Alkenes, alcohols and carboxylic acids · GCSE Chemistry

Separate sciences only (not Combined Science). GCSE Chemistry revision notes, key terms and practice questions.

Alkenes

Alkenes are hydrocarbons with a carbon–carbon double bond (C=C). Their general formula is CₙH₂ₙ.
The first four are ethene (C₂H₄), propene (C₃H₆), butene (C₄H₈) and pentene (C₅H₁₀).
Alkenes are unsaturated: they have two fewer hydrogen atoms than the alkane with the same number of carbon atoms.
Alkenes burn with a smoky flame because of incomplete combustion.

Alkenes react by adding atoms across the double bond, which becomes a single bond.
With hydrogen (using a nickel catalyst): ethene + hydrogen → ethane.
With water as steam (using a catalyst): ethene + steam → ethanol.
With halogens: ethene + bromine → dibromoethane. This is why bromine water is decolourised.

Alcohols contain the functional group –OH. The first four are methanol (CH₃OH), ethanol (C₂H₅OH), propanol and butanol.
They react with sodium to produce hydrogen, burn in air, and dissolve in water to form a neutral solution.
They can be oxidised to form carboxylic acids.
Alcohols are used as fuels and solvents. Ethanol is the alcohol in alcoholic drinks.
Ethanol can be made by fermenting sugar solutions with yeast, in warm conditions without oxygen.

Carboxylic acids contain the functional group –COOH. The first four are methanoic, ethanoic (CH₃COOH), propanoic and butanoic acid.
They dissolve in water to form acidic solutions, and react with carbonates to produce carbon dioxide.
They react with alcohols, with an acid catalyst, to make esters: ethanoic acid + ethanol → ethyl ethanoate + water.
Esters have fruity smells and are used in perfumes and flavourings.
(Higher) Carboxylic acids are weak acids because they only partially ionise in water.

Unsaturated: Containing a carbon–carbon double bond.
Functional group: A group of atoms that decides how a compound reacts.
Addition reaction: A reaction in which atoms add across a C=C double bond.
Hydrogenation: Adding hydrogen to an alkene, using a nickel catalyst.
Alcohol: An organic compound containing the –OH functional group.
Carboxylic acid: An organic compound containing the –COOH functional group.
Ester: A compound formed when a carboxylic acid reacts with an alcohol.
Fermentation: Using yeast to convert sugar into ethanol and carbon dioxide.
Homologous series: A family of compounds with the same general formula and similar reactions.