F331 Chemistry

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  • Created by: Georgia
  • Created on: 23-02-14 18:53

Octane numbers and fuels

  • Octane number measures the tendancy to auto-ignite/pre-ignite/knock
  • A cycloalkene/cycloalkane molecule is produced during reforming
  • Heterogeneous means that the catalyst and reactants are in different states
  • A catalyst speeds up the reaction giving a different pathway to lower activation energy 
  • Using hydrogen as a fuel only produces water on combustion so no greenhouse gases
  • The reforming process is endothermic because:
    • energy in / endothermic to break bonds
    • energy given out / exothermic when bonds form
    • less energy given out than taken in 
  • During a cracking reaction:
    • larger hydrocarbons are split 
    • to form shorter alkanes or alkenes
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Absorption/Emission Spectra

  • An atomic absorption spectra has a visable spectra background and black lines to indicate where the particles have been absorbed. 
  • An atomic emission spectra is the opposite, has a black background and visible spectra lines. 
  • Why is an elements absorption spectra unique?
    • Energy absorbed allows electrons to move up energy levels
    • Energy levels are unique to each independent element
    • Energy absorbed is related to frequency ( Δ E = hv)
  • The main stages in a time of flight mass spec:
    • Sample is ionised 
    • All ions are accelerated in magnetic field to the same kinetic energy
    • Move into the drift region
    • Heavier ions move more slowly
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  • Adding an oxygenate to fuels increases octane number and lowers the chance of knocking 
  • Calculating energy density: eg: The energy change of combustion of C8H16 is -5400kJmol-1
    • Work out relative atomic mass of C8H16 = 112
    • Moles in 1kg : 112 / 1000 = 8.93
    • kJ per kg = 8.93 x 5400 = 47329
  • A structural isomer is molecule with the same molecular formula but different structural formulae
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  • Energy transferred to water: 
    • vol of water x shc x temp change 
  • Empirical formula: (eg: Cerium forms an oxide containing 81.4% by mass of cerium, Ar Ce: 140.1)
    • Moles Ce = 81.4 / 140.1 = 0.58
    • Moles O = 18.6 /16.0 = 1.16
    • Emperical formula = CeO2
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