AS chemistry unit one point 6

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ghoran
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254394
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AS chemistry unit one point 6
Updated:
2013-12-28 14:14:13
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alkanes
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unit one
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revision
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  1. alkanes are
    saturated hydrocarbons
  2. petroleum is a
    mixture consisting of mainly of alkane hydrocarbons
  3. different fractions of petroleum can be drawn off at different levels in a fractionating column because of the
    temperature gradient
  4. cracking involves the
    breaking of C-C bonds
  5. cracking takes place at
    high temperature and pressure and produces a high percentage of alkenes
  6. catalytic cracking takes place at
    slight pressure , high temperature and in the presence of a zeolite catalyst and is used mainly to produce motor fuels and aromatic hydrocarbons
  7. alkanes are used as
    fuels
  8. pollutants can be removed using
    catalytic converters
  9. hydrocarbons containing sulphur leads to
    sulphur dioxide that causes air pollution
  10. sulphur dioxide can be removed from flue gases using
    calcium oxide
  11. the combustion of fossil fuels (including alkanes) results in the release of
    carbon dioxide to the atmosphere
  12. carbon dioxide , methane and water vapour are referred to as
    greenhouse gases and that these gases may contribute to global warming
  13. crude oil isn't very useful as it , but you can separate it into more useful bits (or fractions) by fractional distillation . Here's how fractional distillation works
    • first the crude oil is vaporised at about 3500
    • the vaporised crude oil goes into the bottom of the fractionating column and rises up through the trays 
    • the largest hydrocarbons don't vaporise at all , because their boiling points are too high - they just run to the bottom and form a gooey residue 
    • as the crude oil vapour goes up the fractionating column it gets cooler creating temperature gradient 
    • because boiling points of alkanes increase as the molecules get bigger , each fraction condenses at a different temperature . the fractions are therefor drawn off at different levels in the column .
    • the hydrocarbons with the lowest boiling points don't condense . they're drawn off as gases at the top of the column
  14. draw a diagram of a fractionating column showing the different fractions
  15. what are gases used for
    camping gas
  16. what is gasoline used for
    petrol fuel for cars
  17. what is naphtha used for
    chemical feedstock
  18. what is kerosene (paraffin used for)
    • jet fuels 
    • central heating fuel
  19. what is gas oil used for
    • diesel fuel for cars 
    • central heating fuel
  20. what is fuel oil used for
    fuel for power stations and ships
  21. what is bitumen used for
    road surfacing
  22. what are the characteristics of the small molecules
    • low boiling points 
    • volatile 
    • flow easily 
    • ignite easily
  23. what are the characteristics of the large molecules
    • high boiling points 
    • not very volatile 
    • don't flow easily 
    • don't ignite easily
  24. the fractional distillation of petroleum produces a range of fractions with different boiling points. the mixture of fractions obtained often contain
    a higher proportion of heavier fractions than needed . the petrochemical industry carriers out the processing of these heavier fractions in order to make useful ones which are perhaps in higher demand than supply
  25. cracking is a
    decomposition reaction which involves the breaking of C-C bonds . this results in the formation of shorter chain alkanes and alkenes
  26. the shorter alkanes are more useful as fuels than the original alkane because they are
    more volatile (evaporate more readily) and burn more easily .
  27. the alkenes are used
    as raw materials for making polymers
  28. show the cracking of heptane in displayed formula
  29. there are two ways that cracking can be carried out
    • thermal 
    • catalytic
  30. whats the raw material for thermal cracking
    long chain alkane
  31. whats the raw material for catalytic cracking
    long chain alkane
  32. wat temperature does thermal cracking take place at
    800-9000C
  33. what pressure does thermal cracking take place at
    up to 7000kPa
  34. what catalyst is used in thermal cracking
    none
  35. what are the products of thermal cracking
    alkene , short chain alkane
  36. what are the uses of the products from thermal cracking
    making polymers
  37. what are the important notes about thermal cracking
    • air is excluded from this process 
    • heating is carried out for less than a minute to prevent total decomposition
  38. what temperature does catalytic cracking occur at
    5000C
  39. what pressure does catalytic cracking occur at
    slightly above atmospheric pressure
  40. what catalysts are used in catalytic cracking
    silica and aluminium oxide or zeolite
  41. what are the products of catalytic cracking
    aromatic hydrocarbons
  42. what notes
  43. what are the important notes about catalytic cracking
    • more efficient than thermal cracking 
    • produces more branched cyclic , and aromatic hydrocarbons
  44. economic reasons for cracking
    • the factional distillation produces disproportionate amounts of some of the fractions 
    • the amount of petrol produced isn't enough for our needs 
    • too much of the naphtha fraction is produced 
    • the cracking process helps us to make maximum use of petroleum by converting the less useful fractions into more useful ones . this is an essential process , as petroleum is a nonrenewable resource
  45. petrol is a mixture of
    hydrocarbons
  46. petrol is a mixture of hydrocarbons :
    • petrol manufacturers mix hydrocarbons to obtain petrol with the right properties 
    • the petrol must vaporise readily enough to ignite the engine 
    • it must not be so volatile that excess vapour enters the engine 
    • it must not ignite in the cylinders of the engine too early 
    • these properties are temperature dependant
  47. longer chains are not so volatile so are
    used for when the external temperature is warmer . winter blends would vaporise to readily in the summer
  48. you may have heard of the term octane rating . this tells us
    how easily the petrol ignites
  49. a low octane rating implies
    that a fuel will readily auto-ignite . this means that petrol ignites in the engine before it should . this causes a knocking sound and can lead to engine damage
  50. processing hexane forms
    three different molecules , each of which has a unique octane rating
  51. by processing hexane (low octane rating) we can get
    • 2) 3-methylpentane 
    • 1) cylcohexane - which can be reformed to produce benzene 3)
  52. hexane
  53. cyclohexane
  54. benzene
  55. for complete combustion to occur
    an unlimited supply of oxygen is needed
  56. complete combustion equation
    • alkane + oxygen --> carbon dioxide + water 
    • + a release of energy
  57. for methane (the main fossil fuel in natural gas) the complete combustion equation is
    CH4 (g) + 2O2(l) --> CO2(g) + 2H2O(l) 
  58. for hydrocarbons the complete combustion equation
    C8H18(g) + 12.5O2(g) --> 8CO2(g) + 2H2O(l)
  59. when does incomplete combustion occur
    in a limited supply of oxygen
  60. incomplete combustion :
    • in a limited supply of oxygen incomplete combustion occurs 
    • the hydrogen in the hydrocarbon still forms water 
    • the carbon undergoes partial oxidation to form carbon monoxide 
    • in some cases unburnt carbon particles are released as soot during incomplete combustion these particles are known as particulates
  61. carbon monoxide is a
    toxic gas that binds to the haemoglobin in red blood cells and prevents them carrying oxygen
  62. why is carbon monoxide difficult to detect
    its colourless , odourless and tasteless
  63. incomplete combustion of methane equation
    • CH4(g) + 1.5O2 (g)  --> 2H2O(l) +CO(g)
  64. incomplete combustion of octane equation
    • C8H18(g) + 8.5O2(g)  --> 8CO(g) + 9H2O(l)
  65. inside an engine the combustion temperature of  a fuel can be
    10000C
  66. under high temperatures in the engine
    • enough energy is available for the nitrogen and oxygen in the air to react together forming a number of oxides of nitrogen
    • these are referred to as NOx

