reveals the single C–C and C–H bonds that make the molecule saturated. A structural formula such as CH₃CH₃ condenses the same information onto one line, grouping atoms bonded to each carbon. A skeletal formula omits C and H labels, showing a zig-zag carbon chain. Whichever representation is used, the molecule is unchanged: the same atoms are joined in the same order. Choosing a form depends on the purpose, for example counting atoms, checking bonding or naming the alkane. A strong answer earns marks for these points. Recognise that a molecular formula gives the number of each type of atom but not the arrangement or bonding. Interpret a displayed formula by counting each drawn line as one covalent bond and each labelled atom as one atom. Convert between a displayed formula and a condensed structural formula such as CH₃CH₃ for ethane. Explain that all representations of a given alkane describe the same molecule with the same molecular formula. A common slip: Treating a molecular formula as if it showed how atoms are joined. Instead, it shows only the numbers of atoms, so a displayed or structural formula is needed to show bonding. Exam tip: When asked to draw a displayed formula, show every atom and every bond, and check each carbon has four bonds.
See it
Key words
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Saturated hydrocarbons
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Structure and formula relationships
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See every definition
- Saturated hydrocarbons
- Molecules in which every carbon is joined by single bonds only, so the molecule is saturated.
- Structure and formula relationships
- Whichever representation is used, the molecule is unchanged, with the same atoms joined in the same order.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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