Hydrocarbons and the alkanes

Crude oil formed over millions of years from the remains of tiny sea creatures buried in mud. It is a finite resource and a mixture of a huge number of compounds. Most of them are hydrocarbons: compounds made of hydrogen and carbon atoms only.

Most of the hydrocarbons in crude oil are alkanes. The alkanes form a homologous series: a family with the same general formula, where each member has one more CH2 than the last, and all react in similar ways.

alkanes: CnH2n+2

  • 1 carbon: methane, CH4
  • 2 carbons: ethane, C2H6
  • 3 carbons: propane, C3H8
  • 4 carbons: butane, C4H10

The start of the name counts the carbons: meth 1, eth 2, prop 3, but 4. The ending -ane means an alkane. Alkanes are saturated: every carbon to carbon bond is a single bond.

  1. An alkane in petrol has 8 carbon atoms. What is its formula?
  2. n = 8, so H = 2 × 8 + 2 = 18
  3. Formula: C₈H₁₈ (octane)

Double first, then add 2. A common slip is 8 + 2 = 10 hydrogens.

Count the hydrogens

Use the general formula: double the carbons, then add 2.

Splitting crude oil into fractions

Crude oil is separated by fractional distillation. Molecules with different chain lengths have different boiling points, so they condense at different temperatures.

hot crude oil vapourrefinery gasespetrolkerosenediesel oilheavy fuel oilbitumencool (about 25 °C)hot (about 350 °C)vapoursrise
  1. Heat the crude oil so most of it evaporates
  2. The vapour enters near the bottom of the column, which is hot at the bottom and cool at the top
  3. The vapours rise and cool
  4. Each hydrocarbon condenses where the temperature drops below its boiling point
  5. Fractions (groups of similar chain length) are piped off at different heights

A fraction is still a mixture, but of hydrocarbons with similar numbers of carbon atoms and similar boiling points.

Chain length decides the properties

The longer the molecule, the stronger the forces between molecules. So as chain length increases:

  • boiling point goes up (they condense lower down the column)
  • viscosity goes up (thicker, slower to pour)
  • flammability goes down (harder to light)

That is why short chains make good fuels. Uses: refinery gases for bottled gas, petrol for cars, kerosene for aircraft, diesel oil for lorries and trains, heavy fuel oil for ships, bitumen for roads.

"Higher up the column" means lower boiling point. The top of the column is the cool end.