9780008160869

Page 138

Organic Side Header Chemistry

Cracking Hydrocarbons You must be able to: Describe the cracking of alkanes Describe how bromine water can be used to differentiate between an alkane and an alkene.

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Cracking Hydrocarbons • • • •

Longer-chain hydrocarbons can be broken down into shorter, more useful hydrocarbons. This process is called cracking. Cracking is a useful industrial process. The two main methods of cracking are steam cracking and catalytic cracking. During steam cracking: – the hydrocarbons are mixed with steam – they are heated to 850°C. During catalytic cracking: – the hydrocarbons are heated until they vaporise – the vapour is passed over a hot aluminium oxide catalyst – a thermal decomposition reaction then takes place. The products of cracking include alkanes and another type of hydrocarbon called alkenes.

Key Point Cracking breaks down long-chain hydrocarbons into useful short-chain alkanes and reactive alkene molecules.

Aluminium oxide or broken pot (catalyst) Gaseous short-chain hydrocarbon molecules (alkene)

Heat Long-chain hydrocarbon – liquid paraffin (alkane) soaked into ceramic wool

Liquid short-chain hydrocarbon molecules (alkane)

Cold water

No atoms are made or destroyed during the cracking reaction, so the chemical equation must always balance. heat + catalyst

long-chain alkane decane C10H22

138

short-chain alkane + alkene octane + ethene C8H18 + C 2 H4

The total number of carbon atoms and the total number of hydrogen atoms must be the same on both sides of the equation.

GCSE Combined Science Revision Guide

227841 AQA Higher Tier Combined Science All-in-One_P120_157.indd 138

6/29/16 12:56 PM


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