Chapter 3: Global Warming

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Chapter 3 Global Warming
Chem 100 Lecture 5 February
1, 2005
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Questions

• What is the greenhouse effect?
• Is the concentration of CO2 really increasing?
• How much CO2 do we produce?
• Is the earth really warming?
• What molecules contribute to the greenhouse
  effect?
• Molecular structure

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Earths atmosphere traps solar energy

• Radiation from sun hits the atmosphere
• Some reflected back
• Some absorbed
• Ex molecules such as O3 absorb UV radiation
• Most visible radiation is unabsorbed passes
  thru atmosphere (eventually absorbed by the
  earth)
• Earth warms radiates heat as IR radiation

Earth basically converts visible radiation
to IR radiation
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The atmosphere traps energy

• Certain compounds in the atmosphere (such as CO2)
  absorb IR radiation and then re-emit the
  radiation
• This traps the energy that would have been
  radiated away from Earth
• 84 of the radiated heat is absorbed by the
  atmosphere reradiated back to the earth.
• A balance (steady-state) exists between
  maintaining the earths temperature and climate.

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What might change the balance?

• Decreasing levels of O3
• Would allow more UV radiation to reach the earth
• This radiation can be transformed to IR that will
  be radiated away from earth
• Increased levels of CO2 (and other compounds that
  absorb emit IR radiation)

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Sources/sinks of atmospheric CO2

• Sources
• Respiration
• CO2 byproduct
• Decomposition
• Microbial byproduct
• Combustion
• Forest fires
• Fossil fuels

• Sinks
• Oceans
• CO2 dissolves in waterthe amt depends on temp
  currents
• Minerals
• Limestone CaCO3
• Photosynthesis
• 6CO2 H2O ? C6H12O6 6O2

Ideally, sources and sinks are balanced
atmospheric CO2 concentrations remains constant.
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Sources and sinks roughly balanced on a per/year
basis
Over time, amount increases
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The Carbon Cycle
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Connection b/w CO2 global temp?

• Historical records
• Estimate of atmospheric CO2 vs. time temp vs.
  time show a similar pattern

Deuterium is heavier than hydrogen ?Deuterium
wont evaporate as easily as hydrogen ?As
temperature increases, more evaporation
occurs, and the concentration of deuterium
is higher High D/H ratio correlates to high temps
Does this prove that CO2 increases cause temp
increases?
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Temperature can affect CO2

• Oceans can store more CO2 at cooler temps
• Note that sodas go flat faster at higher temps
  (higher temps allow gas to escape more quickly)

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Trend in Recent Times

• CO2 concentrations seem to have risen by 25 from
  1957 1993
• Global temperature has increased between 0.4 and
  0.8oC from 1880 1990
• Are these changes significant, or just a blip
  in the historical record?

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Identifying a greenhouse gas

• Some
• Greenhouse gases
• CO2, CH4, H2O
• All absorb emit IR radiation

• Some
• non-greenhouse gases
• O2, N2
• Do not absorb emit IR radiation

What gives a molecule the ability to absorb
emit IR radiation? Before we get to this, we
need to understand molecular structure
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Intro to Molecular Structure
Diatomic atoms can ONLY have a linear shape
If there are 3 or more atoms, the molecule may
not be linear

H O H
O
H
H

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General Rule for structure

• The most stable arrangement is the one that keeps
  repelling e- as far away as possible.
• This includes e- in bonds, lone pair e-, and
  single e-

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Stepwise procedure

• Determine of e- groups around the central atom
  (draw Lewis Structures)

CO2
H2O
CH4
2) Most stable shape is the one in which the e-
are as far apart as possible
Ex for 4 e- groups
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Molecular structure of NH3
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How many e- groups around N are there?

• Lone pair e- are less tightly held to the central
  atom than the e- in bonds, so lone pair of e- are
  extra repulsive.
• Repel the e- in the N-H bonds pushing the H atoms
  closer together

107.5o
Triangular pyramidal structure
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Molecular structure of water
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How many e- groups around O are there?
Oxygen has 2 lone pairs of e- so the H atoms are
pushed together even more
104.5o
Bent shape
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Molecular structure of ozone
How many e- groups?
3

• Furthest apart 3 e- groups can be is 120o
• Again, lone pair of e- have more repulsion and
  force O atoms to be slightly closer than 120o

Trigonal planar
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Molecular structure of CO2
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How may e- groups around C are there?
linear
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Summary Table
Total e- pairs
2
3
4
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Absorption of different energy radiation leads to
different effects

• IR radiation (l 10-5 m) has less energy than
  UV radiation (l 10-8 m)
• Not enough energy to break bonds
• But this IS enough energy to cause
• bond vibration

• Similarly, microwaves (l 10-2 m) have less
  energy than IR.
• This energy is enough to rotate molecules
• Microwave ovens work by exciting (through
  rotation) water molecules in food.

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Molecular Vibrations

• Bonds can be thought of as springs
• Different bond strengths correspond to different
  spring tensions.
• Springs with more tension require absorption of a
  higher energy IR photon for vibration to occur
• Photon frequency (n) vibration frequency
• Only certain types of vibrations are able to
  absorb radiation

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Types of vibrations possible for CO2

• Symmetric vibration

2. Asymmetric vibration

• Bending vibration
• in-plane

4. Bending vibration out of plane

• Different vibrations correspond to different
  energies of IR radiation
• It is easier to bend a molecule than stretch it
• Therefore, bending will occur with absorption of
  lower energy IR
• radiation

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Vibrations that absorb radiation

• In order to absorb radiation, the vibration must
  involve a change in the overall electron charge
  distribution
• Radiation is a changing electric field
• Only a changing electric field in the molecule
  can interact with it
• Atoms have different abilities to attract bonding
  e-
• In , bonding e- are shared but they
  tend to be more around O than the C
• O has more electronegativity
• (O has a stronger pull for the e- than C)

-
-

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Electronegativity trend in periodic table
Increasing electronegativity
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Only Vibrations with a change in charge
distribution absorb radiation
-
-

• Symmetric vibration

-
-

2. Asymmetric vibration

• Bending vibration
• in-plane

-
-

4. Bending vibration out of plane
-
-

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Some requirements for absorbing IR radiation

• Molecule must have more than 1 atom
• Need more than 1 type of atom
• These molecules dont absorb IRbecause the atoms
  in the molecules have the same electronegativity,
  there is no partial and partial to the bond
• Thus, there would be no change in the charge
  distribution during vibration

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Greenhouse effect
The greenhouse gas molecule absorbs IR energy,
and the molecule vibrates. The molecule can go
back to a normal (non-vibrational) state by
emitting energy back to Earth.
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IR spectroscopy
IR spectrum of CO2
A 1/T A absorbance T transmission Low T
High A
An instrument called an IR spectrometer can
measure the IR absorbed by a molecule
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IR tutorial with molecular vibration animations
From http//chemistry.beloit.edu/Warming/pages/i
nfrared.html