A-Level Chemistry: Alkanes, Fuels and Pollution

Alkanes are the first organic family studied in A-level Chemistry, and the course looks at them mainly as fuels: where they come from, how the petroleum industry turns crude oil into useful products, and what burning them does to the air. This material covers that Year 12 content and the environmental chemistry that goes with it.

It starts with crude oil as a mixture of mostly alkane hydrocarbons and with fractional distillation: the temperature gradient in the column, why fractions condense at different heights, and why boiling point rises with chain length as van der Waals forces between molecules get stronger. You then look at cracking, the breaking of C–C bonds in long alkanes. You explain the economic reason for it (long-chain fractions are produced in greater amounts than are needed, while shorter alkanes and alkenes are more valuable), compare thermal cracking (high temperature and high pressure, a high proportion of alkenes) with catalytic cracking (high temperature, slight pressure and a zeolite catalyst, giving motor fuels and aromatic hydrocarbons), and balance cracking equations to find a missing product.

The second half deals with combustion and pollution. You write equations for complete and incomplete combustion, then connect each pollutant to its source: nitrogen oxides formed from nitrogen and oxygen at the high temperature of an engine, carbon monoxide and soot from incomplete combustion, unburned hydrocarbons, and sulfur dioxide from fuels that contain sulfur. You see how catalytic converters remove carbon monoxide, nitrogen monoxide and hydrocarbons, and why calcium oxide or calcium carbonate can remove sulfur dioxide from power-station flue gases. Finally you study the ozone layer: why ozone in the upper atmosphere is beneficial, how ultraviolet radiation releases chlorine atoms from CFCs, the two equations by which a chlorine atom catalyses the breakdown of ozone, and how research evidence led to the ban on CFCs. The link between infrared absorption by carbon dioxide, methane and water vapour and global warming completes the picture.

Four formats are offered. The 12-question quiz mixes equations with explanation questions. The flashcards give key terms, conditions and equations. The written work has eight questions to answer by hand, from balancing combustion equations to explaining ozone depletion, each with a reference answer. The oral exam lets you explain these processes to an examiner, one question at a time. The free-radical mechanism of methane chlorination is covered in a separate material of this category and is not repeated here.

The content is based on the A-level chemistry content on alkanes and on the environmental impact of halogenoalkanes, as set out for example in the AQA A-level specification. It is revision practice written by Zestly, not an exam board resource.

• Explain how fractional distillation separates crude oil and why boiling point rises with chain length
• Explain the economic reasons for cracking and compare thermal and catalytic cracking
• Balance cracking and combustion equations, including incomplete combustion
• Identify the pollutants from engines and power stations and explain how catalytic converters and calcium compounds remove them
• Explain how chlorine atoms from CFCs catalyse the decomposition of ozone, with equations
• Link infrared absorption by CO₂, CH₄ and H₂O to global warming

Practice material written by Zestly, based on the A-level chemistry content on alkanes, combustion and ozone depletion (for example AQA A-level Chemistry 7405, sections 3.3.2, 3.3.3.3 and 3.3.6.3).