Name the fossil fuels: coal, natural gas and petroleum

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IGCSE Chemistry – Topic 11: Fuels (Cambridge 0620)

Learning Objectives

  • Identify and name the three main fossil fuels.
  • Explain the origin, composition and main uses of coal, natural gas and petroleum.
  • Describe fractional distillation of crude oil and list all refinery fractions required by the syllabus.
  • Recall the general formulas, functional groups and naming rules for alkanes, alkenes, alcohols and carboxylic acids.
  • Write and balance the key reactions (combustion, substitution, addition, cracking, hydration, esterification).
  • Understand the link between fossil fuels, fertilisers, air‑quality issues and climate change.

1. What are Fossil Fuels?

Fossil fuels are natural resources that contain large amounts of hydrocarbons – molecules made only of carbon (C) and hydrogen (H). They were formed from the remains of ancient plants and animals that were buried, heated and compressed over millions of years.

2. The Three Main Fossil Fuels

Fuel Physical State Dominant Hydrocarbon(s) Typical Uses
Coal Solid Mostly carbon (C) with trace alkanes (CH4, C2H6 …) Power‑station electricity, steel making, domestic heating
Natural gas Gas Methane (CH4) – about 85–95 % Cooking, space heating, electricity generation, chemical feedstock
Petroleum (crude oil) Liquid Mixture of alkanes, cyclo‑alkanes and aromatics (e.g. C8H18, C12H26) Transport fuels, lubricants, plastics, pharmaceuticals

2.1 Origin of Each Fuel

  • Coal: Formed from ancient swamp‑type vegetation that was buried and transformed under pressure and heat (carbonisation).
  • Natural gas: Produced from the same organic matter as petroleum but under higher temperature and pressure, allowing the lighter hydrocarbons to remain gaseous.
  • Petroleum: Derived from marine plankton and algae that settled on the sea floor, were buried in sedimentary rock and transformed into liquid hydrocarbons.

3. Fertilisers – NPK

Fertilisers supply the essential nutrients nitrogen (N), phosphorus (P) and potassium (K) that plants need for growth.

  • Nitrogen – supplied as ammonium nitrate (NH4NO3) or urea (CO(NH2)2).
  • Phosphorus – supplied as calcium phosphate, e.g. Ca3(PO4)2.
  • Potassium – supplied as potassium sulphate (K2SO4) or potassium chloride (KCl).

These fertilisers are often manufactured from natural gas (for ammonia) and from phosphate rock, linking them directly to fossil‑fuel chemistry.

4. Air Quality, Climate Change & Fossil Fuels

  • Greenhouse gases: CO2 (from combustion of all fossil fuels), CH4 (natural gas leaks), N2O (from fertiliser use).
  • Other pollutants: SO2 and NOx (from coal and oil combustion), particulate matter (PM2.5).
  • Mitigation strategies:
    • Switch to low‑carbon energy sources (renewables, nuclear).
    • Use catalytic converters to reduce NOx from vehicles.
    • Improve energy efficiency and capture CO2 from power stations.

5. Formulae, Functional Groups & Terminology

  • Homologous series: A group of compounds that differ by a constant unit (‑CH2‑). Each series has a general formula.
Family General Formula Functional Group Suffix (IUPAC)
Alkanes CnH2n+2 None (saturated C–C) -ane
Alkenes CnH2n Carbon‑carbon double bond (C=C) -ene
Alcohols CnH2n+1OH Hydroxyl group –OH -ol
Carboxylic acids CnH2n+1COOH Carboxyl group –COOH -oic acid

6. Naming Organic Compounds (Displayed Formulas)

Tip: Write the displayed (structural) formula first, then add the appropriate suffix.

CompoundDisplayed FormulaName (IUPAC)
MethaneCH4methane
EthaneCH3–CH3ethane
EtheneCH2=CH2ethene
EthanolCH3–CH2–OHethanol
Ethanoic acidCH3–COOHethanoic acid

7. Fractional Distillation of Crude Oil

Crude oil is a complex mixture of hydrocarbons with a wide range of boiling points. In a refinery a tall vertical column is heated at the base; vapour rises, cools and condenses at heights that correspond to its boiling point. This process is called fractional distillation.

7.1 Main Fractions (Syllabus‑required list)

Fraction Boiling Range (°C) Main Hydrocarbons Key Use
Refinery gas ≤ 30 CH4, C2H6, C3H8, C4H10 LPG and petrochemical feedstock
Petrol (gasoline) 30–200 C4–C12 alkanes & aromatics Motor‑vehicle fuel
Naphtha 150–200 C5–C10 alkanes Feedstock for ethene & propene production
Kerosene 150–250 C10–C15 alkanes Aviation fuel & heating
Diesel 250–350 C12–C20 alkanes Road & marine transport, generators
Fuel oil 350–400 Long‑chain alkanes, heavy aromatics Power‑station boilers, ship fuel
Lubricating oil ≈ 400 Very high‑molecular‑weight alkanes & aromatics Engine & machinery lubricants
Bitumen ≈ 500 Very heavy, high‑viscosity hydrocarbons Road surfacing & roofing

8. Chemistry of the Main Hydrocarbon Families

8.1 Alkanes (Saturated Hydrocarbons)

General formula: CnH2n+2 (n ≥ 1)

Properties: colourless, non‑polar, low reactivity; boiling point rises with chain length.

