Fuels and crude oil

What is organic chemistry?

It is the study of complex carbon compounds with the exception of carbon dioxide and carbon monoxide. Examples of carbon compound are plastic, oil, nylon.

Organic compounds mainly contain hydrogen and carbon and some contain oxygen and nitrogen.

Hydrocarbons are organic compounds that ONLY contain carbon and hydrogen. Alkane and alkene are hydrocarbons.

Organic compounds can be grouped into four families/homologous series:

  1. alkane
  2. alkene
  3. alcohol
  4. carboxylic acid


Petroleum is a mixture of hydrocarbons and must be separated by fractional distillation. Different hydrocarbons have different uses.

Steps that occur during fractional distillation

  1. The crude oil is heated and the liquid evaporate.
  2. The column is cooler at the top.
  3. As the vapour rise up the column, they cool.
  4. Each fraction condenses at a different temperature, the fraction with the lowest boiling point will condense at the top while the fraction with the highest boiling point will condense at the bottom. The more the carbon atoms in a compound, the higher the boiling point.
  5. All the hydrocarbons with different boiling point will condense at the same time and the fractions contain mainly alkanes.
Fractions Boiling point range/°C Uses
1 Petroleum Gas Around 25 Fuel for cooking and heating
2 Petrol (Gasoline) 30 - 90 Fuel for vehicles
3 Naptha 90 - 150 Feedstock (raw materials) for making petrochemicals
4 Paraffin (Kerosene) 150 - 230 Fuel for aircraft engines, cooking and heating
5 Diesel 230 - 340 Fuel for diseal engines in buses, lorries and trains
6 Lubricating oil 400 - 500 Lubricating machines and making waxes and polishes
7 Bitumen More than 500 Making road surfaces and roofing

Some larger long chain hydrocarbons are in lower demand compared to smaller short chain hydrocarbons. Some fractions from the fractional distillation of crude oil can be further converted to other useful products via cracking.


Cracking is a process whereby large long chain alkane molecules are broken down to a mixture of smaller, short chain alkanes and sometimes hydrogen. During cracking, an alkene is always produced.

Conditions for cracking:

  1. Temperature: 600°C
  2. Aluminium oxide or silica (sand) *sometimes porous pot is used in lab as it contains both silica and aluminium oxide

Purpose of cracking

  1. To provide smaller longer chain hydrocarbons that are in higher demand.
  2. To provide a source of alkene.
  3. To provide hydrogen for the Haber process to manufacture ammonia.

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