Fuels

O Level / IGCSE Chemistry · Organic chemistry. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Coal, natural gas and petroleum are the fossil fuels. Natural gas is mainly methane. Petroleum (crude oil) is a mixture of hydrocarbons, which are compounds that contain hydrogen and carbon only.

Petroleum is separated by fractional distillation. It is heated until most of it is vapour, and the vapour rises up a column that is hot at the bottom and cooler at the top. Each hydrocarbon condenses at the level where the temperature is just below its boiling point. Each group collected is called a fraction.

Small molecules have low boiling points, so they rise to the top. Large molecules condense near the bottom.

Rules to remember

Common mistake

Students say the top of the column is the hottest part. It is the coolest part, so only the smallest molecules with the lowest boiling points reach it.

Worked example

Fraction P contains molecules with 5 to 10 carbon atoms. Fraction Q contains molecules with 20 to 30 carbon atoms. Which fraction is collected higher in the column, and which is more viscous?

  1. P has shorter chains, so the forces between its molecules are weaker and its boiling point is lower.
  2. A lower boiling point means P stays a vapour for longer and condenses higher up, where the column is cooler.
  3. Q has longer chains, so it flows less easily and is the more viscous liquid.

Answer: P is collected higher in the column; Q is more viscous.

Practice questions

Try each one, then open the answer.

1. How do the fractions change going up the fractionating column?

  1. A
    shorter chains, lower boiling points and more volatile
  2. B
    longer chains, higher boiling points and more viscous
  3. C
    higher boiling points, longer chains and less volatile
  4. D
    more viscous, darker in colour and less volatile
Show answer

Answer: A. Small molecules have weak forces between them, so they have low boiling points, are volatile and runny, and rise to the cool top of the column.

2. Which fraction collects at the bottom of the fractionating column and is used for making roads?

  1. A
    gasoline (petrol)
  2. B
    lubricating oil
  3. C
    bitumen
  4. D
    refinery gas
Show answer

Answer: C. Bitumen has the longest chains and highest boiling point, so it stays at the bottom. Refinery gas is at the top.

3. Which list contains only fossil fuels?

  1. A
    coal, wood, hydrogen
  2. B
    hydrogen, coal, petroleum
  3. C
    coal, natural gas, petroleum
  4. D
    ethanol, petroleum, natural gas
Show answer

Answer: C. Coal, natural gas and petroleum formed from ancient living things over millions of years. Wood and ethanol are renewable; hydrogen is not a fossil fuel.

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