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Sheets Page 1 Lecture 25 Lecture 25: Atmosphere & environment Read: BLB 18.1–4 HW: BLB 18:9,11,15,29,69 Sup 18:1–3 Know: • ozone chemistry • chemistry of the lower atmosphere • sulfer compounds & acid rain • nitrogen oxides & smog check out the environmental & climate change folder under “Lessons” on our Angel site for more links & movies, fyi only Need help?? Get help!! TAs in CRC (211 Whitmore) and SI—hours on Chem 110 website; my office hours (Mon 12:30-2 & Tues 10:30-12 in 324 Chem Bldg [or 326 Chem]) Bonus deadline for BST #8: Intermolecular forces, March 26 Bonus deadline for BST #9: Solutions & dilutions, April 2 Exam 3: Monday, April 6 @ 6:30Sheets Page 2 Lecture 25 Pressure & altitude • atmospheric gases: 78% N2, 21% O2, some Ar, CO2 & other gases • pressure at a given altitude depends upon weight of gas above it • pressure decreases exponentially as altitude ↑ altitude P collision frequency (km) (atm) (s–1) 0 1 1010 50 10–3 107 100 10–6 103 200 10–13 1Sheets Page 3 Lecture 25 Composition of the atmosphere • mole fraction: Xa = • N2 & O2 represent ~99% of atmosphere • ppm: parts per million: ppm = × 106 example: neon XNe = 0.00001818 Ne concentration = 18.18 ppm (see BLB Table 18.1) • if you know barometric pressure, you can determine partial pressure of the gas; recall those partial pressures? Example: What is the partial pressure of neon if the barametric pressure is 0.987 atm? PNe =Sheets Page 4 Lecture 25 Atmospheric layers based on temperature profile • troposphere: T ↓ as altitude ↑; where we live, weather, planes • stratosphere: T ↑, warming caused by ozone cycle; UV light absorbed • mesosphere: T ↓, low density of gases • thermosphere: T ↑, high energy radiation is absorbed; ions formed • present life could NOT survive (above ground) if all solar radiation reached Earth • recall, pressure profile of atmosphere: low pressure ⇒ ↓ molecular collisions, thus, chemical reactions occur frequently • temperature profile of atmosphere: T means average KE of molecules is highSheets Page 5 Lecture 25 Photochemistry in the atmosphere • photoexcitation: electronic excitation (Chap 6) hν •NO2 → •NO2* (* = excited state) • photodissociation: bond broken by absorption of a photon hν O2 → •O• + •O• bond E O2 = 495 kJ/mol need sufficient energy: λ < 242 nm; verify this for yourself! recall, E = hν & c = λν (Chap 6) • O2 and •O• concentrations vary with altitude • photoionization: removal of a valence e– from a molecule by absorption of a photon hν N2 → N2+ + e– • need energy ≥ the ionization potential (1495 kJ/mol), λ < 80.1 nmSheets Page 6 Lecture 25 Solar radiation penetration in the atmosphere higher energy lower energy < 100nm 170–300nm > 330nm _______________________________________ thermosphere NO+ photo- O2+ ionization O+ e– mesophere NO+ O2+ stratosphere O2 O photodissociation NO O3 troposphere O2 photoexcitation N2 earthSheets Page 7 Lecture 25 Ozone • resonance: MG = , ° (VSEPR) • bond length: 1.28 Å (O2 is 1.21 Å) • light blue gas; BP = –111.3°C; pungent odor (electrical discharges) • ΔHf° = 142.3 kJ/mol (reactive, less stable than O2) (see ~Lecture 39) • in troposphere: O3 is an irritant (see smog) • in stratosphere: O3 is essential; peak of [O3] is at ~25 km; [O3] ~ 10 ppm • λ < 350 nm (UV) induces photochemistry in many organic molecules (skin cancer) • ozone in stratosphere most radiation with λ = 240–320 nm OOOOOOSheets Page 8 Lecture 25 The natural ozone cycle • formation of O3 O2 + hν → 2O λ < 242 nm O + O2 →O3 • UV-blocking by O3 O3 + hν →O2 + O λ < 320 nm • the (small) amount of O3 in stratosphere reflects the delicate balance between creation & destruction http://www.theozonehole.com/Sheets Page 9 Lecture 25 Chlorofluorocarbons (CFCs) • examples: CFCl3 CF2Cl2 freon-11 freon-12 properties uses relatively inert aerosol propellants easily liquified refrig. coolants non-combustible cleaning solvents volatile polymer mfg. • BUT CFC destroys ozone CF2Cl2 + hν → CF2Cl• + Cl• (λ < 240 nm) 2Cl• + 2O3 → 2ClO• + 2O2 ClO• + ClO• → ClOOCl ClOOCl + hν → •ClOO + •Cl •ClOO + hν → •Cl + O2 NET: 2O3 → 3O2 • Cl atom from CFC catalyzes O3 destruction (speeds up reaction but is not consumed or used up during the reaction); 1 Cl atom destroys > 100,000 O3 molecules!?!!Sheets Page 10 Lecture 25 Ozone hole • see BLB Fig. 18.5 • special conditions at the South Pole create dramatic seasonal loss of ozone • complex reactions related to presence of polar stratospheric clouds explain the seasonal nature of the loss • nearly complete loss of ozone at some altitudes near the poles, and ~50% total reduction at other latitudes near the poles • away from the poles, depletion is not as great and is seasonal • global O3 concentration has declined since 1980 • you can burn in <7 min in Chile & Argentina; Australia 6% lethal skin cancer vs. 0.3% general world population ⇒ bring your sunblockSheets Page 11 Lecture 25 Ozone hole (cont.) http://www.nasa.gov/vision/earth/environment/ozone_resource_page.html Ozone hole on 7 Oct 2008 http://www.atm.ch.cam.ac.uk/tour/part2.htmlSheets Page 12 Lecture 25 Ozone hole (cont.) the good news: 1987: Montreal Protocol called for virtual elimination of ozone depleting substances; signed by 160 nations; 1996: production and use of CFCs banned worldwide; recovery by 2100?? See http://video.google.com/videoplay?docid=-8206266264728754527&q=ozone+hole&total=212&start=0&num=10&so=0&type=search&plindex=0 the bad news: CFCs are stable & will remain in troposphere and continue to diffuse up to stratosphere if CFC production (or release into the atmosphere) DID stop, ozone depletion would continue for at least several decades effects include 2–10% increase in skin cancers per 1% decrease in ozone, possible


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PSU CHEM 110 - LECTURE NOTES

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