GCSE level organic chemistry exam revision notes: Part 1.

FOSSIL FUELS and the CARBON CYCLE

1. Fossil Fuels - where do they come from? and the Carbon Cycle

See also

Carbon cycle, nitrogen cycle, water cycle, decomposition - decay investigation, biogas  gcse biology

GCSE level m/c QUIZ on oil products & organic chemistry (easier-foundation-level)

GCSE level m/c QUIZ on oil products & organic chemistry (harder-higher-level)

1. The origin of oil and other fossil fuels - what are they formed from?

What is a fossil fuel? What is the origin of coal, peat, oil, natural gas? What is the Carbon Cycle? These revision notes on the use of fossil fuels and relationship with the carbon cycle.

• Crude oil is formed from organic material of the remains of plant and animal organisms that lived millions of years ago.  These remains form sediments e.g. at the bottom of seas, and  become buried under layers of sedimentary rock. They  decay, without air (oxygen), under the action of heat and pressure to form crude oil over millions of years. Coal is formed in a similar way from plant material.

  • Crude oil is a non-renewable energy resource taking millions of years to form from degraded organic biomass, so we are consuming fossil fuels at a much faster rate than they are being formed.

  • The vast majority of compounds found in crude oil are hydrocarbons, that is molecular compounds made up of carbon atoms combined with hydrogen atoms.

• It is a fossil fuel because it is formed from once living organisms and the Sun is the original source of energy. It is a non-renewable and finite (limited reserves) energy resource because it takes millions of years to form and we burn it faster than its is formed! It is also known as a finite energy resource because it will eventually run out! We do not have unlimited oil reserves!

  • The majority of fossil fuels (e.g. oil and gas) are made of hydrocarbon compounds - molecules composed of carbon and hydrogen atoms, though coal is mainly carbon with small quantities of other elements.

• Coal, peat and natural gas are the other principal non-renewable fossil fuels formed from the remains of plants or animals.

  • Coal, formed millions of years from the remains of tropical plant material, mainly consists of carbon,  Burning coal produces a lot of pollution as the greenhouse gas carbon dioxide. The pollutants include soot particles (black deposits of carbon), sulphur dioxide (lung irritant and acid rain gas) and poly-aromatic hydrocarbons which are carcinogenic.

    • The main reaction on burning is ...

    • carbon + oxygen ====> carbon dioxide

    • C_(s) + O_2(g) ====> CO_2(g)

    • 

    • ... so the main product of burning coal is carbon dioxide

    • This is an example of an oxidation reaction - an atom (carbon) has gained oxygen (become combined with oxygen).

  • Natural gas, mainly the hydrocarbon methane CH₄, is often found with oil. It consists of 25% by mass of hydrogen and 75% carbon, and, apart from the 'greenhouse' CO₂, produces far less pollution than coal on combustion.

    • The main reaction on burning is ...

    • methane + oxygen ====> water + carbon dioxide

    • CH_4(g) + 2O_2(g) ====> CO_2(g) + 2H₂O_(l)

    • 

    • See 3. ALKANES - saturated hydrocarbons and combustion

  • Peat ('turf') is formed over hundreds-thousands of years from the decay of plant material in the absence of oxygen, in boggy-water logged ground. It is a poor quality fuel since the carbon content is much less than in coal and large amount of ash formed on combustion. However, there is a peat fired power station in Ireland.

• THE CARBON CYCLE: When the fossil fuels are burned the 'carbon', as carbon dioxide, is returned to the atmosphere of the Earth's environment. There, it gets absorbed by plant leaves and used up in photosynthesis with the help of sunlight energy and green chlorophyll. The plant material decays reforming carbon dioxide, or, is eaten by animals and used in respiration to form carbon dioxide. Either way, this completes the carbon cycle. See also evolution of Earth's atmosphere

  • photosynthesis: carbon dioxide + water ==> glucose + oxygen

    • 6CO₂ + 6H₂O ====> C₆H₁₂O₆ + 6O₂ 

  • respiration: glucose + oxygen ===> carbon dioxide + water

    • C₆H₁₂O₆ + 6O₂  ====> 6CO₂ + 6H₂O

    • and fossil fuel combustion, forest fires etc. all return carbon dioxide to the atmosphere

  • Some of the carbon ends up as coal from decayed plants or oil from decayed animal remains i.e. fossil fuel formation of oil, gas and gas, which ultimately is also becomes part of the carbon cycle.

  • Until the industrial revolution, the processes in the carbon cycle were in equilibrium, however, the large-scale burning of fossil fuels is disturbing the balance between these same processes and carbon dioxide levels have been steadily rising, particularly over the last 200 years.

• POLLUTION PROBLEMS from burning fossil fuels are dealt with in section 4.

