CCEA GCSE Chemistry 2.5 Organic Chemistry Exam Questions

About CCEA GCSE Chemistry 2.5 Organic Chemistry

Organic Chemistry is the largest topic in Unit 2 of CCEA GCSE Chemistry. You need to understand homologous series and name, draw and write formulae for alkanes, alkenes, alcohols and carboxylic acids. The topic covers fractional distillation of crude oil, cracking, the test for alkenes, addition polymerisation, complete and incomplete combustion and the pollution caused by burning fuels. The prescribed practical investigates the reactions of carboxylic acids. It links to energy changes, quantitative chemistry and the gas tests. Examiners frequently ask for structural formulae, equations for combustion and cracking, the bromine water test and drawing a repeat unit of a polymer from its monomer.

What you need to know

• Homologous series A family of compounds with the same general formula and functional group, similar chemical properties, a gradual trend in physical properties and successive members differing by CH2.
• Alkanes and crude oil Alkanes, CnH2n+2, are saturated hydrocarbons (methane, ethane, propane, butane). Crude oil is separated by fractional distillation: it is vaporised and the fractions condense at different heights in the column according to boiling point, giving refinery gas, petrol, naphtha, kerosene, diesel, fuel oil and bitumen.
• Cracking and alkenes Cracking breaks long alkanes into shorter, more useful alkanes and alkenes using heat and a catalyst, for example C10H22 → C8H18 + C2H4. Alkenes, CnH2n, are unsaturated with a C=C double bond. Alkenes decolourise orange bromine water; alkanes do not.
• Addition polymers Many alkene monomers join to form a polymer, for example ethene forms poly(ethene). The double bond opens and the repeat unit is drawn with extension bonds. Polymers are hard to dispose of because they are non-biodegradable.
• Alcohols Alcohols, CnH2n+1OH, contain the -OH group (methanol, ethanol, propanol). They burn to form carbon dioxide and water. Ethanol can be oxidised to ethanoic acid.
• Carboxylic acids Carboxylic acids contain the -COOH group (methanoic, ethanoic, propanoic acid). They are weak acids that react with metals to give a salt and hydrogen and with carbonates to give a salt, water and carbon dioxide (fizzing).
• Combustion and pollution Complete combustion gives carbon dioxide and water. Incomplete combustion in limited oxygen gives carbon monoxide (toxic) and soot. Sulfur impurities form sulfur dioxide, which causes acid rain; nitrogen oxides also form at high temperatures in engines.

Common mistakes in 2.5 Organic Chemistry

• Writing that bromine water turns colourless with alkanes. Only alkenes decolourise it; with alkanes it stays orange.
• Saying bromine water goes clear. Use the word colourless; clear does not mean the colour has gone.
• Drawing a polymer repeat unit with the C=C double bond still present. In the polymer the carbons are joined by single bonds.
• Writing the alcohol formula as CnH2n+2OH. The general formula is CnH2n+1OH.
• Explaining fractional distillation by saying fractions boil at different heights. The column has a temperature gradient and fractions condense where the temperature is below their boiling point.

Try three 2.5 Organic Chemistry exam-style questions