Earth

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Earths Atmosphere
2
Atmosphere

• Envelope of gases that surround the Earth
• Protects the Earth
• Provides materials necessary to support all forms
  of life

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4 Regions based on Temperature

• Troposphere
• Stratosphere
• Mesosphere
• Thermosphere

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Troposphere

• 0-12 km
• Temperature decreases with altitude
• Minimum of 215 K
• Weather
• Upper limit is the tropopause

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Stratosphere

• 10-50 km
• Temperature increases with altitude
• Maximum of 275 K
• Upper limit is the stratopause

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Mesosphere

• 50-85 km
• Temperature decreases with altitude
• Minimum of 190 K
• Upper limit is the mesopause

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Thermosphere

• Above 85 km
• Temperature increases with altitude

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Pressure

• Decreases in a regular way with increasing
  elevation
• Troposphere and stratosphere account for 99.9 of
  the mass of the atmosphere

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Earths Composition

• N2 and O2 make up 99 of atmosphere
• CO2
• Noble gases
• TABLE 18.1

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Parts per Million

• Unit of concentration
• One part by volume in 1 million volume units of
  the whole
• Volume is proportionate to mole
• Volume fraction mole fraction

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N2 versus O2

• N2 has a triple bond and O2 has a double bond
• O2 much more reactive and has lower bond energy
  than N2

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Outer Regions of the Atmosphere

• Beyond stratosphere
• Outer defense against radiation and high-energy
  particles

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Photodissociation

• Shorter-wavelength/ higher-energy radiations in
  the ultraviolet range of spectrum cause chemical
  changes
• For radiation to fall on Earths atmosphere
• Photons with sufficient energy
• Molecules absorb photons

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Photodissociation

• Rupture of a chemical bond resulting from
  absorption of a photon by a molecule
• No ions formed
• ½ of electrons stay with one of the atoms ½
  stay with the other
• 2 neutral particles

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Photodissociation of O2

• Bond energy 495 kJ/mol

hv ?


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Photoionization

• Occurs when a molecule absorbs radiation (a
  photon) and the absorbed energy causes an
  electron to be ejected from the molecule
• Becomes positively charged ion

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Ozone in the Upper Atmosphere

• O3 is the key absorber of photons having
  wavelengths from 240-310 nm
• Below altitude of 90 km, most short-wavelengths
  (lt 240 nm) have been absorbed by N2, O2, and
  atomic O

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Continued

• 30-90 km

O2 ? O3 excess energy (releases 105
kJ/mol)
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Continued

• O3 collides with other atoms or molecules, M
  (usually N2 or O2), transfers energy
• O O2 ? O3
• O3 M ? O3 M
• O O2 M ? O3 M

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Effects on Rate of O3 Formation

• Presence of O atoms (favored at higher altitudes)
• Molecular collisions (favored at lower altitudes)

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Continued

• Highest rate of O3 formation occurs in a band at
  50 km altitude
• 90 of O3 is found in the stratosphere

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After Formation

• O3 does not last long
• It absorbs solar radiation and decomposes back
  into O and O2

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Cyclic Process

• O2 hv ? O O
• O O2 M ? O3 M (heat released)
• O3 hv ? O2 O
• O O M ? O2 M (heat released)
• 1 3 photochemical (initiated by a solar
  photon)
• 2 4 exothermic chemical reactions
• Net result solar radiant energy converts to
  thermal energy

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Depletion of O3 Layer

• 1970s CFCs depleting ozone
• CF Cl3 and CF2Cl2
• Used in refrigerators, propellants, foaming
  agents
• CFCs diffuse in stratosphere
• Exposed to radiation
• Photodissociation occurs

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Photodissociation of CFCs

• C-Cl bond is weaker than C-F bond
• Free Cl atoms are formed when ? 190-225 nm
• Greatest at altitude of 30 km
• CF2Cl2 hv ? CF2Cl Cl

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Continued

• Cl reacts with ozone
• Cl O3 ? ClO O2
• Sometimes ClO regenerates free Cl atoms
  (photodissociation)
• ClO hv ? Cl O

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Cl-catalyzed decomposition of O3 to O2

• 2Cl 2O3 ? 2ClO 2O2
• 2ClO hv ? 2Cl 2O
• O O ? O2
• 2Cl 2O3 2ClO 2O ? 2ClO 2Cl 3O2 2O
• 2O3 ? 3O2

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Limiting use of CFCs

• 1987 Montreal Protocol on Substances that Deplete
  the Ozone Layer
• 1992 100 nations agreed to ban CFC production by
  1996

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Replacing CFCs

• Hydrofluorocarbons
• C-H bond replaces that of C-Cl
• ex CH2FCF3 (HFC-134a)

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Natural Depletion

• Natural sources that contribute Cl and Br to
  atmosphere (the methyl's)
• CH3Cl and CH3Br
• 1/3 of depletion (2/3 human activities)

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Homework

• Page 797
• 15-21 odd only