  67. one of the gases produced in the engine is nitrogen monoxide which can undergo further oxygen in the air to form nitrogen dioxide what are the equations
    N2(g) + O2(g) -->2NO(g)

    2NO(g) + O2(g) --> 2NO2(g)
  68. some fuel passes through the car engine without
    undergoing oxidation , these gases are called unburned hydrocarbons or volatile organic compounds
  69. in the sunlight unburned hydrocarbons compounds
    can react with NOx to form ground level ozone (O3) which is a major component of smog
  70. ground level ozone irritates
    peoples eyes , aggravates respiratory problems and even causes lung damage
  71. you will already be aware that a catalyst is a
    substance that changes the rate of reaction but is chemically unaltered at the end of the reaction
  72. a catalytic converter is fitted to the exhaust system of a car . explain how it works
    • it removes pollutants from the exhaust gases before they are released through the end of the exhaust 
    • the converter contains a honeycomb mesh of two catalysts (platinum-rhodium and platinum palladium) 
    • using a honeycomb mesh provides a large surface area for the gases to pass over 
    • the catalytic converter can convert more than 90% of the pollutant gases into less harmful gases 
    • at the platinum-rhodium catalyst the NOis reduced to nitrogen 
    • at the platinum palladium catalyst two oxidation reactions can occur . 
    • any carbon monoxide formed is oxidised to carbon dioxide 
    • unburned hydrocarbons are oxidised to carbon dioxide and water
  73. our atmosphere is made up of several gases , a number of which play an important role in controlling our climate . some of these gases are described as
    • greenhouses gases e.g. water vapour , carbon dioxide and methane 
    • these are good at absorbing infrared radiation and reemitting in a different direction which warms the atmosphere . the world needs a greenhouse effect to keep the planet warm enough sustain life . 
    • however , processes such as the burning of fossil fuels have released large amounts of greenhouse gases 
    • as a result our planet is getting warmer . this is global warming and can lead to climate change
  74. people don't tend to worry about water vapour in the atmosphere this is because
    there's always been lots of it , and unlike carbon dioxide , the levels have stayed pretty constant - and some of it gets removed every time it rains
  75. our rain is naturally acidic because
    it contains dissolved carbon dioxide from the atmosphere . we use the term acidic rain for rain that is more acidic than normal
  76. acid rain may be formed naturally or by human activity
    • nitrogen dioxide formed in engines forms nitric acid : 2NO2(g) + H2O(l) + 0.5O2(g) --> 2HNO3(aq) 
    • fossil fuels contain sulphur which forms sulphur dioxide during combustion 
    • this then dissolves in water in a reversible reaction forming sulphurous acid : SO2(g) + H2O(l) <--> H2SO3(aq) 
    • a series of reactions then lead to the formation of sulphuric acid H2SO4
    • acid rain can cause chemical weathering and acidification 
  77. flue gas is the
    waste gas from boilers and furnaces
  78. flue-gas desulphurisation is
    the process that removes sulphur dioxide from waste gases . It is used in coal fired power stations , which release huge volumes of sulphur dioxide
  79. a number of reactions occur during the flue gas desulphurisation process . The key equation is that for the reaction
    • of calcium oxide , CaO , with sulphur dioxide gas forming calcium sulphite : CaO(s) + SO2(g) --> CaSO3(s) 
    • the calcium oxide is produced by heating limestone , calcium carbonate , CaCO: CaCO3(s) --> CaO(s) + CO2(g) 
    • note that this is a far from ideal solution because the production of calcium oxide involves the release of carbon dioxide . the calcium sulphite formed is almost insoluble in water and presents a disposable problem . it can however be oxidised to make hydrated calcium sulphate which is gypsum , used to make make plasterboard : CaSO3(s) + 0.5O2(g) +2H2O(l) --> CaSO4.2H2O(s)
  80. flue gas desulphurisation diagram
  81. catalytic converter diagram

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