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10.3.3Chemistry of the Environment

Alkenes — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 6 of 7 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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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
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Section 1 of 5Structure of Alkenes

Alkenes are unsaturated hydrocarbons, and their double bond is the reactive part of the molecule.

Double bond
  • Double bond Every alkene contains at least one carbon–carbon double bond, shown as C=C.
Unsaturated
  • Unsaturated Alkenes are unsaturated because they can add other atoms across the double bond.
General formula
  • General formula Alkenes have the general formula CₙH₂ₙ, which differs from the alkane formula CₙH₂ₙ₊₂.
Functional group
  • Functional group The C=C double bond is the functional group that gives alkenes their characteristic reactions.
NameMolecular formulaDisplayed formula
EtheneC₂H₄H₂C=CH₂
PropeneC₃H₆H₂C=CH–CH₃

Section 2 of 5Reaction with Bromine Water

The double bond is what allows alkenes to react with bromine water.

1
Bromine water is orange because it contains dissolved bromine.
2
The bromine adds across the double bond of the alkene, forming a saturated compound.
3
The bromine is no longer free in solution, so the orange colour disappears.
4
Alkanes have no double bond and do not react, so bromine water stays orange with them.
5
This colour change is why bromine water is used as a simple test to show whether a hydrocarbon is an alkene.

Section 3 of 5Recognising Alkenes

Recognising Alkenes
Formula test
  • Formula test An alkene has the formula CₙH₂ₙ, so C₅H₁₀ is an alkene and C₅H₁₂ is an alkane.
Naming
  • Naming Alkene names end in -ene, such as ethene and propene.
Reactivity
  • Reactivity Alkenes react in ways alkanes cannot, because of the reactive double bond.
Drawing
  • Drawing Always draw the double bond as two lines between the two carbon atoms.

Section 4 of 5Worked Example

Question Show whether C₅H₁₀ is an alkene or an alkane.

  1. Alkanes fit CₙH₂ₙ₊₂; for n = 5 this gives C₅H₁₂.
  2. Alkenes fit CₙH₂ₙ; for n = 5 this gives C₅H₁₀.
  3. C₅H₁₀ matches CₙH₂ₙ exactly, so it is an alkene and will decolourise bromine water.

Answer C₅H₁₀ is an alkene.

Section 5 of 5Exam Checklist & Tips

10.3.3 – Key Points for the Exam

  • Alkene An unsaturated hydrocarbon containing at least one carbon–carbon double bond.
  • Unsaturated Containing at least one carbon–carbon double bond.
  • Addition reaction A reaction in which atoms are added across a double bond, leaving a single bond.
  • Functional group The atom or group of atoms in a molecule that controls its characteristic reactions.

Common Mistakes

  • Calling alkenes saturated hydrocarbons; they contain a double bond, so they are unsaturated.
  • Using CₙH₂ₙ₊₂ as the formula for alkenes; that is the formula for alkanes.
  • Stating that bromine water turns colourless with alkanes; it stays orange.
  • Drawing the double bond as a single line, which changes the molecule.

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