Energies of molecules - Energiesofmolecules Rotational...

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Rotational Translational Energy Energy Translational Electronic Rotational Vibrational Energies of molecules Translational energy – kinetic energy  (½ m v 2 ) Rotational energy – kinetic energy too (but in a different form) Vibrational energy – both kinetic AND potential energy Electronic energy – both kinetic AND potential energy (“motion” of electrons is hard to picture or draw) A typical molecule “possesses” energy in each of these four categories. The energy is spread around through collisions between molecules, but because of  quantum mechanics, energy is not “partitioned” equally among these types of motion. The figures below help us compare these energies on the same energy scale.
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The comparison in this second diagram is very different. Molecules are free to capture  essentially any amount of translational kinetic energy between zero and about  5.0  kJ/mol, but the average is around 2.5 kJ/mol. On the other hand, the same molecule is restricted to very well-defined energy states  corresponding to rotational motion, vibrational motion, and electronic motion shown as  ENERGY LEVELS  in the above two pictures. Many of you will learn (or have learned)  how to calculate these energy levels in Chemistry 3442. If a molecule is found in one energy state, it can only gain more energy it if gets enough  energy (by colliding with another molecule or the wall of the container) to get to the  next higher energy level . In the case of electronic motion that is a lot of energy indeed! Unfortunately, the only way a molecule can gain energy to get to the next level is by  COLLISION which means it can only gain the translational energy illustrated by the size  of the  orange  rectangles in the above figures. Assume the size of these rectangles reflects the  AVERAGE  translational energy a  molecule can have, and is roughly the amount of energy that might be exchanged when  two molecules collide! Note that compared to typical rotational energy levels there is plenty of energy available  for molecules to get to the next level since the rectangle is considerably larger that the  first 10 or so levels. Contrast this with the next figure showing a few of the lowest vibrational levela in a 
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This note was uploaded on 09/11/2011 for the course CHEMISTRY 3441 taught by Professor Lobue during the Spring '11 term at Georgia Southern University .

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Energies of molecules - Energiesofmolecules Rotational...

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