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K324 / 2027
Organic Chemistry

Lesson 1 of 9

Fuels, fractions and bioethanol

Follow carbon from a resource to useful products.

O-Level 6092 (2026) / SEC G3 K324 (2027)

  • I can identify non-renewable fuel sources.Syllabus

    Syllabus K324, 11.1(a). Identify non-renewable fuel sources

  • I can explain crude-oil fractionation.Syllabus

    Syllabus K324, 11.1(b). Explain crude-oil fractionation

  • I can recognise renewable biofuel.Syllabus

    Syllabus K324, 11.1(c). Recognise renewable biofuel

  • I can compare carbon impacts of fuels.Syllabus

    Syllabus K324, 11.1(d). Compare carbon impacts of fuels

  • I can describe homologous series.Syllabus

    Syllabus K324, 11.2(a). Describe homologous series

  • I can explain cracking.Syllabus

    Syllabus K324, 11.2(h). Explain cracking

Fuels, fractions and bioethanol

Follow carbon from a resource to useful products.

Make a guess. You are not marked.

Petrol molecules have about 5 to 10 carbon atoms. Bitumen molecules have more than 70. Which fraction has the higher boiling point?

  1. Petrol, as its smaller molecules pack closer
  2. Bitumen, as its larger molecules attract more
  3. Neither; they boil at the same temperature
Show the answer

Bitumen, as its larger molecules attract more

Bigger molecules have stronger attractive forces between them, so more energy is needed to separate them and they boil at a higher temperature.

A hydrocarbon contains carbon and hydrogen only. Natural gas is mainly methane; crude oil is a mixture of hydrocarbons. Both are non-renewable because they form far more slowly than they are consumed. Burning a fuel releases energy, while the same hydrocarbon feedstock may also be valuable for manufacturing chemicals and plastics.

Fractionate crude oil
  1. Heat the oil

    Many components vaporise.

  2. Vapour rises through the column

    The column is hotter at the bottom and cooler at the top.

  3. Condense different fractions

    Higher-boiling, generally larger molecules condense lower down; smaller, more volatile molecules travel higher.

  4. Collect useful mixtures

    Each fraction contains hydrocarbons with a range of boiling points, not necessarily one pure compound.

Crude oil fractions, from the top of the column to the bottom
FractionMolecules and boiling rangeMain use
Petroleum gasSmallest molecules (1 to 4 C); lowest boiling pointsFuel for cooking and heating
Petrol (gasoline)Small moleculesFuel for cars
NaphthaSmall to medium moleculesFeedstock for making chemicals and plastics
Paraffin (kerosene)Medium moleculesFuel for aircraft
DieselMedium to large moleculesFuel for lorries, buses and trains
Lubricating oilLarge moleculesLubricants, waxes and polishes
BitumenLargest molecules; highest boiling points; very viscousRoad surfaces and roofing

As hydrocarbon molecules get larger, melting and boiling points generally rise overall, viscosity increases, and volatility decreases. Smaller fractions are useful fuels; larger fractions may be cracked into more sought-after smaller molecules. The amount supplied by fractional distillation need not match the demand for each fraction. Larger molecules generally have stronger intermolecular attractions, so more energy is needed to separate them and the liquid resists flow more. These are trends within comparable members, not a claim that every individual melting point rises smoothly.

Bioethanol can be made from sugars in a crop such as sugarcane and is renewable if the crop is regrown. Growing plants take in carbon dioxide, partly offsetting carbon dioxide released when the ethanol burns. Fossil fuels instead add carbon stored underground to the active carbon cycle. Biofuel is not automatically emission-free: cultivation, processing, transport and land-use change also affect its impact.

Check your understanding

Why is a crude-oil fraction not a pure substance?

  1. It still contains water and sand that the distillation column cannot remove.
  2. It is a mix of hydrocarbons with similar boiling points that condense together.
  3. Its molecules break apart in the hot column, so new compounds keep forming.
Show the answer

It is a mix of hydrocarbons with similar boiling points that condense together.

Fractional distillation groups compounds by boiling range. Several hydrocarbons boil in the same range, so each fraction is still a mixture.

Check your understanding

Why are natural gas and crude oil described as non-renewable?

  1. They took millions of years to form and are used far faster than they form
  2. They release carbon dioxide into the air whenever they are burned as fuel
  3. They are mixtures of many different hydrocarbons that must be separated
  4. They are found deep underground, so they have to be drilled for
Show the answer

They took millions of years to form and are used far faster than they form

A non-renewable resource cannot be replaced within a human lifetime. Fossil fuels formed from ancient remains over millions of years, so once used they are gone.

Check your understanding

Compared with the bitumen fraction, the petrol fraction has:

  1. smaller molecules, lower boiling points and lower viscosity
  2. larger molecules, higher boiling points and higher viscosity
  3. larger molecules, lower boiling points and higher flammability
  4. smaller molecules, higher boiling points and lower viscosity
Show the answer

smaller molecules, lower boiling points and lower viscosity

Small molecules have weaker attractions between them, so they boil at lower temperatures and flow easily. Petrol condenses near the top of the column.

Check your understanding

Bioethanol is made from sugarcane. Why can burning it add less CO2 overall than burning petrol?

  1. Ethanol contains no carbon atoms, so burning it gives water but no carbon dioxide.
  2. Ethanol burns at a lower temperature, so less of its carbon turns into carbon dioxide.
  3. The growing cane took in CO2, which partly offsets the CO2 released on burning.
Show the answer

The growing cane took in CO2, which partly offsets the CO2 released on burning.

Sugarcane absorbs CO2 for photosynthesis, so burning its ethanol returns carbon recently taken from the air. Petrol releases carbon locked away for millions of years. Farming and transport still use fuel, so it is not fully carbon neutral.

Exam-style question

A refinery records the percentage of its crude oil that ends up in two fractions, and the percentage its customers want.
Petrol: supply 15%, demand 35%.
Fuel oil (large molecules): supply 40%, demand 20%.
(a) Use the data to explain why the refinery cracks some fuel oil. [2]
(b) A molecule of C12H26 is cracked to give octane, C8H18, and ethene only. Write the balanced equation. [1] [3]

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