Cambridge IGCSE Chemistry 11 Organic Chemistry Exam Questions

About Cambridge IGCSE Chemistry 11 Organic Chemistry

Organic chemistry is one of the largest topics in Cambridge IGCSE Chemistry 0620. It covers formulae, functional groups and naming, fossil fuels and fractional distillation of petroleum, alkanes, alkenes, alcohols, carboxylic acids and polymers. Extended candidates also study structural isomers, esters, condensation polymers such as nylon and PET, and proteins. The topic draws on covalent bonding, energetics and stoichiometry. Examiners regularly ask for displayed formulae, names of fractions and their uses, the bromine water test, the conditions for making ethanol by fermentation and by hydration of ethene, and the structure of repeat units in addition and condensation polymers.

What you need to know

• 11.1 to 11.2 Formulae and naming Write molecular, structural and displayed formulae. A homologous series has the same general formula and functional group, similar chemical properties and a trend in physical properties. Name alkanes, alkenes, alcohols and carboxylic acids with up to four carbon atoms. Extended candidates draw structural isomers.
• 11.3 Fuels Coal, natural gas (mainly methane) and petroleum are fossil fuels. Petroleum is separated by fractional distillation into refinery gas, gasoline/petrol, naphtha, kerosene/paraffin, diesel oil/gas oil, fuel oil, lubricating oil and bitumen. Down the column, chain length, boiling point and viscosity increase.
• 11.4 Alkanes Alkanes are saturated hydrocarbons, generally unreactive apart from combustion. They undergo substitution with chlorine in the presence of ultraviolet light, e.g. CH4 + Cl2 → CH3Cl + HCl.
• 11.5 Alkenes and cracking Cracking larger alkanes with high temperature and a catalyst gives smaller alkanes, alkenes and hydrogen. Alkenes are unsaturated and decolourise bromine water (orange to colourless). They undergo addition with bromine, hydrogen (nickel catalyst) and steam.
• 11.6 Alcohols Ethanol is made by fermentation of glucose with yeast at about 25 to 35 °C without oxygen, or by catalytic addition of steam to ethene at 300 °C and 6000 kPa with a phosphoric acid catalyst. Fermentation uses renewable resources but is slow and gives impure ethanol; hydration is fast and continuous but uses non-renewable ethene.
• 11.7 Carboxylic acids Ethanoic acid reacts as a typical weak acid with metals, bases and carbonates. Extended candidates know it can be made by oxidising ethanol with acidified potassium manganate(VII) or by bacterial oxidation, and that carboxylic acids react with alcohols, with a concentrated sulfuric acid catalyst, to form esters such as ethyl ethanoate.
• 11.8 Polymers Addition polymers such as poly(ethene) form from alkene monomers. Non-biodegradable plastics cause problems in landfill and the oceans, and burning them releases toxic gases. Extended candidates draw repeat units and describe condensation polymers: polyamides (nylon), polyesters (PET) and proteins made from amino acids.

Common mistakes in 11 Organic Chemistry

• Saying alkanes react with bromine water. Only alkenes decolourise bromine water quickly; alkanes leave it orange.
• Drawing a repeat unit with the double bond still present. In addition polymers the C=C becomes a single bond and the repeat unit has bonds extending out each side.
• Mixing up fermentation conditions. Fermentation needs yeast, a warm temperature and no oxygen; too high a temperature denatures the enzymes in yeast.
• Saying fractional distillation breaks down molecules. It separates fractions by boiling point; cracking is what breaks large molecules down.
• Naming esters the wrong way round. The alcohol part comes first, so ethanol and ethanoic acid give ethyl ethanoate.

Try three 11 Organic Chemistry exam-style questions