3.2.3Additional lessons
Homologous Series — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 60 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Features of a Series
Same functional group
- Same functional group Every member has the same functional group, which controls the characteristic chemical reactions of the series.
General formula
- General formula All members fit one general formula, such as CₙH₂ₙ₊₂ for alkanes and CₙH₂ₙ for alkenes.
CH₂ difference
- CH₂ difference Neighbouring members differ by one CH₂ unit, which adds 14 to the relative molecular mass.
Trend in properties
- Trend in properties Physical properties such as boiling point change gradually along the series as the chain lengthens.
Similar chemistry
- Similar chemistry Members react in similar ways because the functional group is the reactive site, although rates may vary.
| Series | General formula | Functional group |
|---|---|---|
| Alkane | CₙH₂ₙ₊₂ | C–C single bonds |
| Alkene | CₙH₂ₙ | C=C |
| Alcohol | CₙH₂ₙ₊₁OH | –OH |
| Carboxylic acid | CₙH₂ₙ₊₁COOH | –COOH |
Section 2 of 5Why Boiling Point Rises
1
Longer chains have more electrons, so the instantaneous dipoles that cause London dispersion forces are larger.
2
A longer molecule has a greater surface area of contact with its neighbours.
3
Stronger intermolecular forces need more energy to overcome, so boiling point rises with chain length.
4
Alcohols have higher boiling points than alkanes of similar mass because hydrogen bonds act in addition to dispersion forces.
5
Solubility in water falls with chain length, as the non-polar chain grows larger relative to the polar group.
Section 3 of 5Common Series
Common Series
Halogenoalkanes
- Halogenoalkanes Contain a carbon-halogen bond and are named with a chloro, bromo or iodo prefix.
Aldehydes and ketones
- Aldehydes and ketones Both contain C=O, but aldehydes have it at the end of the chain and ketones within the chain.
Esters
- Esters Contain –COO– and are formed from an alcohol and a carboxylic acid.
Amines and amides
- Amines and amides Amines contain –NH₂ on a carbon chain, while amides contain –CONH₂.
Section 4 of 5Worked Example
Question Deduce the molecular formula and relative molecular mass of the fifth member of the alkene series.
- The fifth alkene has n = 5 carbon atoms.
- Formula CₙH₂ₙ gives C₅H₁₀.
- Mᵣ = 5 × 12.01 + 10 × 1.008 = 60.05 + 10.08.
- Mᵣ = 70.13.
Answer C₅H₁₀ (pentene), Mᵣ = 70.13
Section 5 of 5Exam 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 gives the molecular formula of any member of a series.
- Functional group The atom or group that determines the characteristic reactions.
- Dispersion forces Weak forces between temporary dipoles in molecules.
Common Mistakes
- Saying members of a series have identical physical properties.
- Describing the difference between members as a CH₃ group instead of CH₂.
- Forgetting that the general formula for cycloalkanes differs from that of alkanes.
- Claiming that boiling point rises because covalent bonds break on boiling.
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