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Lecture 22

CHMA10H3 Lecture Notes - Lecture 22: Infrared Spectroscopy, Vsepr Theory, Bond Length

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Marco Zimmer

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CHMA10- Lecture 22 (Mar. 8th)
- Estimating Reaction Enthalpies
o A reaction is exothermic when weak bonds break and strong bonds form
o A reaction is endothermic when strong bonds break and weak bonds form
- Bond Lengths
The distance between the nuclei of bonded atoms is called the bond length
Because the actual bond length depends on the other atoms around the bond, we often
use the average bond length
Average for similar bonds from many compounds
Longer bonds tend to be weaker
Length: Single bonds < double bonds < triple bonds
Bonds between bigger atoms are larger
- Trends in Bond Lengths
In general, the more e- two atoms share, the shorter the covalent bond
Must be comparing bonds between like atoms
Generally, bond length decreases from left to right across period
Generally, bond length increases down the column
Generally, as bonds get longer, they also get weaker
- Bond Vibrations
Bond length are not staticbonds vibrate!
Different vibrational motions have specific categories with energies corresponding to ns
is in the infrared frequency range
- Infrared (IR) Spectroscopy
Bond vibrations can be detected by IR absorption
Bond strength affects the vibrational energy
o The unit of IR spectroscopy is the wave number cm-1 (reciprocal of wavelength)
o Easy to determine energy from E=hv
- Lewis Structure of Water
Lewis theory predicts there are regions of e- in an atom
Some regions result from placing shared pairs of valence e- between bonding
Other regions result from placing unshared valence e- on a single nucleus
Lewis structures predict a linear (180 degrees) or right angle (90 degrees) shape for
- VSEPR Theory
Valence-shell electron-pail repulsion (VSEPR) Theory predict molecular geometry by
arranging e- pairs (groups) so as to minimize electrostatic repulsions
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