3.2.3Additional lessons
Homologous Series — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 45 of 98
Part 1 of 6What Is a Homologous Series
A homologous series is a family of compounds with a regular pattern in structure and properties.
Same functional group
- All members share the same functional group, so they react in a similar way.
General formula
- Members fit one general formula, for example CₙH₂ₙ₊₂ for alkanes.
CH₂ difference
- Consecutive members differ by a CH₂ unit, which changes the relative molecular mass by 14.
Gradual trends
- Physical properties such as boiling point change gradually as the chain length increases.
Part 2 of 6Common Series
Common Series
Alkanes
- Saturated hydrocarbons with general formula CₙH₂ₙ₊₂, such as methane, ethane and propane.
Alkenes
- Hydrocarbons with a C=C double bond and general formula CₙH₂ₙ, such as ethene.
Alcohols
- Compounds with an –OH group and general formula CₙH₂ₙ₊₁OH, such as ethanol.
Carboxylic acids
- Compounds with a –COOH group and general formula CₙH₂ₙ₊₁COOH, such as ethanoic acid.
| Series | Functional group | General formula | Example |
|---|---|---|---|
| Alkane | C–C | CₙH₂ₙ₊₂ | ethane |
| Alkene | C=C | CₙH₂ₙ | propene |
| Alcohol | –OH | CₙH₂ₙ₊₁OH | propan-1-ol |
| Carboxylic acid | –COOH | CₙH₂ₙ₊₁COOH | ethanoic acid |
Part 3 of 6Trends in Boiling Point
1
Moving along a series each extra CH₂ increases the number of electrons in the molecule.
2
More electrons make stronger London dispersion forces between molecules.
3
Stronger intermolecular forces need more energy to overcome, so boiling points increase with chain length.
4
For the alkanes the boiling points are methane −162 °C, ethane −89 °C, propane −42 °C, butane −1 °C and pentane 36 °C.
5
Early members are therefore gases, and members become liquids and then solids as the chain lengthens.
Part 4 of 6Chemical Similarity
Same reactions
- All alkenes undergo addition across the double bond, and all alcohols burn in excess oxygen to give carbon dioxide and water.
Different series differ
- Members of different series have very different chemistry because their functional groups differ.
Prediction
- Knowing one member allows prediction of the reactions of others in the series.
Solubility
- Smaller alcohols and acids dissolve in water through hydrogen bonding, but solubility falls as the carbon chain grows.
Part 5 of 6Worked Example
Question Deduce the molecular formula of the alkane with 8 carbon atoms and the alcohol with 3 carbon atoms.
- Alkanes follow CₙH₂ₙ₊₂, so for n = 8 the formula is C₈H₁₈.
- Alcohols follow CₙH₂ₙ₊₁OH, so for n = 3 the formula is C₃H₇OH, written C₃H₈O.
- Check hydrogen counts2(8) + 2 = 18, and 2(3) + 1 + 1 = 8.
Answer Octane is C₈H₁₈ and propanol is C₃H₈O (C₃H₇OH).
Part 6 of 6Exam Checklist & Tips
3.2.3 – Key Points for the Exam
- Homologous series A family of compounds with the same functional group and general formula, differing by CH₂.
- General formula An algebraic formula that fits all members of a series.
- Functional group The atom or group that determines the characteristic reactions of a compound.
- Saturated Containing only single bonds between carbon atoms.
Common Mistakes
- Saying that members differ by CH₃ rather than CH₂.
- Stating that boiling points decrease along the series.
- Forgetting that different series have different functional groups and reactions.
- Using the alkane general formula for alkenes.
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