C1bL1 Cracking

Key Words

Alkane - Compound that consists of hydrogen and carbon in which the carbons are linked by single bonds.

Alkene - Compound that consists of hydrogen and carbon in which some of the carbons are linked by double bonds.

Catalyst - A substance that enables a chemical reaction to happen without being used up itself.

Chemical formula - a way of writing down the numbers of each kind of atom in a substance.

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

Hydrocarbons - compounds consisting of hydrogen and carbon.

Plastics - materials made from polymers that can be easily moulded into shape.

Polymer - very long chain molecule made up of many repeating sub-units.

Resource - useful material.

Thermal decomposition - compound breaking down when heated.

Vapour - substance that is in a gaseous state.

Grade E

In Chemistry 1a we saw that crude oil was a mixture of many substances and fuels and oils could be separated by fractional distillation.  More substances can be made by breaking the alkane chains using a process called cracking.

Alkanes are hydrocarbons, molecules consisting of carbon and hydrogen only.  If these are heated to form a hydrocarbon vapour, they can be broken down into smaller molecules, by passing them over a catalyst.  This process is called cracking.  The breaking down of big molecules into smaller ones using heat is called thermal decomposition.  So cracking is a form of thermal decomposition.

When alkanes are cracked, they form shorter chain alkanes and a new kind of hydrocarbon, an alkene.

Grade C

Remember how alkanes were separated by fractional distillation:

The alkanes can be broken to form a short chain alkane and an alkene by cracking.  Alkenes are more reactive than alkanes, and can be used to make a wide range of  useful materials, such as plastics.

Plastics are polymers of small molecules, forming giant molecules consisting of several million atoms joined together.

Grade A

If we crack hexane, we end up with butane and ethene:

The hexane can be cracked into other alkanes and alkenes.  The longer the alkane, the more little molecules of all different types there will be.