Chemical Reaction vs' Physical Interactions Chapter 12

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Chemical Reaction vs. Physical Interactions
(Chapter 12)
Boiling point of NH3 versus PH3 Why is NH3
boiling point higher? The N H bonds between
NH3 molecules matter, not the N H bonds within
the NH3 molecule.
EN is inversely proportion to atomic radius.
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Lab 2 Using VSEPR to determine GHGs
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What causes changes in climate?
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What is it about CO2 (and other GHGs) that makes
it a greenhouse gas?
It has to do with the interaction of
electromagnetic radiation with matter
Electromagnetic waves travel at the speed of
light (c) characterize by frequency (?),
wavelength (?), and amplitude important
relationships c ? ? E h? hc/?
(Energy of photons)
Atoms and molecules can absorb EM radiation, but
only at certain wavelengths.
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Chemical Energy and the Electromagnetic Spectrum
Heat (Thermal) and Light (Radiant) energy
released or absorbed during formation or breaking
of bonds is made up of different wavelength or
frequencies of energy.
Heat Light
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When atoms and molecules absorb EM radiation,
they are interacting with photons at certain
wavelengths.
Different energy levels within a molecule
http//www.wag.caltech.edu/home/jang/genchem/infra
red.htm
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Another way to visualize rotation and vibration
of molecules
Molecular vibration When the molecule vibrates,
the atoms move towards and away from each other
at a certain frequency. The energy (E h?
hc/?) of the system (molecule) is related to how
much the spring is stretched or compressed.
The amount of energy needed to cause vibration in
molecules is related to (1) the strength of the
chemical bonds and (2) the masses of the atoms
Quantization Only photons with certain energies
will excite molecular vibrations.
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Bond polarity When IR radiation is absorbed by
a molecule and it vibrates, dipole moments
oscillate. No oscillation capacity cannot
absorb IR.
Electronegativity of atoms in bonds
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Types of vibrational motions (stretching and
bending) that result from the absorption of IR
(1) Symmetric stretch atoms move in opposite
directions as the bonds shrink or stretch
(symmertric w.r.t the central atom)
Example CO2
Net dipole moment change?
No change in net dipole moment and, therefore,
this motion cannot result from the absorption of
IR (b/c absorption of IR changes the dipole
moment)
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Types of vibrational motions (stretching and
bending) that result from the absorption of IR
(2) Asymmetric stretch outer atoms move in the
same direction, while the central atom moves in
the opposite direction
Example CO2
Net dipole moment change?
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Types of vibrational motions (stretching and
bending) that result from the absorption of IR
(3) Bend bonds in the molecule bend, resulting
in different bond angles

Example CO2
Net dipole moment change?
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IR spectrum of CO2
CO2 strongly absorbs IR
Absorption strength depends on concentration http
//chemlinks.beloit.edu/Warming/moviepages/IRconc.
htm
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• Summary Will a molecule
  absorb infrared radiation?
• Determine the molecules polarity ( bond
  polarity molecule shape atomic
• composition (EN) )
• (2) A molecule can only absorb IR when it
  experiences a change in net dipole moment
• during a vibration. Draw all of the
  molecules possible vibrational motions. If any
• of these vibrational changes lead to a
  change in the net dipole moment, the
• molecule can absorb IR.

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Atmospheric Gas Composition
N2 78.08 O2 20.95 H2O
1 Ar 0.93 CO2 0.038 CH4 lt
0.002 N2O lt 0.002 SO2 lt 0.002 NH3 lt
0.002 O3 lt 0.002 DMS lt 0.002 CFC-11 lt
0.002 (average, very variable)
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Lab
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Summary So Far Overall Polarity of a Molecule
If a molecule is polar, it will (1) have a net
dipole moment (2) orient itself in an electric
field If a molecule is nonpolar, it will (1)
not have a net dipole moment (2) will move about
randomly in an electric field

• Steps used to determine molecular polarity
• Draw the 2-D Lewis dot structure to determine the
• number and types (single, double, triple) of
  bonds
• present, and any lone pairs present.
• When dealing with several options (resonance
  structures),
• determine the dominant resonant structure by
• 1. calculating formal charge for the atoms of
  each
• molecular possibility that you are
  evaluating
• 2. using the three criteria for selecting the
  dominant
• resonance structure based on formal charge
• (2) Determine the 3-D shape of the molecule
• (3) Determine the overall polarity of the molecule

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In addition to the wavelength of energy
interaction with the molecule The symmetry of the
molecule (Lewis dot structure VSEPR!!!) will
also determine whether a photon can be absorbed.
Symmetry is NOT a net dipole moment
Symmetrical mirror image Asymmetrical not
mirror image
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A GHG is a molecule that (1) Has a change in
its net dipole moment during molecular
vibrations caused by interaction with
electromagnetic IR wavelengths (2) Has molecular
symmetry