GCSE & A Level Revision Notes

Subject: Chemistry
Level: IGCSE
Exam Boards: EDEXCEL

Section 3 (3.1-3.12)

Organic Chemistry


Homologous series: A family of compounds that have similar properties and bonding, such as the alkenes or alcohols

Hydrocarbon: A compound containing hydrogen and carbon only

Saturated: Contains only single bonds

Unsaturated: Contains a double bond

General Formula: e.g. CnH2n A formula that fits all the members in a homologous series

Isomerism: Where molecules have the same molecular formula but a different structural or displayed formula.



Alkanes have the general formula of CnH2n+2


Methane Ethane Propane Butane
Methane (CH4) Ethane (C2H6) Propane (C3H8) Butane (C4H10)


Different products come from combustion of hydrocarbons.

  • Complete combustion – Carbon Dioxide
  • Incomplete combustion – Carbon Monoxide
  • Very Incomplete combustion – carbon (soot)


CH4 + Br2 → CH3Br + HBr Under UV light, bromine substitutes with hydrogen into methane to produce bromomethane.



Alkenes are another homologous series. They have a general formula of CnH2n.


Ethene Propene
Methane (CH4) Ethane (C2H6)


A test for alkenes is an addition reaction with bromine water. The bromine water, which starts brown, goes colourless. C2H4 + Br2 → C2H4Br2


Ethanol can be manufactured by passing ethene and steam over phosphoric acid, which acts as a catalyst, at a temperature of around 300˚C and a pressure of around 60-70 atmospheres.


Ethanol can also be manufactured by the fermentation of sugar by yeast, which takes place at a temperature of around 30˚C. C6H12O6 → 2C2H5OH + 2CO2

  Fermentation Hydration of Ethene
Raw Materials Renewable resources Non-renewable (crude oil)
Process Batch Continuous
Rate of reaction Very slow Fast
Quality Dilute solution- needs processing Pure ethanol
Conditions Low temperature + pressure Very high temperature and pressure – incurs costs


Ethanol can be dehydrated, by passing it over hot AlO3. This acts as a catalyst to speed up the reaction.

C2H5OH → C2H4 + H2O

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