Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

3.2.1Organic Chemistry

Describing Alkanes — Edexcel AS Chemistry Revision Notes & Topical Questions

5 min read 4 parts Lesson 19 of 42

📚 Edexcel AS Chemistry Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
0%read

Part 1 of 4Structure and Naming

General formula
  • General formula Alkanes have the general formula CₙH₂ₙ₊₂, so an alkane with five carbons has twelve hydrogens.
Saturated
  • Saturated They contain only single bonds, so each carbon is bonded to the maximum number of other atoms.
Shape
  • Shape Each carbon has four bonding pairs and no lone pairs, giving a tetrahedral shape with bond angles of 109.5°.
Sigma bonds
  • Sigma bonds All the C–C and C–H bonds are σ bonds formed by direct overlap of orbitals along the bond axis.
Naming
  • Naming The stem shows the chain length (meth, eth, prop, but, pent, hex) and the ending –ane shows single bonds only.
AlkaneFormulaBoiling point / °C
MethaneCH₄−162
EthaneC₂H₆−89
PropaneC₃H₈−42
ButaneC₄H₁₀−1
PentaneC₅H₁₂36
HexaneC₆H₁₄69

Part 2 of 4Physical Properties

1
Alkanes are non-polar molecules because carbon and hydrogen have very similar electronegativities.
2
Only weak London forces act between their molecules, so they have low melting and boiling points compared with polar compounds.
3
Longer chains have more electrons and a larger surface area, so the London forces are stronger and the boiling point rises along the series.
4
Branched isomers are more compact, so they boil at lower temperatures than straight-chain isomers.
5
Alkanes do not dissolve in water because they cannot form hydrogen bonds, but they mix with other non-polar liquids.

Part 3 of 4Chemical Reactivity

Chemical Reactivity
Strong bonds
  • Strong bonds The C–C and C–H bonds are strong, with mean bond enthalpies of about 347 and 413 kJ mol⁻¹.
Non-polar bonds
  • Non-polar bonds The bonds are not polar, so neither nucleophiles nor electrophiles are strongly attracted to them.
Combustion
  • Combustion Alkanes burn readily in oxygen and are widely used as fuels.
Substitution
  • Substitution They react with halogens in ultraviolet light by free-radical substitution, which is covered later.
No reaction
  • No reaction Alkanes do not react with acids, alkalis or oxidising agents such as acidified potassium manganate(VII) at room temperature.

Part 4 of 4Exam Checklist & Tips

3.2.1 – Key Points for the Exam

  • Alkane A saturated hydrocarbon with the general formula CₙH₂ₙ₊₂.
  • Saturated Containing only single carbon-carbon bonds.
  • σ bond A covalent bond formed by end-on overlap of orbitals along the line between the nuclei.
  • London forces Weak intermolecular forces caused by temporary dipoles in molecules.
  • Tetrahedral A shape with four bonding pairs at bond angles of 109.5°.

Common Mistakes

  • Saying that alkanes are unreactive because they are non-polar and forgetting the strength of the C–C and C–H bonds.
  • Writing the general formula as CₙH₂ₙ, which is the formula for alkenes.
  • Explaining the rise in boiling point by saying covalent bonds get stronger, when it is the London forces that increase.
  • Quoting bond angles of 120° for alkanes instead of 109.5°.

Last updated:

\n