EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 复习笔记:4.2.4 Cracking

EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 复习笔记:4.2.4 Cracking

Cracking

 

  • Saturated molecules contain single bonds only whereas unsaturated molecules contain double bonds between their carbon atoms
  • Alkanes are saturated compounds and alkenes are unsaturated compounds
  • Long chain alkane molecules are further processed to produce other products consisting of smaller chain molecules
  • A process called cracking is used to convert them into short chain molecules which are more useful
  • Small alkenes and hydrogen are produced using this process
  • Kerosene and diesel oil are often cracked to produce petrol, other alkenes and hydrogen
  • There are two methods used to crack alkanes: catalytic cracking and steam cracking
  • As the names suggest, one method uses a catalyst and the other uses steam

 

 

Cracking-Decane_

 

Decane is cracked to produce octane for petrol and ethene for ethanol synthesis

 

 

 

  • Catalytic cracking involves heating the hydrocarbon molecules to around 470 – 550°C to vaporise them
  • The vapours then pass over a hot powdered catalyst of aluminium oxide
  • This process breaks covalent bonds in the molecules as they come into contact with the surface of the catalyst, causing thermal decomposition reactions
  • The molecules are broken up in a random way which produces a mixture of smaller alkanes and alkenes
  • Hydrogen and a higher proportion of alkenes are formed at higher temperatures and higher pressure

 

Fraction Supply & Demand

 

  • Crude oils vary considerably in their composition and some need more refining than others
  • Supply is how much of a particular fraction can be produced from refining the crude oil
  • Demand is how much customers want to buy
  • General the demand for certain fractions outstrips the supply so this is why cracking is necessary to convert surplus unwanted fractions into more useful ones
  • This is mostly larger, heavier fractions that are cracked into smaller lighter fractions

 

4.2.4-Supply-_-Demand-of-Crude-oil-Fractions

 

Supply & demand graph for crude oil fractions

  • You can see from the chart that fuel oil and bitumen are surplus fractions so they are cracked and modified to produce petrol, kerosene and diesel

 

Exam Tip

Remember that cracking is an endothermic reaction.

转载自savemyexams

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