Families of organic compounds Cambridge IGCSE Combined Science revision
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In plain words
Carbon atoms can link into chains, and that makes millions of compounds possible. To keep track, chemists sort them into families called homologous series. Members of a family share a feature, the functional group, so they react alike, and each member has one more CH₂ than the one before.
Three things to know
- A homologous series has the same general formula and functional group, similar chemical properties, and a steady trend in physical properties. Each member differs from the next by CH₂.
- Alkanes CₙH₂ₙ₊₂ (no functional group). Alkenes CₙH₂ₙ (C=C). Alcohols CₙH₂ₙ₊₁OH (–OH). Carboxylic acids (–COOH).
- Saturated means only single bonds between the carbon atoms. Unsaturated means there is a C=C double bond. Isomers have the same molecular formula but different structures.
Worked example
What is the molecular formula of the alkane with 6 carbon atoms?
- Alkanes have the general formula CₙH₂ₙ₊₂.
- With n = 6, the number of hydrogens is (2 × 6) + 2 = 14.
- C₆H₁₄.
Tips and tricks
- To use a general formula, put the number of carbons in for n and work out the hydrogens.
- A displayed formula shows every atom and every bond. Check each carbon has exactly four bonds.
It lands in your notebook with its questions as flashcards.
Families of organic compounds: 6 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What is the general formula of the alkenes?
Alkenes have two fewer hydrogens than the matching alkane.
What is the functional group of the carboxylic acids?
It is what makes them acids.
By what do neighbouring members of a homologous series differ?
Each has one more carbon and two more hydrogens.
What does unsaturated mean?
Alkenes are unsaturated.
What is the formula of the alkane with three carbon atoms?
(2 × 3) + 2 = 8 hydrogens.
What do isomers have in common?
The same atoms are joined in a different arrangement.
Quiz
6 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
4 questions, 7 marks. Write your answers on paper, then check them.
Families of organic compounds
Cambridge IGCSE Combined Science 0653 · 7 marks · papermunch.org
Name ______________________________ Date ______________
Give two features of a homologous series.[2]
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Any two: same general formula; same functional group; similar chemical properties; a trend in physical properties; each member differs from the next by CH₂.
State the general formula of the alkanes, and the formula of the alkane with five carbon atoms.[2]
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CₙH₂ₙ₊₂. C₅H₁₂.
State what is meant by structural isomers.[2]
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Compounds with the same molecular formula but different structural formulae.
Name the functional group in alcohols.[1]
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–OH (the hydroxyl group).
Answers: Families of organic compounds
- 1. Any two: same general formula; same functional group; similar chemical properties; a trend in physical properties; each member differs from the next by CH₂.
- 2. CₙH₂ₙ₊₂. C₅H₁₂.
- 3. Compounds with the same molecular formula but different structural formulae.
- 4. –OH (the hydroxyl group).