Infra-red Spectroscopy

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  • Infra-red Spectroscopy
    • Molecules can absorb infra-red radiation, the energy absorbed causes the covalent bonds in the molecule to vibrate
      • Only specific energies are absorbed by  a particular molecule. Each absorption band is due to the vibration of a specific bond or set of bonds
        • An infra-red spectrum shows the wavenumber ranges for the absorption band of a substance
    • Uses of infra-red spectroscopy
      • To identify compounds by comparison of the pattern of peaks with a spectral database. The peaks with wavenumbers less than about 1600cm-1 especially are due to excitiation of complex vibrations involving many atoms
        • The patterns may be unique to a specific compound so this is called the fingerprint region of the spectrum. A computer can search for an exact match between this pattern and those recorded for known compounds in the database
          • This is called fingerprinting. It is used for example to detect; traces of drugs in forensic science and environmental pollutants
      • To measure the concentration of a substance from the strength of their characteristic absorption bands  e.g. in modern brethalysers
        • The instrument measures IR absorption at a chosen wavelength, where ethanol absorbs strongly but other normal breath components  such as water do not. The instrument is calibrated to relate the extent of IR absorption to the ethanol concentration in the breath
      • Atmospheric pollutants can be detected from their characteristic infra-red fingerprints and their concentrations determined from the strength of these absorption bands
    • Infra-red radiation and climate
      • When infra-red radiation is emitted at the earths surface, some of it will be absorbed by molecules in the atmosphere instead of escaping directly into space. This is called the greenhouse effect and it tends to warm the atmosphere
        • Global warming is causing climate change and this trend is likely to continue. Chemists are helping to address the problems of climate change by;
          • Carbon Capture and Storage (CCS)
            • This involved reducing CO2 emissions into the atmosphere by capturing CO" from major sources such as power stations and either;
              • Injecting it into deep geological formations where it can be trapped beneath rock
              • Injecting it into the deep ocean where it can slowly dissolve
              • Reacting it with natural oxide minerals to form stable carbonates
          • Developing alternative energy sources to reduce dependency on fossil fuels
        • Chemists are continuously monitoring the composition of the atmosphere and the amounts of CO2 and other greenhouse gases that we are releasing into it
          • This helps us to check that we are on course to meet targets for reducing emissions such as those agreed under the Kyoto Protocol
      • The contribution of any individual substance to the greenhouse effect depends on the concentration of the substance in the atmosphere and how good it is at absorbing infra-red radiation
        • If the concentration of any of these compounds in the atmosphere increases, more infra-red radiation will be absorbed and the warming effect will be greater
          • CO2 levels in particular have been rising as the result of increased use of fossil fuels and this is almost certainly the origin of the global warming we are currently witnessing


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