chemistry cracking and Alkenes

What is cracking?

Cracking is the process of breaking down large hydrocarbons molecules into smaller more useful molecules.

→ it breaks bonds in longer hydrocarbons

→ the products include smaller alkanes and alkenes.

Why is cracking needed?

1. there is a higher demand for small hydrocarbons because they are more useful as fuels

2. It produces alkenes which are useful for making polymers and other chemicals

What are the two methods of cracking?

catalytic cracking: uses a high temperature and a catalyst

steam cracking: hydrocarbon vapour mixed with steam and heated to a very high temperature

What is an Alkene?

An alkene is an unsaturated hydrocarbon that contains a double carbon bond (C=C). Alkenes are also more reactive than alkanes

How do you test for an Alkene?

1. add bromine water

2. if an alkene is present, the bromine changes colour from orange/red → colourless

3. this happens because the alkenes react with bromine

What happens in catalytic cracking?

• the hydrocarbon is heated to a high temperature and vapourised

• the vapour is passed over a hot catalyst

• the large hydrocarbon molecules break into smaller molecules

What happens in steam cracking?

• the hydrocarbon is vapourised

• it is mixed with the steam

• the mixture is heated to a very high temperature

• the large molecules break down into smaller ones

What is the aim of the cracking practical?

• to crack a long-chain hydrocarbon and investigate the products formed

• a product can then be tested to see whether an alkene has been produced

Why are alkenes useful?

• they can be used to make polymers

• they can also be used as starting materials to make other chemicals

Why are Alkenes more reactive than Alkanes?

Alkenes contain a C=C bond which is more reactive than the single C-C bond that alkanes contain. This means alkenes react more easily with other substances.