Crude Oil and Hydrocarbons Notes

AQA GCSE Chemistry: Revision notes

Key facts

  • Crude oil is a finite resource: a mixture of hydrocarbons, mostly alkanes (CnH2n+2\mathrm{C}_n\mathrm{H}_{2n+2}).
  • Fractional distillation separates it into fractions using differences in boiling point.
  • Fractions become fuels and feedstock for the petrochemical industry.
  • As molecules get bigger: boiling point rises, viscosity rises and flammability falls.
  • Complete combustion of a hydrocarbon makes carbon dioxide and water only.

What is crude oil?

Crude oil is a finite mixture of hydrocarbons found in rock.

Crude oil is a finite resource. It is a mixture, so its parts can be separated by physical methods without any chemical reaction.

It contains mostly hydrocarbons: molecules made of only hydrogen and carbon. Most are alkanes. The first four are methane, ethane, propane and butane.

−160−140−120−100−80−60−40−20MethaneEthanePropaneButaneAlkaneBoiling point (°C)
Boiling points of the first four alkanes (approximate, °C): bigger molecules boil at higher temperatures.
  • Alkane formulaCnH2n+2\mathrm{C}_n\mathrm{H}_{2n+2}

Which of these is a hydrocarbon?

Fractional distillation

Hydrocarbons with different boiling points condense at different heights in a column.

Fractional distillation separates crude oil into fractions, each a mixture of hydrocarbons with similar boiling points.

The fractions are processed into fuels (petrol, diesel oil, kerosene, heavy fuel oil, liquefied petroleum gases) and feedstock for the petrochemical industry, which makes solvents, lubricants, polymers and detergents.

  1. 1

    Heat

    crude oil is heated until it evaporates

  2. 2

    Rise

    vapours rise up a column, hot at the bottom and cooler at the top

  3. 3

    Condense

    each hydrocarbon condenses at a height set by its boiling point

  4. 4

    Collect

    each liquid is collected as a separate fraction

Fractional distillation of crude oil

What property lets crude oil be separated by fractional distillation?

Properties across fractions

As molecules get bigger, boiling point and viscosity rise but flammability falls.

Bigger molecules have stronger forces between them, so they are harder to boil. Viscosity (how thick a liquid is) also rises, and the liquid becomes harder to ignite.

So petrol suits an easily ignited, free-flowing fuel, and heavy fuel oil suits large-scale burning.

Short chains

Boiling point:
Low
Viscosity:
Low
Flammability:
High

Long chains

Boiling point:
High
Viscosity:
High
Flammability:
Low
−150−100−50050MethaneEthanePropaneButanePentaneHexaneAlkaneBoiling point (°C)
Boiling points of the first six alkanes rise with chain length

Which fraction has the highest viscosity?

Burning hydrocarbons

With plenty of oxygen they burn completely, giving only carbon dioxide and water.

Complete combustion needs a good supply of oxygen.

hydrocarbon + oxygen → carbon dioxide + water

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

Always check that the atoms balance on both sides.

  1. 1

    Hydrocarbon and plenty of oxygen

  2. 2

    Burn completely

  3. 3

    Carbon dioxide and water made

  4. 4

    Energy released as heat

Complete combustion of a hydrocarbon.
  • Hydrocarbon + oxygencarbon dioxide + water

Worked example

Write the balanced equation for the complete combustion of propane.

What are the only products of complete combustion of a hydrocarbon?

Try an exam question

Describe how crude oil is separated into fractions in a fractionating column.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.