Topics for Today

1
Topics for Today

• Greenhouse Gases How do they keep us warm?

2
Readings for Today

• 3.4 Vibrating Molecules and the Greenhouse Effect

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Topics for Friday

• Climate Change A Global Problem?
• Quiz 7 (of 11)!!

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Readings for Friday
May even bleed into Monday.

• 3.8 Methane and other greenhouse gases 
•  3.10 Responding to science with policy changes
•  3.11 The Kyoto Protocol on Climate Change
•  3.12 Global warming and ozone depletion

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Announcements!!!
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Bring back your radon disks!!
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Announcements!
Nice, warm weather
Please remember Do not wear shorts or sandals to
lab!!
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Exam Regrading
Same process as Exam 1.
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Topics for Today

• Greenhouse Gases How do they keep us warm?

10
Review from Monday
11
Review
The Greenhouse Effect
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Review from a long time ago.
(
Lecture 15.
(
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What happens when a molecule absorbs light?
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What happens to a molecule when it absorbs IR?
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Among other motions, this includes stretching and
bending!
Figure 3.17
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What happens to a molecule when it absorbs IR?
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Q When will IR cause a molecule to vibrate?
A When there is a dipole moment!
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• Dipole Moment Unequal distribution of charge!

Where have we seen dipole moments before?
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• Polar Bonds Unequal sharing of electrons in a
  chemical bond. Still a covalent bond!

RESULTS IN AN UNEQUAL DISTRIBUTION OF CHARGE!!
From Lecture 19 or so
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A bunch of polar molecules
d-
d
H
Cl
N
d-
That is, they have dipoles
H
H
d
d
H
d
d-
O
H
H
d
d
25
What about some other molecules?
..
..
O
C
O
..
..
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Carbon Dioxide
Polar or nonpolar bonds?
..
..
O
C
O
..
..
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Carbon Dioxide
Polar bonds
..
..
d
O
C
O
d-
d-
..
..
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Carbon Dioxide
Polar or nonpolar molecule?
..
..
d
O
C
O
d-
d-
..
..
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So, no overall dipole moment
Carbon Dioxide
Nonpolar Molecule
..
..
Why does it absorb IR radiation?
d
O
C
O
d-
d-
..
..
But CO2 is a greenhouse gas!
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The IR can cause (or induce) a dipole moment!
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Possible Vibrations for CO2
Think ELECTRONEGATIVITY!!!
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• Symmetric Stretch
• Both out, both in
• Keeps linear shape

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• Symmetric Stretch
• Both out, both in
• Keeps linear shape

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Does this induce a dipole moment?

• Symmetric Stretch
• Both out, both in
• Keeps linear shape

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• Asymmetric Stretch
• One in and one out
• Keeps linear shape

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• Asymmetric Stretch
• One in and one out
• Keeps linear shape

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Does this induce a dipole moment?

• Asymmetric Stretch
• One in and one out
• Keeps linear shape

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• Bend
• Bends in one plane
• Loses linear shape

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• Bend
• Bends in one plane
• Loses linear shape

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• Bend
• Bends in one plane
• Loses linear shape

Does this induce a dipole moment?
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• Bend
• Bends in another plane
• Loses linear shape

Same as the other bend, just in a different plane
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IR vibrations for a linear 3 atom molecule
Figure 3.17
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Bending in each plane
http//chemistry.boisestate.edu/rbanks/spectroscop
y/co2vibrations.gif
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CO2 bending and stretching

• Summary
• Symmetric Stretch
• Asymmetric Stretch
• Bend 1
• Bend 2

Which of these is able to produce a dipole moment?
CO2 has THREE possible ways to absorb IR!!
How do we measure this?
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Spectrophotometry
Sort of
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What are wavenumbers?
Figure 3.15 in your Text
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What is transmittance?
4 µm 4000 nm!
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Absorbance
The opposite of absorbance!
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Review
The Greenhouse Effect
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http//www.spaceguarduk.com/cd/dict/dictionary/inf
rared.htm
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Where is visible light?
Higher Energy
Lower Energy
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What happens when a molecule absorbs light?
It depends on the wavelength
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What happens to a molecule when it absorbs IR?
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What happens when a molecule absorbs light?
It depends on the wavelength
AND on the molecule.
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Summary
For a molecule to absorb IR radiation, it must
1) Permanently have an uneven distribution of
charge (dipole)
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Polar molecules have dipole moments
d-
d
H
Cl
N
d-
H
H
d
d
H
d
d-
O
H
H
d
d
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Summary
For a molecule to absorb IR radiation, it must
1) Permanently have an uneven distribution of
charge (dipole) OR 2) Vibrate in a way that
can CREATE an uneven distribution of charge
(dipole)
Nonpolar molecules MAY or MAY NOT absorb IR
radiation
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CO2 bending and stretching

