Mary J. Bojan Chem 110 Gases and the atmosphere Chemical reactions in the atmosphere Ozone cycle Ozone depletion Greenhouse effect Global Warming Pollution in the troposphere 1Mary J. Bojan Chem 110 Gases and the atmosphere Review Composition of the atmosphere Use mole fraction and ppm Pressure profile of atmosphere Low pressure means few molecular collisions Chemical reactions occur less frequently Temperature profile of atmosphere High temperature means avg. KE of molecules is high! 2Mary J. Bojan Chem 110 Composition of the atmosphere mole fraction: N2 and O2 represent > 99% of atm 3 € xi=moles of itotal molesMary J. Bojan Chem 110 Composition of the atmosphere Eg. Neon xNe = 0.00001818 Ne concentration = 18.18 ppm For other components use parts per million (ppm). ppm = xi × 106 4 If you know the barometric pressure, you can determine the partial pressure. PBAR = 0.987 atm= PTOT PNe = xNe PTOT = 18.18×10-6 ( 0.987 atm) = 17.94×10-6 atmMary J. Bojan Chem 110 Pressure decreases exponentially with altitude. WHY? What is pressure? 5 Pressure at any altitude depends on weight of gas above it.Mary J. Bojan Chem 110 Pressure and altitude Altitude P collision frequency (km) (atm) (sec-1) 0 1 1010 50 10-3 107 100 10-6 103 200 10-13 1 6 • Low pressure means fewer collisions • When collision frequency is low, chemical reactions occur less frequentlyMary J. Bojan Chem 110 Based on Temperature Profile Thermosphere: High energy radiation is absorbed Ions formed. Mesosphere: Density of gases is small. Stratosphere: Warming caused by ozone cycle Ultraviolet light absorbed Troposphere: Life: where we live! – weather – planes 7 Regions of Atmosphere Present life forms could not survive (above ground) if all solar radiation reaches earth.Mary J. Bojan Chem 110 Distribution of Radiation 8Mary J. Bojan Chem 110 Penetration of Solar Radiation 9Mary J. Bojan Chem 110 Reactions in the atmosphere Photochemical Reactions: reactions that involve the absorption or emission of a photon. Electronic excitation of molecules NO2(g) + hν → NO2*(g) Photodissociation bond broken by absorption of a photon (light) O2 + hν → O + O E = hν Photon needs sufficient energy to break bond λν = c (Bond energy for O2 = 495kJ/mol) λ < 242 nm Photoionization – removal of an outer (valence electron) from a molecule by absorption of a photon N2 + hν → N2+ + e- 10Mary J. Bojan Chem 110 Thermosphere (Ionosphere) 11Mary J. Bojan Chem 110 Warming of the Stratosphere Ozone concentration peaks in stratosphere ~25km, concentration ≈ 10ppm Presence of ozone causes temperature increase in stratosphere. 12Mary J. Bojan Chem 110 Ozone O3 Properties: • Light blue gas • Pungent odor (smell near electrical discharges) • ΔHf°= 142kJ/mol (reactive… less stable than O2) 13 Structure • 2 resonance forms: bent, bond angle = 117° • bond length = 1.28Å (O2 is 1.21Å) (in agreement with VSEPR predictions)Mary J. Bojan Chem 110 Why is ozone so important? Ozone cycle formation of O3 O2 + hν→ 2O λ < 242nm O + O2 →O3 uv –blocking by O3 O3 + hν →O2 + O λ < 320nm Solar radiation (ultraviolet) is converted to heat 14 The (small) amount of O3 in stratosphere reflects the delicate balance between creation and destruction.Mary J. Bojan Chem 110 Problem: Ozone depletion What causes it? Chlorofluorocarbons (CFCs) Examples: CFCl3 CF2Cl2 freon-11 freon-12 Properties − relatively inert − easily liquefied − Non-combustible − volatile 15 Uses − aerosol propellants − coolants for refrig. − solvents for cleaning − polymer manufacturing evaporate and diffuse upward easilyMary J. Bojan Chem 110 CFC destruction by ozone CF2Cl2 + hν→ CF2Cl + Cl (λ<240nm) Cl + O3 → ClO + O2 ClO + O → Cl + O2 NET: O3 + O →2O2 Global concentration of O3 has been on the decline since 1980. 16 Cl atom from CFC catalyzes O3 destruction (speeds up reaction, but is not used up). 1 Cl atom destroys ~ 2 million O3 moleculesMary J. Bojan Chem 110 Greenhouse Effect Energy comes to earth from the sun. • Some is reflected by atmosphere • Some is absorbed • Most of uv and visible reaches the earth’s surface: causes warming. Earth radiates heat back out. mainly infrared energy 17Mary J. Bojan Chem 110 The earth’s energy balance Earth radiates infrared light back Solar radiation warms the earth 18 Distribution of RadiationMary J. Bojan Chem 110 Greenhouse effect Absorption of infrared radiation: Activates stretching and bending modes keeps heat from being released back into space Without the greenhouse effect, Earth would be ~33° colder (water → ice) H2O and CO2 thermally regulate the atmosphere. 19Mary J. Bojan Chem 110 What is Global Warming? Over the past century atmospheric CO2 has increased ~25% Mean global temperature has increased ~1° Increased quantities of CO2 may be causing global warming. 20Mary J. Bojan Chem 110 Need to know about Carbon cycle: interconversion of C in the environment How serious is the problem? 21Mary J. Bojan Chem 110 The Carbon Cycle Sources of atmospheric CO2 Natural volcanic eruption sources decaying organic materials respiration Man-made fuel combustion sources deforestation cement manufacturing 22 Sinks for CO2 Formation of rock Dissolves in oceans PhotosynthesisMary J. Bojan Chem 110 Summary ~200 billion metric tons (bmt) of C in the form of CO2 is removed each year from the atmosphere. ~110 bmt removed by photosynthesis ~ 90 bmt removed by absorption in ocean (seashells, coral, limestone…) ~203 bmt of C are released into the atmosphere respiration decomposition deforestation ~3.5% of CO2 released fossil fuel use 23Mary J. Bojan Chem 110 Summary Projections CO2 levels are expected to double in 25 years. 1.5-4.5 change in global T (?) 1-5m rise in ocean levels (but then more CO2 dissolves!) shift in climate and crop production See National Geographic’s Sept. 2004 Edition
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