ATAR Notes: Forum
VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: Mao on April 12, 2011, 11:48:40 am
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I received this question via pm. Posting so everyone can have a read (if they want).
in IR spec, why don't we get discrete lines, if the bonds in the molecule absorb specific quantas of energy in order to alter vibrational energy. Hieneman seemed to stress the point that the bonds absorb specific "discrete" energies but didn't explain why the peaks are broad.
By 'discrete', we really mean they are related to the energy of an atom. Part of this energy is the kinetic energy (vibrational energy is part of kinetic energy), which depends on temperature and it is distributed according to the boltzmann distribution. That means for any molecule, its ground state vibrational mode is not a single level of energy, rather it is some distribution of energy around some average energy. The result is these all have slightly different energy shifts to the first overtone, thus a broad peak.
The breadth of the peak depends on the molecular structure. Some ground state energies can vary a lot (O-H or N-H, where the H can move around a lot), thus giving very wide peaks. Some can only vary by a small amount (C=O, tightly bound, C and O rarely have much kinetic energy), so we see a sharp peak that is closer to the 'discrete' description.
and the same thign with Uv-VIs spec, however i think they don't absorb specific amounts because the enegry levels in molecules are altered when in aqueous form or molecular form
Similar thing, molecules vibrate in space due to non-zero temperature. The absorption of light is discrete, but the molecules absorb slightly different energies (due to distortions), thus the absorption of light is over a wide range.
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thanks!