C1aL16  Crude Oil

Key Words

Boiling Point - temperature at which a liquid boils to form a gas.

Compounds - substances mad of two or more elements.

Covalent - Bonds that share electrons.

Crude Oil - Stick brown liquid pumped from the ground..

Fractional distillation - process that separates hydrocarbons from the mixture in crude oil.

Fractionating Column - a tower in which fractional distillation takes place.

Hydrocarbons - compounds consisting of hydrogen and carbon.

Petrol - a fuel used by petrol (spark ignition) engines in cars.

 

Test Yourself

Homework
Chemistry GCSE
Home

Grade E

Crude oil is a nasty, sticky liquid that comes from the ground.  It was formed by millions of tiny organisms that fell to the bottom of warm seas millions of years ago.  As such it's really not much use at all, but it can be turned into a range of very useful products.  The petrochemical industry does this.

Crude oil is a mixture of hydrocarbons, compounds formed of hydrogen and carbon.  Some hydrocarbons form very small molecules.  Methane (CH4) is the smallest hydrocarbon possible.  Other hydrocarbons are very big molecules, consisting of chains of 50 carbon atoms, or more.  This is summed up in the table:

Fraction Number of Carbon atoms Use
Petroleum gas 1 to 4 heating and cooking
Naphtha 5 to 9 making other chemicals
Petrol 5 to 10 fuel for cars and light aeroplanes
Kerosene (paraffin) 10 to 16 fuel for jet or turbine aeroplanes
Diesel 14 - 20 Diesel fuel or heating
Oil 20 - 50 Lubricating oil
Bitumen 50 or more Making roads

 The fractions in the table are separated in a huge tower called a fractional distillation column, as shown in the picture:

PAUL RAPSON / SCIENCE PHOTO LIBRARY

Each fraction consists of hydrocarbons that have similar boiling points.

 

Grade C


Fractional distillation is the process in which fractions are separated in a column like this:

The crude oil is heated strongly.  The smaller molecules have a lower boiling point, so they go to the top of the column and condense out onto the trays, before being removed for further processing.  The longer molecules have higher boiling points, so condense further down the column.

Since crude oil is a mixture, no chemical reactions are happening:

  • Within the hydrocarbon molecules, there are strong covalent bonds.  However the forces between the molecules are quite small.  The boiling point depends on these.

  • Short hydrocarbon chains have weak intermolecular bonds, so the boiling point is low.

  • Between longer chain molecules, the bonds are stronger, so the boiling point is higher.

 

Grade A

Long chain molecules tend to align with each other in parallel chains.  This makes the bonding between the molecules a lot stronger, so that a higher temperature is needed to boil the liquid.