Combustion (complete):

CnH2n+2 + (3n + 1) O2 → n CO2 + (n + 1) H2O

Substitution (radical) reactions (photochemical):

  • With chlorine: CH4 + Cl2 → CH3Cl + HCl
  • With bromine (slower but more selective): CH4 + Br2 → CH3Br + HBr

These reactions proceed via homolytic cleavage of the halogen bond, formation of radicals, and propagation steps – the mechanism required by the syllabus.

8.2 Alkenes (Unsaturated Hydrocarbons)

General formula: CnH2n (one C=C double bond).

Key test for unsaturation: Bromine water (Br2) decolourises as the double bond adds bromine:

C=C + Br2 → C–Br–C–Br

Addition reactions (all follow the electrophilic addition mechanism):

  • Hydrohalogenation: C2H4 + HCl → C2H5Cl
  • Hydration (acid‑catalysed): C2H4 + H2O → CH3CH2OH
  • Hydrogenation (catalytic): C2H4 + H2 → C2H6

Cracking (thermal or catalytic) breaks long‑chain alkanes to give shorter alkanes + alkenes. Example:

C10H22 → C8H18 + C2H4

Cracking supplies the ethene and propene needed for the petrochemical industry.

8.3 Alcohols (Derived from Alkenes)

General formula: CnH2n+1OH.

Production routes – advantages & disadvantages
RouteReactionAdvantagesDisadvantages
Fermentation of sugars C6H12O6 → 2 CH3CH2OH + 2 CO2 Uses renewable feedstock; low‑temperature process. Limited to small‑scale, requires careful control of microbes.
Hydration of ethene (industrial) C2H4 + H2O → CH3CH2OH  (catalyst: phosphoric acid, 300 °C) High yield, uses cheap ethene from petroleum. Requires high temperature, catalyst handling, and natural‑gas‑derived ethene.

Combustion (complete):

CH3CH2OH + 3 O2 → 2 CO2 + 3 H2O

Uses: alcoholic beverages, solvent, fuel for spirit burners, feedstock for ethyl acetate and other esters.

8.4 Carboxylic Acids (e.g., Ethanoic Acid)

General formula: CnH2n+1COOH.

Industrial preparation of ethanoic acid (Cambridge‑approved route):

  1. Ethene → Ethylene oxide (oxidation).
  2. Ethylene oxide + H2O → Ethanol.
  3. Ethanol (oxidation) → Acetaldehyde → Ethanoic acid.

Typical reactions (IGCSE level):

  • With a base: CH3COOH + NaOH → CH3COONa + H2O
  • With a metal carbonate: 2 CH3COOH + CaCO3 → (CH3COO)2Ca + CO2 + H2O
  • Esterification (acid‑catalysed): CH3COOH + CH3CH2OH ⇌ CH3COOCH2CH3 + H2O

Esters such as ethyl acetate are important solvents and are used in paints, adhesives and nail‑polish removers.

9. Summary Table – Linking Fossil Fuels to Key Chemical Concepts

Fuel Major Hydrocarbon Family Key Reactions (IGCSE) Representative Products / Uses
Coal Carbon (solid) + trace alkanes Combustion → CO2 + H2O Electricity, steel production
Natural gas Alkanes (mainly CH4) Combustion; radical substitution with Cl2 / Br2 Cooking, heating, chemical feedstock
Petroleum Mixture of alkanes, alkenes, cyclo‑alkanes, aromatics Fractional distillation, cracking, addition (alkenes), substitution (alkanes), esterification (acids) Transport fuels, plastics, lubricants, solvents

10. Key Points to Remember

  1. All fossil fuels originate from ancient organic matter and consist mainly of hydrocarbons.
  2. Coal – solid carbon‑rich; natural gas – gaseous mixture dominated by methane; petroleum – liquid mixture separable by fractional distillation.
  3. Fractional distillation yields eight syllabus‑required fractions, each with a characteristic boiling range and a principal use.
  4. Alkanes (CnH2n+2) are saturated; they combust completely and undergo radical substitution with halogens.
  5. Alkenes (CnH2n) are unsaturated; they give a positive bromine test and participate in electrophilic addition reactions (hydrohalogenation, hydration, hydrogenation).
  6. Ethanol can be produced by fermentation (renewable) or by acid‑catalysed hydration of ethene (industrial); both routes have pros and cons.
  7. Carboxylic acids (e.g., ethanoic acid) react with bases, metals and alcohols; esterification provides many useful solvents.
  8. Fossil‑fuel chemistry is linked to fertiliser production (N from natural gas) and to air‑quality issues (CO2, SO2, NOx); mitigation involves cleaner technologies and renewable energy.
Suggested diagram: Cross‑section of the Earth showing the formation zones of coal (shallow terrestrial), natural gas (deeper, hotter zones) and petroleum (intermediate depth). Include a simple sketch of a fractional distillation column with temperature gradient.

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