A7LAM CSCA ACADEMYCSCA Academy / Chemistry / Substances and their reactions
CSCA Chemistry · Lesson 8 of 21Hydrocarbons
Hydrocarbures · الهيدروكربونات
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Hydrocarbons contain only carbon and hydrogen. Recognising alkanes, alkenes and benzene by their bonds is the key skill.
Key ideas
- Alkanes CₙH₂ₙ₊₂ have only single bonds (methane, ethane, propane).
- Alkenes CₙH₂ₙ have a C=C double bond.
- Benzene C₆H₆ has a stable ring; alkenes add Br₂ and decolourise bromine water.
- Isomers have the same formula but different structures.
Formulas to know
- Methane CH₄ · Ethane C₂H₆ · Propane C₃H₈
- Ethene C₂H₄
- CH₂=CH₂ + Br₂ → CH₂Br−CH₂Br
Worked example
Give the formula of butane, the alkane with 4 carbons.
- Alkanes follow CₙH₂ₙ₊₂.
- With n = 4: H = 2 × 4 + 2 = 10.
Answer: C₄H₁₀
Practice questions
1. Which is an alkene?
C₂H₆ · C₃H₆ ✓ · CH₄ · C₄H₁₀
C₃H₆ fits CₙH₂ₙ (propene).
2. Which decolourises bromine water?
Ethane · Ethene ✓ · Methane · Benzene
The C=C double bond adds Br₂.
3. How many hydrogen atoms in pentane (5 carbons)?
10 · 12 ✓ · 14 · 8
CₙH₂ₙ₊₂ with n = 5 gives 12.