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Ch_12_summary - CHAPTER 12 MODERN ANALYTICAL CHEMISTRY(IB...

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CHAPTER 12 MODERN ANALYTICAL CHEMISTRY (IB OPTION A) SUMMARY © IBID Press 2007 1 Analytical chemistry involves qualitative and quantitative analysis of a sample to determine its chemical composition and structure and to determine purity. Analytical methods are usually faster, more precise and easier to automate than ‘wet’ methods, and a combination of analytical techniques is often used to obtain complete structural information. Spectroscopy Many analytical techniques involve spectroscopy - the way in which the absorption or emission of electromagnetic radiation by substances varies with frequency involving radiation such as UV, IR and microwave. The energy of a quantum of radiation is directly proportional to its frequency and the energy is inversely proportional to its wavelength. Thus high frequency (and hence short wavelength) radiation carry a great deal of energy and those of low frequency radiation carry much less. A particle (atom, molecule or ion) can absorb a quantum of light and this will affect its state depending on the amount of energy that the quantum carries: γ -rays can bring about changes in the nucleus. X-rays cannot cause changes in the nucleus, but have enough energy to remove electrons in inner filled shells of atoms. Ultraviolet and visible light have enough energy to affect the valence electrons. UV radiation causes sunburn with too much exposure to sunlight. Microwaves affect the rotational state of molecules. Radio waves can alter the spin state of some nuclei when they are exposed to magnetic fields and are used in NMR spectroscopy. In emission spectroscopy the frequency of the radiation emitted by excited particles dropping to a lower energy state is studied. In absorption spectroscopy radiation of a wide range of frequencies is passed through the sample and the way in which the absorption of radiation varies with its frequency is studied. Energy of particular frequencies is absorbed and used to enable a particle to move from a lower to a higher energy state. A double-beam spectrometer allows radiation from a source that is split into two equal beams, one passed through the sample and the other through a reference containing the same solvent but without the substance being studied. The two beams are then recombined at the detector. The signals from the sample and reference beams are then compared electronically to see if the sample absorbs radiation of any frequency. As the spectrum of the sample is scanned, the frequency of the radiation is varied and a graph of absorption against frequency, wavelength or wavenumber is drawn. Comparison of the spectrum of the unknown compound with a data bank enables its identification. IR: A quantum of infrared radiation has sufficient energy to excite a molecule to a higher vibrational level. In order to absorb infrared light a vibrational motion must result in a change in the dipole moment of the molecule. The information from an IR spectrum can be used to identify bonds present, but not always the functional groups present.
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