Fuels Notes

Edexcel GCSE Chemistry: Revision notes

Key facts

  • Hydrocarbons contain carbon and hydrogen only. Crude oil is a finite mixture of them.
  • Fractional distillation separates crude oil by boiling point.
  • Longer chains have higher boiling points, are harder to ignite and are more viscous.
  • Incomplete combustion makes toxic carbon monoxide and soot.
  • Cracking turns long alkanes into shorter, more useful alkanes and alkenes.

Hydrocarbons and crude oil

Crude oil is a finite mixture of hydrocarbons, mostly alkanes.

Hydrocarbons are compounds of carbon and hydrogen only. Crude oil is a complex mixture of them, mostly alkanes, in chains or rings.

It is a source of fuels and of feedstock for the petrochemical industry. It is a finite resource: it cannot be replaced once used.

HHHHC
Methane, CH₄, the simplest hydrocarbon: carbon bonded only to hydrogen.
  • Hydrocarboncarbon and hydrogen only

Which elements does a hydrocarbon contain?

Fractional distillation

Fractional distillation separates crude oil into fractions because chains of different length have different boiling points.

The oil is heated and the vapour enters a fractionating column that is hot at the bottom and cooler at the top. Each fraction condenses at its own level and is drawn off.

Hydrocarbons with higher boiling points condense lower down. Fuel oil is used for large ships and some power stations, and bitumen for roads and roofs.

  1. 1

    Heat

    crude oil is heated until most of it turns to vapour

  2. 2

    Rise and cool

    vapours rise up the column, which is cooler towards the top

  3. 3

    Condense

    high boiling point fractions condense lower down; low boiling point ones rise further

  4. 4

    Collect

    each fraction is drawn off at its own level

Fractional distillation of crude oil

Gases

  • Domestic heating and cooking

Petrol

  • Fuel for cars

Kerosene

  • Fuel for aircraft

Diesel oil

  • Fuel for some cars and trains

Where in the column is a fraction with a high boiling point collected?

Fractions and homologous series

Longer chains, lower down the column, have higher boiling points, are more viscous and are harder to ignite.

Most hydrocarbons in crude oil are in the alkane homologous series. Members share a general formula, differ by CHX2\ce{CH2} between neighbours, change gradually in physical properties and have similar chemical properties.

From the top of the column to the bottom, molecules have more carbon and hydrogen atoms.

−150−100−5005010012345678Number of carbon atomsBoiling point (°C)
Boiling points of the first alkanes: they rise with chain length, so longer chains condense lower down the fractionating column.

Top of column

Molecules:
Short chains
Boiling point:
Lower
Ignition:
Easier
Viscosity:
Runny

Bottom of column

Molecules:
Long chains
Boiling point:
Higher
Ignition:
Harder
Viscosity:
Thick

Neighbouring members of the alkane homologous series differ by...

Burning fuels

Plenty of oxygen gives carbon dioxide and water; too little oxygen gives toxic carbon monoxide and soot.

Complete combustion needs plenty of oxygen:

CHX4+2 OX2→COX2+2 HX2O\ce{CH4 + 2O2 -> CO2 + 2H2O}

Incomplete combustion has limited oxygen and can make carbon monoxide and soot (carbon). Carbon monoxide is colourless and odourless, and binds to haemoglobin so blood cannot carry oxygen.

A yellow, sooty flame on a faulty boiler signals incomplete combustion.

  1. 1

    Fuel burns

    A hydrocarbon reacts with oxygen

  2. 2

    Plenty of oxygen

    Complete: carbon dioxide and water

  3. 3

    Limited oxygen

    Incomplete: carbon monoxide, soot (carbon) and water

The amount of oxygen decides the products.

Complete combustion

Oxygen:
Plenty
Products:
Carbon dioxide and water

Incomplete combustion

Oxygen:
Limited
Products:
Carbon monoxide and/or carbon (soot), as well as water

Carbon monoxide

Cause:
Incomplete combustion
Problem:
Toxic: stops blood carrying oxygen

Sulfur dioxide

Cause:
Sulfur impurities in fuel burn
Problem:
Dissolves in rain to cause acid rain

Oxides of nitrogen

Cause:
Air heated in an engine
Problem:
Pollutants

Which gas produced by incomplete combustion is toxic?

Cracking and hydrogen

Cracking turns long alkanes, which are in surplus, into shorter and more useful alkanes and alkenes.

Cracking breaks large saturated hydrocarbons into smaller molecules, some of them unsaturated alkenes. It is needed because distillation gives more long chains than are in demand, and more demand for short-chain fuels such as petrol. Alkenes are feedstock for polymers.

Petrol, kerosene, diesel oil and methane are non-renewable fossil fuels.

  1. 1

    Long-chain alkane

    large and saturated

  2. 2

    Cracking

    breaks the molecule apart

  3. 3

    Shorter alkane

    a more useful fuel

  4. 4

    Alkene

    unsaturated, used to make polymers

Cracking

Hydrogen: advantages

  • Only water is made when it burns
  • Can be made from renewable sources

Hydrogen: disadvantages

  • Hard to store and transport safely
  • Little refuelling infrastructure
  • Production often uses fossil fuels

Why is cracking needed?

Try an exam question

Evaluate the use of hydrogen instead of petrol as a fuel for cars.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.