• Summary
• Symmetric Stretch
• Asymmetric Stretch
• Bend 1
• Bend 2

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Summary
For a molecule to absorb IR radiation, it must
1) Permanently have an uneven distribution of
charge (dipole) OR 2) Vibrate in a way that
can CREATE an uneven distribution of charge
Nonpolar molecules MAY or MAY NOT absorb IR
radiation
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Do N2, O2, or Ar absorb IR radiation?
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1) Polar or nonpolar molecules?
Do N2, O2, or Ar absorb IR radiation?
2) Can we vibrate these to produce a dipole
moment?
..
..
..
O
N
N
O
..
Ar
..
..
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These molecules / atoms are NOT GHGs.
1) They are nonpolar 2) They cannot vibrate to
create an unequal distribution of charge.
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Summary
For a molecule to absorb IR radiation, it must
1) Permanently have an uneven distribution of
charge (dipole) OR 2) Vibrate in a way that
can CREATE an uneven distribution of charge -A
molecule with 3 atoms
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How about water?
Is water polar?
..
..
But it can also vibrate!!
O
H
H
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And a very potent GHG!!
Water vapor is an extremely strong absorber of IR
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Water Vapor
Figure 3.16
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CO2
Figure 3.15 in your Text
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Water Vapor
Figure 3.16
CO2 absorbs here
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Water Vapor
Figure 3.16
Different molecules absorb different wavelengths!
CO2 absorbs here
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Does an increase in water vapor lead to an
increase in Earths temperature?
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On one hand
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Water Vapor
Figure 3.16
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On one hand
On the other hand
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Interactions with Clouds

• Clouds Condensed water on a seed particle
• Reflects solar radiation

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Review
What is the Greenhouse Effect?
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The Greenhouse Effect

• Solar radiation

• Greenhouse gases are transparent to incoming
  solar radiation
• Greenhouse gases absorb the infrared light which
  is emitted from the earth
• This traps energy in the earths atmosphere

Long-wave radiation
Greenhouse Gases
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About 80 of the heat emitted from the surface is
trapped in the atmosphere
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The Earths atmosphere is about 380 ppm CO2
The average temperature is about 59oF
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About -4 oF!
What would the average temperature of Earth be
with no CO2?
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About 80 of the heat emitted from the surface is
trapped in the atmosphere
Any more than this 80 is called the enhanced
greenhouse effect.
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Some people are worried that this will lead to
climate change (global warming)
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Greenhouse gases

• CH4
• CO2
• H2O
• N2O (nitrous oxide)
• CFCs and replacements

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Greenhouse gases

• CH4
• CO2
• H2O
• N2O (nitrous oxide)
• CFCs and replacements

DONE!
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Water Vapor
Figure 3.16
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Water Vapor

• Extremely strong dipole moment (Very good at
  absorbing IR)
• Most abundant greenhouse gas (varies from 0-4 of
  the atmosphere)

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• Some GHGs

N2O
HCFCs
CH4
CFCs
O3
HFCs
CO2
H2O
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What makes greenhouse gases more potent?
N2O
HCFCs
CH4
CH4
CFCs
O3
HFCs
CO2
H2O
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1) Atmospheric Lifetime
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Methane
Atmospheric lifetime about 10 years
Removed by the hydroxyl radical we will see this
reaction Monday!
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CFCs
Atmospheric lifetime gtgt 100 years
Destroyed in the stratosphere by UV-C
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1) Atmospheric Lifetime
2) Atmospheric Concentration
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Concentration of CO2 is about 370 ppm.
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Concentration of CO2 is about 370 ppm.
CFC concentrations are around 0.007 ppm
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3) How efficiently can the molecule absorb IR?
1) Atmospheric Lifetime
2) Atmospheric Concentration
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CO2 bending and stretching
CO2 has only 3 vibrations

• Summary
• Symmetric Stretch
• Asymmetric Stretch
• Bend 1
• Bend 2

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CFCs
This is due to the more complicated SHAPE
CFCs have at least 10 possible vibrations!!
CFCs are much more efficient at trapping IR than
CO2.