Crude Oil and Fuels

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Crude Oil

Crude oil on own not very useful. Mixture of large number of compounds - some useful. Limited resource - used to produce fuels & other chemicals. 

Mixture consists of 2 or more elements or compounds that are not chemically combined together. Properites of the substances in a mixture remain unchanged, so can be seperated by physical methods, such as distillation. 

Most compounds in crude oil consist of molecules made up of only carbon & hydrogen atoms- called hydrocarbons. Hydrocarbon molecules vary in size, - affects properties & how used as fuels. 

Larger the hydrocarbon; 

  • less easily it flows 
  • higher its boiling point
  • less volatile it is 
  • less easily it ignites 
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Fractional distillation

Crude oil can be seperated into different fractions by fractional distillation. Each fraction contains hydrocarbon molecules with a similar number of carbon atoms. Most of the hydrocarbons obtained are alkanes. 

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Ethanol as a fuel

Advantages: 

  • Renewable energy source
  • Sugar cane grows quickly in a country with a hot climate
  • Sugar cane absorbs CO2 from the atmosphere as it grows 

Disadvantages: 

  • Sugar cane can only be grown in countries with a hot climate 
  • CO2 is produced as a product of combustion
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Hydrogen as a fuel

Advantages: 

  • Water is the only product of combustion so its a 'clean' fuel
  • Water is potentially a source of plentiful supplies of hydrogen 

Disadvantages: 

  • There are currently no 'low energy' ways to extract hydrogen from water in large quantities
  • Hydrogen is a gas, so is difficult to store in large quantities
  • When hydrogen is mixed with air and ignited its explosive, so there are safety issues to consider
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Alkanes (Saturated Hydrocarbons)

'Spine' of a hydrocarbon - made up of chain of carbon atoms. When chains are joined together by single carbon-carbon bonds - hydrocarbon saturated - alkane. 

  • Hydrogen atoms make one bond each 
  • Carbon atoms can make four bonds each 
  • Simplest alkane, methane, made up of 4 hydrogen atoms and 1 carbon atom 

General formula for alkanes is: CnH2n+2

Carbon atoms in alkenes are linked to 4 other atoms by single bonds - alkane saturated - alkanes fairly reactive, burn wel. 

Shorter-chain hydrocarbons release energy more quickly by burning - greater demand for them as fuels

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Burning fuels

Most fuels contain carbon & hydrogen, many also contain sulfur. Fuels burn - produce waste products, which are released into atmosphere. 

Combustion of hydrocarbon fuel releases energy. During combusition, both carbon & hydrogen oxidised. If combustion not complete, solid particles containing soot & unburnt fuels may be released. Due to high temperature reached when fuels burned, nitrogen in air can react with oxygen - nitrogen oxides formed. Can cause acid rain. 

Carbon dioxide causes global warming due to greenhouse effect. Solid particles in air cause global dimming. 

Sulfur can be removed from fuel before burning. Sulfur dioxide can be removed from waste gases after combustion - both processes add to costs. 

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