Edexcel A Level Chemistry:复习笔记3.2.2 Alkane Fuels

Alkane Fuels

 

  • Alkanes are obtained from the fractional distillation and cracking of crude oil

Fractional distillation of crude oil

  • Crude oil is a mixture of hydrocarbons containing alkanes, cycloalkanes and arenes (compounds with a benzene ring)
  • The crude oil is extracted from the earth in a drilling process and transported to an oil refinery
  • At the oil refinery the crude oil is separated into useful fuels by fractional distillation
    • This is a separating technique in which the wide range of different hydrocarbons are separated into fractions based on their boiling points

Fractional-Distillation

Crude oil is initially separated into fractions with similar boiling points in a process called fractional distillation

 

Cracking of crude oil fractions

  • However, the smaller hydrocarbon fractions (such as gasoline fractions) are in high demand compared to the larger ones
  • Therefore, some of the excess heavier fractions are broken down into smaller, more useful compounds
  • These more useful compounds include alkanes and alkenes of lower relative formula mass (Mr)
  • This process is called cracking

3.2-Hydrocarbons-Cracking

The heavier fractions that are obtained in fractional distillation are further cracked into useful alkane and alkenes with lower Mr values

 

  • When a large hydrocarbon is cracked, a smaller alkane and alkene molecules are formed
    • E.g.. octane and ethene from decane

     

3.2-Hydrocarbons-Cracking-of-Long-Alkanes

Long hydrocarbon fraction is cracked into two smaller ones

 

  • The low-molecular mass alkanes formed make good fuels and are in high demand
  • There are two types of cracking:
  • Thermal cracking requires high temperatures (up to 1000 oC) and high pressure (up to 70 atmospheres) and produces alkanes and a lot of alkenes
  • Catalytic cracking uses a lower temperature (around 450 oC) and slight pressure in the presence of a catalyst such as a zeolite or aluminium oxide to produce mainly aromatic hydrocarbons

Reforming alkanes

  • Many vehicles run on petrol or diesel which are both a mixture of alkanes along with other hydrocarbons, impurities and additives
  • Many of the alkanes in these fuels are straight chain alkanes
  • These straight chain alkanes are more likely to explode, rather than combust, inside the engine
    • This is known as knocking and makes the combustion less efficient
  • To reduce this straight chain alkanes are reformed into:
    • Branched alkanes, e.g. octane → 2,5-dimethylhexane

CH3CH2CH2CH2CH2CH2CH2CH3 → CH3CH(CH3)CH2CH2CH(CH3)CH3

    • Cycloalkanes, e.g. hexane → cyclohexane

CH3CH2CH2CH2CH2CH3 → C6H12 + H2

  • Reforming often uses a platinum catalyst

 

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