How did fossil fuels form, why are they so widely used, and what are the environmental costs of burning them?
How coal, oil and natural gas form, their extraction and use, why they are non-renewable and finite, and the environmental impacts of extracting and burning fossil fuels.
A focused answer to AQA A-Level Environmental Science 3.3.1, covering the formation of coal, oil and natural gas, their extraction and use, why they are finite, and the environmental impacts of using fossil fuels.
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What this dot point is asking
AQA wants you to explain how coal, oil and natural gas form, how they are extracted and used, why they are non-renewable, and the environmental impacts of extracting and burning them. Command words include Describe, Explain and Calculate, so be ready to handle a combustion calculation as well as the descriptive content.
How fossil fuels form
The locked-up energy is, in effect, ancient sunlight: photosynthesis fixed solar energy into the organisms' tissues as chemical energy, and burial prevented decomposers from releasing it. Burning the fuel today reverses that storage, returning the carbon to the atmosphere as carbon dioxide.
Why fossil fuels are finite
This is why supplies are finite and reserves will eventually be exhausted. As the easiest, highest-grade reserves are used first, extraction moves to deeper, more remote or lower-grade deposits, raising both the financial cost and the energy needed to extract each unit (a falling energy return on energy invested).
Extraction and use
- Coal is mined by surface (open-cast) mining for shallow seams or deep mining for buried seams.
- Oil and gas are extracted by drilling wells on land and offshore, and increasingly by hydraulic fracturing (fracking), which pumps high-pressure fluid into shale to fracture it and release trapped gas.
Fossil fuels dominate global energy because they are energy-dense, easily transported and stored, and historically cheap, and because the entire infrastructure of power stations, vehicles and heating systems was built around them.
Environmental impacts
Extraction causes habitat destruction (open-cast mining and infrastructure), oil spills that devastate marine and coastal ecosystems, and water pollution, including concerns over fracking fluids and methane leakage. Burning fossil fuels releases:
- Carbon dioxide, the main greenhouse gas driving the enhanced greenhouse effect and climate change.
- Sulfur dioxide and nitrogen oxides, which form acid rain and cause respiratory harm.
- Particulates (soot), which cause smog and lung disease.
- Carbon monoxide from incomplete combustion, a toxic gas.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20184 marksDescribe how coal forms and explain why coal is classed as a non-renewable resource.Show worked answer →
Markers split this 2 for formation and 2 for the non-renewable reasoning.
Formation: coal forms from the remains of land plants that grew in swamps, were buried under sediment, and were compressed and heated over millions of years. Increasing heat and pressure drive off water and volatiles, progressing peat to lignite to bituminous coal to anthracite.
Non-renewable: it forms over millions of years, vastly slower than humans extract and burn it, so the stock is effectively fixed on a human timescale and cannot be replaced once used. The discriminating point examiners want is the rate comparison, formation rate against use rate, not just the word slow.
AQA 20215 marksA power station burns coal containing carbon. Calculate the mass of carbon dioxide produced when 2.0 tonnes of carbon is completely burned, and explain two further environmental impacts of burning coal.Show worked answer →
A quantitative item: markers reward the relative-mass method and two developed impacts.
Calculation: the equation is . Relative atomic mass of carbon is 12; relative formula mass of carbon dioxide is 44. So each kilogram of carbon gives of carbon dioxide. For of carbon: , about of carbon dioxide.
Further impacts: sulfur in the coal forms sulfur dioxide, which causes acid rain that acidifies lakes and soils and corrodes buildings; high combustion temperatures form nitrogen oxides and particulates that cause smog and respiratory disease. Award the ratio method and two clear cause-effect impacts.
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