Chemistry · Both
Organic Chemistry – Hydrocarbons
Alkanes and alkenes, homologous series, combustion and cracking of crude oil fractions.
Organic chemistry is the chemistry of carbon compounds. Crude oil is a mixture of hydrocarbons that can be separated and processed into useful fuels and materials.
Alkanes
**Alkanes** – saturated hydrocarbons (single bonds only).
General formula: CₙH₂ₙ₊₂
First members: methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀
As molecules get larger: higher boiling point, less flammable, more viscous.
Alkenes
**Alkenes** – unsaturated hydrocarbons (contain a C=C double bond).
General formula: CₙH₂ₙ
Ethene C₂H₄, propene C₃H₆
Test for unsaturation: bromine water decolourises (orange → colourless) with alkenes.
Worked example
How do you test for an alkene?
Solution
Add bromine water – it **decolourises** if a double bond is present.
Combustion and cracking
**Complete combustion** (plenty of oxygen):
hydrocarbon + oxygen → carbon dioxide + water
**Incomplete combustion** → carbon monoxide / soot (dangerous)
**Cracking** – breaks long-chain hydrocarbons into shorter, more useful ones (e.g. fuels and alkenes for plastics).
Needs heat and often a catalyst.
Practice questions
Try these without looking at the answers first.
Q1.General formula of alkanes?
Show answer
CₙH₂ₙ₊₂
Q2.What is the test for alkenes?
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Bromine water decolourises
Q3.What does cracking produce?
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Shorter hydrocarbons / alkenes and fuels