Chapter 3: Global Warming

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Chapter 3 Global Warming
A sound-bite culture cant discuss science very
well. Exactly what were losing when we reduce
biodiversity, the causes and consequences of
global warmingthese traumas cant be adequately
summarized in an evening news wrap-up. -Barbara
Kingsolver- Small Wonder, 2002
2
The Evidence

• The past 2 decades have been the warmest in
  recorded history ( heat island effects
  accounted for)
• Ice sheets in Greenland and Antarctica are
  melting and moving at unexpectedly high rates
• Growing seasons in temperate forests have
  increased by two weeks over the last 40 years
• First and Last frosts are occurring later and
  earlier in their respective seasons.

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What we need to know

• Isotopes
• Lewis structure
• molecular shape
• electronegetivity
• dipoles
• Moles/Molar Mass
• Molecular composition

4
Where we are

• Chapter 3
• lecture will focus on chapter 3 sect 1-3
• Work problems 1-13
• Due dates for paper
• 17Oct Bibliography31 Oct Outlines 14 Nov
  Papers due

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The Earths Energy Balance
Greenhouse effect Our atmospheric gases trap and
return a major portion of the heat radiating from
the Earth.
30
25
45
3.1
6
Earths energy is roughly balanced

• The temperature is regulated by the incoming
  radiation vs out-going radiation
• as we increase CO2 and other greenhouse gasses
  the out-going radiation is reduced until a
  temperature rise occurs and the out going
  radiation then balances out again.

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CO2 Concentration vs. Earths temperature
3.2
8

Atmospheric CO2 level on the rise
3.2
9

Review How to draw Lewis structures

• Determine the sum of valence electrons
• Draw the stucture placing the lone atom in the
  middle

Draw Lewis Structures for O2 CH4 SO2 C2H4 SO4
2- CO H2SO4 N2-------------------------------
---NO3- O3
3. Use a pair of electrons to form a bond
between each pair of bonded atoms
4. Arrange the remaining electrons to satisfy
octet rule (duet rule for H) starting with the
outer atoms- If needed bring in pairs to form

• Assign formal charges (we wont yet do this)

Formal charge of v.e. of non-bonding e-
½ bonding e- or, F.C. of v.e.
non-bonding e- of bonds to the atom
Remember Resonance, relative lengths and bond
order!
3.3
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Valence Shell Electron Pair Repulsion Theory
Rules for VSEPR Draw a valid Lewis Structure
Ignoring double bonds, determine the number of
electron pairs around the central atom Arrange
the pairs as far apart on the central atom as
possible. Ignoring lone pairs of electrons
determine the shape of the molecules.
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Valence Shell Electron Pair Repulsion Theory
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Valence Shell Electron Pair Repulsion
Theory Consider methane (CH4), where the central
carbon atom has 4 electron pairs around it
A tetrahedral shaped molecule has bond angles of
109.5o.
Four electron pairs as far from each other as
possible indicates a tetrahedral arrangement.
3.3
13
Representations of methane
CH4 molecular formula does not express
connectivity
Structural formulas show how atoms are connected
Lewis structures show connectivity
This Lewis structure is drawn in 3-D
Space-filling Charge-
density
3.3
14
The central atom (O) in H2O also has four
electron pairs around it,
The non-bonding electron pairs take up more space
than bonding pairs, so the H-to-O-to-H bond angle
is compressed.
but unlike methane, two electron pairs are
bonding and two are non-bonding.
The electron pairs are tetrahedral arranged, but
the shape is described only in terms of the atoms
present water is said to be bent shaped.
3.3
15
We can use the VSEPR model to allow us to predict
the shape of other molecules.
Other predictions can be made based on other
electron pair arrangements .
3.3
16
Now look at the central atom of CO2
Two groups of four electrons each are associated
with the central atom.
The two groups of electrons will be 180o from
each other the CO2 molecule is linear.
3.3
17
Molecular response to different types of
radiation
3.4
18
Molecular geometry and absorption of IR radiation
Molecular vibrations in CO2. Each spring
represents a CO bond. (a) no net change in
dipole - no IR absorption. (b, c, d) see a net
change in dipole (charge distribution), so
these account for IR absorption
3.4