OxfordAQA International GCSE Chemistry 3.10 Organic Chemistry Exam Questions

About OxfordAQA International GCSE Chemistry 3.10 Organic Chemistry

Organic Chemistry is the final section of OxfordAQA International GCSE Chemistry and covers carbon compounds as fuels and materials. You study crude oil and fractional distillation, the alkanes and their combustion products, cracking to make alkenes, the bromine water test and addition polymers such as poly(ethene) and poly(propene). You also meet thermosoftening and thermosetting polymers, biofuels, alcohols and carboxylic acids. The separate Chemistry course adds ethanol production by hydration and fermentation, and esters. Examiners regularly ask you to compare the two ways of making ethanol, explain incomplete combustion and its effects, and draw structures for the named members of each homologous series.

What you need to know

• Crude oil Crude oil is a mixture, mostly of hydrocarbons. Fractional distillation separates it continuously in a fractionating column: vapour rises, the column is hotter at the bottom, and fractions condense at different heights according to boiling point.
• Alkanes and combustion Alkanes have the general formula CnH2n+2 and only single bonds (saturated); you need methane, ethane and propane. Boiling point and viscosity increase and flammability decreases with chain length. Complete combustion gives carbon dioxide and water; incomplete combustion gives carbon monoxide and particulates. Sulfur dioxide and oxides of nitrogen cause acid rain and particulates cause global dimming.
• Cracking and alkenes Cracking breaks large hydrocarbons into smaller, more useful ones, using a hot catalyst or steam at very high temperature. Products include alkenes (CnH2n, with a C=C double bond), such as ethene and propene. Required practical: alkenes turn bromine water from orange to colourless.
• Polymers Alkene monomers join to form addition polymers such as poly(ethene) and poly(propene). Thermosoftening polymers have tangled chains with weak intermolecular forces and melt; thermosetting polymers have cross-links and do not melt. Many polymers are not biodegradable; cornstarch based plastics break down more easily.
• Making ethanol (separate Chemistry) Hydration of ethene: C2H4 + H2O → C2H5OH, phosphoric acid catalyst, continuous, non-renewable raw material. Fermentation: C6H12O6 → 2C2H5OH + 2CO2, yeast enzymes, 20 to 35 °C, batch process, renewable sugar, slower and less pure.
• Alcohols Functional group -OH; methanol, ethanol and propanol. They dissolve to give neutral solutions, react with sodium to give hydrogen, burn in air and are used as fuels and solvents. Ethanol is oxidised to ethanoic acid, the acid in vinegar.
• Carboxylic acids Functional group -COOH; methanoic, ethanoic and propanoic acid. They are weak acids that do not fully ionise, so they have a higher pH than strong acids of the same concentration. They react with carbonates to give carbon dioxide and with alcohols, using an acid catalyst, to form esters.
• Esters (separate Chemistry) Functional group -COO-. Ethanol + ethanoic acid → ethyl ethanoate + water. Esters are volatile with distinctive smells and are used in flavourings and perfumes.

Common mistakes in 3.10 Organic Chemistry

• Saying crude oil fractions are separated by boiling the oil at different temperatures one after another. It is a continuous process in one column with a temperature gradient.
• Saying bromine water turns clear. Say it changes from orange to colourless; clear is not a colour change.
• Drawing poly(ethene) with a C=C double bond. The double bond opens up, so the polymer has only single bonds.
• Saying weak acids are dilute. Weak means not fully ionised; dilute means low concentration.
• Writing that incomplete combustion produces only carbon dioxide and water. It also produces carbon monoxide and/or carbon (soot).

Try three 3.10 Organic Chemistry exam-style questions