  • Pollution, carbon monoxide, sulfur/nitrogen oxides, acid rain and global warming-climate change

More on the CARBON CYCLE with reference to the diagram below

• You need to be able to show an understanding of how carbon is recycled in the CARBON CYCLE (diagram above).
  • a) during photosynthesis plants remove carbon dioxide from the atmosphere

    • carbon dioxide + water == light energy/chlorophyll  ==> glucose + oxygen

    • This is the process by which plants make food, for themselves, and for most animal life, including us too!

    • Note that the only way carbon dioxide is removed from the air is photosynthesis in green land based plants or marine organisms like phytoplankton (this point ignores long term formation of carbonate rocks like limestone).

  • b) carbon compounds pass along a food chain
    • .All food chains involve the passing of carbon compounds e.g. sugars, carbohydrates, fats and proteins up to the next trophic level i.e. the consecutive eating along a food chain (and waste produced on the way).
      • e.g. grass ==> cow ==> human
  • c) during plant or animal aerobic respiration organisms release carbon dioxide into the atmosphere
    • sugars e.g. glucose + oxygen ==> carbon dioxide + water (+ energy)
    • this is the main aerobic energy releasing process in most living organisms.
  • d) decomposers release carbon dioxide into the atmosphere - slow aerobic respiration
    • Microorganisms like bacteria and fungi in the soil feed off decaying plant material and animal droppings or remains.
    • Most dead plant matter consists of cellulose which most animals can't digest, but bacteria and fungi, do have the enzymes to break it down and without their help there would be no carbon cycle.
    • Most of these bacteria and fungi respire aerobically so they need a good supply of oxygen to produce the carbon dioxide essential to keeping the carbon cycle going.
  • e) combustion of fossil fuels releases carbon dioxide into the atmosphere

    • Coal, formed millions of years from the remains of tropical plant material, mainly consists of carbon,  Burning coal produces a lot of pollution as the greenhouse gas carbon dioxide.

      • The main reaction on burning is ...

      • carbon + oxygen ====> carbon dioxide

      • C_(s) + O_2(g) ====> CO_2(g)

    • Natural gas (mainly methane) and petrol molecules like octane (and lots of other molecules) from oil and gas reserves.

      • methane + oxygen ===> water + carbon dioxide

      • octane + oxygen ===> water + carbon dioxide
  • f) Volcanic eruptions release huge amounts of carbon dioxide into the atmosphere.
  • g) Some carbon dioxide from the atmosphere ends up as sedimentary rocks like limestone - which is mainly calcium carbonate, so deceasing atmospheric carbon dioxide levels, but this is a slow process.

See also

Carbon cycle, nitrogen cycle, water cycle and decomposition  gcse biology revision notes

and Limestone and lime - their chemistry and uses  gcse chemistry revision notes

and Energy resources: uses, general survey & trends, comparing renewables, non-renewables, generating electricity GCSE Physics revision notes

Key points about the carbon cycle

Carbon Cycle: Core Concepts

The carbon cycle describes how carbon moves between the atmosphere, biosphere, oceans, and geosphere. It is essential for maintaining life and regulating Earth’s climate.

Key Processes in the Carbon Cycle

Typical Exam Board-Specific Emphases

Student revision tips

• Draw the cycle: Practice sketching the carbon cycle with arrows and labels. Diagrams often appear in exams.
• Link processes: Understand how respiration and combustion both return CO₂ to the atmosphere.
• Use keywords: Include terms like photosynthesis, decomposition, fossilisation, and combustion in written answers.
• Practice sequencing: Be able to explain the journey of a carbon atom from the atmosphere into a plant, then into an animal, and back again.

Common Misconceptions to Avoid

• “Only animals respire” → Plants also respire, especially at night.
• “Decomposition removes CO₂” → It actually releases CO₂.
• “Photosynthesis happens in all organisms” → Only plants, algae, and some bacteria photosynthesise.
• “Combustion is natural” → While wildfires are natural, most combustion today is human-driven (fossil fuels).

Exam-Style Questions to Practice

1. Describe the role of photosynthesis and respiration in the carbon cycle.
2. Explain how combustion of fossil fuels affects the carbon cycle.
3. Draw and label a diagram of the carbon cycle.
4. Evaluate the impact of deforestation on the carbon cycle and climate change.

GCSE/IGCSE/O Level Oil Products & Organic Chemistry INDEX PAGE

ALL my Advanced A Level Organic Chemistry revision notes

Multiple Choice Quizzes and Worksheets

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (easier-foundation-level)

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (harder-higher-level)

and 3 linked easy Oil Products gap-fill quiz worksheets

ALSO gap-fill ('word-fill') exercises originally written for ...

... AQA GCSE Science Useful products from crude oil AND Oil, Hydrocarbons & Cracking etc.

... OCR 21st C GCSE Science Worksheet gap-fill C1.1c Air pollutants etc ...

... Edexcel GCSE Science Crude Oil and its Fractional distillation etc ...

... each set are interlinked, so clicking on one of the above leads to a sequence of several quizzes