Edexcel International A Level Chemistry Alkanes Exam Questions

About Edexcel International A Level Chemistry Alkanes

Alkanes are the first homologous series you study in detail in Unit 1 of Edexcel International A Level Chemistry. The topic looks at alkanes as saturated hydrocarbons obtained from crude oil, how they are used as fuels and the environmental problems their combustion causes. You also meet your first organic mechanism, free radical substitution with halogens in UV light. It builds on the naming and formula work in Introduction to organic chemistry and leads into Alkenes, since cracking produces alkenes. Examiners like to test balanced combustion equations, the three stages of the radical mechanism, why cracking and reforming are carried out, and the arguments for and against alternative fuels.

What you need to know

• Structure and properties Alkanes are saturated hydrocarbons with the general formula CnH2n+2, containing only C to C and C to H single bonds. They are fairly unreactive because these bonds are strong and almost non-polar.
• Combustion Complete combustion in excess oxygen gives carbon dioxide and water. Incomplete combustion gives carbon monoxide (toxic, binds to haemoglobin) and soot (carbon particulates).
• Pollutants Oxides of nitrogen form when N2 and O2 react at high engine temperatures, and sulfur dioxide comes from sulfur impurities and causes acid rain. Catalytic converters use platinum or rhodium to convert CO and NOx into CO2 and N2, and unburnt hydrocarbons into CO2 and H2O.
• Cracking and reforming Cracking breaks long-chain alkanes into shorter, more useful alkanes plus alkenes, using heat and a zeolite catalyst. Reforming turns straight-chain alkanes into branched and cyclic hydrocarbons, which burn more smoothly in engines.
• Free radical substitution Alkanes react with chlorine or bromine in UV light. Initiation: Cl2 splits homolytically into two Cl radicals. Propagation: Cl radical + CH4 gives HCl + CH3 radical, then CH3 radical + Cl2 gives CH3Cl + Cl radical. Termination: two radicals join.
• Limitations of substitution Further substitution gives a mixture of products such as CH2Cl2 and CHCl3, and longer alkanes give a mixture of positional isomers, so the method is poor for making a single pure product.
• Alternative fuels Biofuels such as bioethanol and biodiesel are described as carbon neutral in principle, but growing, processing and transporting them releases CO2 and uses land that could grow food.

Common mistakes in Alkanes

• Writing radicals without the single dot, or showing a radical as an ion. A radical has an unpaired electron, shown by a dot, and carries no charge.
• Putting H radicals in propagation steps, for example Cl radical + CH4 giving CH3Cl + H radical. The correct first propagation step gives HCl and a methyl radical.
• Saying catalytic converters remove carbon dioxide. They convert CO, NOx and unburnt hydrocarbons, and they actually produce CO2.
• Forgetting that a cracking equation must produce at least one alkene. Check that carbon and hydrogen atoms balance and that one product fits CnH2n.
• Calling biofuels completely carbon neutral without qualification. Examiners want you to mention CO2 released during farming, processing and transport.

Try three Alkanes exam-style questions