Alcohols & Homologous Series Explained: Complete O Level & IGCSE Chemistry Guide
Master alcohols homologous series, molecular formulas, functional groups, and exam techniques for Cambridge O Level Chemistry 5070 & IGCSE Chemistry 0620 with proven strategies from Cambridge Classroom.
What is a Homologous Series? The Foundation of Organic Chemistry
In organic chemistry, a homologous series is a family of compounds with the same functional group and general formula, where each successive member differs by CH₂. This systematic pattern is crucial for understanding organic compounds, especially for O Level Chemistry 5070 and IGCSE Chemistry 0620 exams.
Key Definition: Homologous Series
A series of organic compounds having the same functional group, similar chemical properties, and a gradual change in physical properties, where each member differs from the next by a CH₂ unit.
Alcohols Homologous Series: Complete Breakdown
Alcohols are a homologous series containing the -OH (hydroxyl) functional group. The general formula for the homologous series of alcohols is CₙH₂ₙ₊₁OH (for saturated alcohols).
General Formula for Homologous Series of Alcohols
The homologous series of alcohols has general formula CₙH₂ₙ₊₁OH, where n is the number of carbon atoms. This is sometimes written as CₙH₂ₙ₊₂O.
Exam-Centered Definition: Alcohols
Alcohols are organic compounds containing one or more hydroxyl (-OH) groups attached to a carbon atom. The simplest alcohol is methanol (CH₃OH), followed by ethanol (C₂H₅OH).
First 4 Alcohols in the Homologous Series
Understanding the first 4 alcohols in the homologous series is essential for O Level Chemistry 5070 and IGCSE Chemistry 0620 exams:
| Name | Molecular Formula | Structural Formula | Common Uses |
|---|---|---|---|
| Methanol | CH₃OH | CH₃-OH | Fuel, solvent, antifreeze |
| Ethanol | C₂H₅OH | CH₃-CH₂-OH | Alcoholic drinks, fuel, solvent |
| Propanol | C₃H₇OH | CH₃-CH₂-CH₂-OH | Solvent, disinfectant |
| Butanol | C₄H₉OH | CH₃-(CH₂)₃-OH | Solvent, plasticizers |
Properties Trend in Alcohols Homologous Series
As you move up the homologous series of alcohols (increasing carbon chain length):
- Boiling point increases due to stronger van der Waals forces
- Viscosity increases - thicker, less runny liquids
- Flammability decreases (harder to ignite)
- Solubility in water decreases as the hydrocarbon chain becomes longer
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Why Alcohols Form a Homologous Series
Alcohols form a homologous series because they:
- Have the same functional group (-OH group)
- Show a gradual change in physical properties
- Have similar chemical properties
- Can be represented by a general formula (CₙH₂ₙ₊₁OH)
- Each successive member differs by CH₂
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FAQs: Alcohols Homologous Series
What is the homologous series of alcohols molecular formula?
The homologous series of alcohols has the general molecular formula CₙH₂ₙ₊₁OH for saturated alcohols, where n represents the number of carbon atoms.
Is ethanol a member of the homologous series of alcohols?
Yes, ethanol (C₂H₅OH) is the second member of the homologous series of alcohols, following methanol. It contains the -OH functional group characteristic of alcohols.
How does viscosity change in the homologous series of alcohols?
Viscosity increases as you go up the homologous series of alcohols. Methanol is the least viscous, while butanol and higher alcohols are more viscous due to stronger intermolecular forces.
What is the first member of the alcohol homologous series?
The first member of the alcohol homologous series is methanol (CH₃OH), which has one carbon atom bonded to the hydroxyl group.
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