Alkanes as fuels: cracking, reforming and pollution

From crude oil to fuels

• Fractional distillation separates crude oil into fractions by boiling point.
• Cracking breaks long-chain alkanes into shorter, more useful alkanes and alkenes, using heat and a catalyst (for example a zeolite): C10H22 -> C8H18 + C2H4.
• Reforming turns straight-chain alkanes into branched alkanes and cyclic hydrocarbons, which burn more smoothly in engines.

Pollutants from combustion

• Carbon monoxide: from incomplete combustion; toxic because it binds to haemoglobin and reduces oxygen transport.
• Oxides of nitrogen: N2 and O2 from air react at high engine temperatures; acidic and cause acid rain and photochemical smog.
• Sulfur dioxide: from sulfur impurities in fuel; acidic, causes acid rain.
• Carbon particulates (soot): from incomplete combustion; harm lungs.
• Unburned hydrocarbons: contribute to smog.

Catalytic converters reduce CO, NOx and unburned hydrocarbons.

Alternative fuels and carbon neutrality

Burning fossil fuels raises atmospheric CO2, a greenhouse gas linked to climate change. A carbon-neutral fuel has no net annual CO2 emission. Bioethanol takes in CO2 as the crop grows, but energy for farming, fertilisers, processing and transport means it is not fully carbon neutral. Hydrogen produces only water when burned, but making it often uses fossil fuels. Petrol is not carbon neutral.

Worked example

Write an equation for the cracking of decane, C10H22, into octane and one other product.

1. Carbon: 10 - 8 = 2; hydrogen: 22 - 18 = 4.

Answer: C10H22 -> C8H18 + C2H4 (ethene)

Check yourself

1. What are the products of cracking?
   • Shorter alkanes and alkenes
   • Only longer alkanes
   • Carbon dioxide and water
   • Alcohols
   Show answer

   Shorter alkanes and alkenes. Cracking breaks long chains, producing a shorter alkane and at least one alkene.

2. C12H26 is cracked into C8H18 and ethene only. How many molecules of ethene form per molecule of C12H26?
   Show answer

   2. Carbon left: 12 - 8 = 4, which is 2 C2H4; hydrogen check: 26 - 18 = 8 = 2 x 4.

3. How are oxides of nitrogen formed in a car engine?
   • Nitrogen and oxygen from the air react at the high temperature
   • Nitrogen impurities in petrol burn
   • Carbon monoxide reacts with nitrogen
   • Sulfur reacts with nitrogen
   Show answer

   Nitrogen and oxygen from the air react at the high temperature. Petrol contains almost no nitrogen; the high temperature lets N2 and O2 from air combine.

4. Why is carbon monoxide toxic?
   • It causes acid rain
   • It is a greenhouse gas
   • It damages the ozone layer
   • It binds to haemoglobin and reduces the blood's ability to carry oxygen
   Show answer

   It binds to haemoglobin and reduces the blood's ability to carry oxygen. CO combines with haemoglobin more strongly than O2 does.

5. What does reforming do?
   • Splits long alkanes into shorter ones
   • Converts straight-chain alkanes into branched and cyclic hydrocarbons
   • Removes sulfur from fuels
   • Separates crude oil by boiling point
   Show answer

   Converts straight-chain alkanes into branched and cyclic hydrocarbons. Branched and cyclic hydrocarbons burn more efficiently in engines.

6. Why is bioethanol not completely carbon neutral?
   • Burning ethanol releases no CO2
   • Plants release CO2 as they grow
   • Ethanol contains sulfur
   • Fossil fuels are used to farm, process and transport it
   Show answer

   Fossil fuels are used to farm, process and transport it. The CO2 absorbed by the crop balances its combustion, but extra energy inputs add CO2.

7. Which pollutant is produced from impurities in the fuel rather than from the hydrocarbon itself?
   • Carbon monoxide
   • Sulfur dioxide
   • Soot
   • Water vapour
   Show answer

   Sulfur dioxide. Sulfur compounds in crude oil burn to SO2.

Practise with instant feedback Print this lesson

Sources

• Pearson Edexcel IAL Chemistry (2018) scheme of work
• Pearson Edexcel International Advanced Level Chemistry (2018) qualification page

Last updated 7 November 2026. Spotted a mistake? Tell us.