Cracking Hydrocarbons

 

 

So what is CRACKING?

It is when big hydrocarbon molecules are broken down into smaller ones.  This is done by heating the hydrocarbon in the presence of a catalyst.  This is a form of thermal decomposition.  The catalyst helps to speed up the reaction.

Hot vapour of hydrocarbons is passed over powdered aluminium oxide catalyst at roughly 400ºC - 700ºC.  The long carbon chain molecules break apart or 'crack' on the surface of the powered catalyst.

Cracking produces short-chain hydrocarbons called alkanes, like petrol and  alkenes, like ethene and propene.  These alkenes are very reactive due to their double bonds and therefore can be used to make other substances.  Ethene, for example can be used to make ethanol and polythene.

 

🧪 Knowledge Check: Cracking Hydrocarbons

Test your understanding of how large hydrocarbons are broken down into smaller, useful molecules.

1. What type of chemical reaction is cracking?

• Combustion
• Thermal decomposition
• Neutralisation

2. Why are large hydrocarbon molecules broken down during the cracking process?

• Because they are too small to be used as fuels.
• Because they are more useful than smaller molecules.
• Because there is a higher demand for smaller molecules like petrol.

3. Which two substances are typically produced when a large alkane is cracked?

• A smaller alkane and an alkene
• Water and carbon dioxide
• Two larger alkanes

4. What two conditions are necessary for the cracking process to take place?

• Low temperature and water
• High temperature and a catalyst
• High pressure and oxygen

5. Which of these is a common use for the alkenes produced during cracking?

• Making plastics
• Lubricating heavy machinery
• Surfacing roads

Click to Reveal Answers

1. Thermal decomposition (Breaking down a substance using heat).
2. Higher demand for smaller molecules (Small molecules like petrol are more useful and in shorter supply).
3. A smaller alkane and an alkene (Cracking splits the long chain into these two types of molecules).
4. High temperature and a catalyst (These are required to break the strong chemical bonds).
5. Making plastics (Alkenes are valuable raw materials for the plastics industry).

 

Tags:Hydrocarbons, Cracking, Petrol, Ethane, Molecules, Diesel, Alkenes, the organic structure methane has what formula, Catalyst