Homologous Series Explained for IB Chemistry

This is why trends across a series are so regular.

4. Gradual Change in Physical Properties

As the chain length increases:

• Boiling point increases
• Viscosity increases
• Density increases
• Volatility decreases

These trends occur because longer chains have stronger London dispersion forces.

5. Similar Chemical Properties

Chemical reactivity is controlled by the functional group, so all members behave similarly in reactions.

For example, all alcohols can undergo oxidation or form esters.

Why Homologous Series Matter in IB Chemistry

Understanding homologous series helps you:

• Predict physical trends
• Identify functional groups
• Determine general formulas
• Classify organic molecules
• Recognize reaction patterns
• Balance equations more easily
• Understand systematic naming (IUPAC rules)

This comes up in topics such as organic structure, isomerism, and reaction mechanisms.

Examples of Common Homologous Series

Alkanes

• General formula: CₙH₂ₙ₊₂
• Functional group: none
• Properties: non-polar, unreactive, undergo substitution

Alkenes

• General formula: CₙH₂ₙ
• Functional group: C=C
• Properties: reactive, undergo addition reactions

Alcohols

• General formula: CₙH₂ₙ₊₂O
• Functional group: –OH
• Properties: hydrogen bonding, higher boiling points

Carboxylic Acids

• General formula: CₙH₂ₙO₂
• Functional group: –COOH
• Properties: acidic, form esters

Understanding these families makes organic chemistry far more systematic.

Chemical vs. Physical Trends

Homologous series show consistent physical changes but constant chemical behavior.

Physical trends:

• Boiling point increases with chain length
• Solubility decreases
• Viscosity increases

Chemical trends:

• Determined by the functional group
• Remain nearly identical across the series

This distinction is crucial for exam questions that ask you to compare compounds.

Why Each Member Is Called a “Homolog”

The term homolog simply means “one step higher in chain length.”
The CH₂ increase is what makes the pattern uniform and predictable.

FAQs

Why do boiling points increase in a homologous series?

Longer carbon chains have more electrons and greater surface area, leading to stronger London dispersion forces. More energy is required to separate molecules, so boiling points rise.

Are isomers part of the same homologous series?

Yes. As long as they contain the same functional group and general formula, structural isomers belong to the same series.

Is methane part of the alkene series?

No. Methane has no carbon–carbon double bond and does not follow the CₙH₂ₙ formula. It belongs only to the alkanes.

Conclusion

A homologous series is a family of organic compounds that share a functional group, follow a general formula, and differ by successive CH₂ units. This structure creates predictable physical trends and consistent chemical behavior. Mastering this concept provides a strong foundation for understanding organic reactions, patterns, and molecular structures in IB Chemistry.

Learn why the mole allows chemists to connect atomic-scale particles to measurable quantities used in real experiments.