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 @ 6:30 Sheets Page 1 Lecture 25
Pressure & altitude atmospheric gases: 78% N 2, 21% O 2, some Ar, CO 2 & 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 10 10 50 10 3 10 7 100 10 6 10 3 200 10 13 1 Sheets Page 2 Lecture 25
Composition of the atmosphere mole fraction: X a = N 2 & O 2 represent ~99% of atmosphere ppm: parts per million: ppm = 10 6 example: neon X Ne = 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? P Ne = Sheets Page 3 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: average KE of molecules is high T means Sheets Page 4 Lecture 25
Photochemistry in the atmosphere photoexcitation: electronic excitation (Chap 6) hν NO 2 NO 2 * (* = excited state) photodissociation: bond broken by absorption of a photon hν O 2 O + O bond E O 2 = 495 kj/mol need sufficient energy: λ < 242 nm; verify this for yourself! recall, E = hν & c = λν (Chap 6) O 2 and O concentrations vary with altitude photoionization: removal of a valence e from a molecule by absorption of a photon hν N 2 N 2 + + e need energy the ionization potential (1495 kj/mol), λ < 80.1 nm Sheets Page 5 Lecture 25
Solar radiation penetration in the atmosphere higher energy lower energy < 100nm 170 300nm > 330nm thermosphere NO + photo- + O 2 ionization O + e mesophere NO + O 2 + stratosphere O 2 O photodissociation NO O 3 troposphere O 2 photoexcitation N 2 earth Sheets Page 6 Lecture 25
O O O Ozone O O O resonance: MG =, (VSEPR) bond length: 1.28 Å (O 2 is 1.21 Å) light blue gas; BP = 111.3 C; pungent odor (electrical discharges) ΔH f = 142.3 kj/mol (reactive, less stable than O 2 ) (see ~Lecture 39) in troposphere: O 3 is an irritant (see smog) in stratosphere: O 3 is essential; peak of [O 3 ] is at ~25 km; [O 3 ] ~ 10 ppm λ < 350 nm (UV) induces photochemistry in many organic molecules (skin cancer) ozone in stratosphere with λ = 240 320 nm most radiation Sheets Page 7 Lecture 25
The natural ozone cycle formation of O 3 O 2 + hν 2O λ < 242 nm O + O 2 O 3 UV-blocking by O 3 O 3 + hν O 2 + O λ < 320 nm the (small) amount of O 3 in stratosphere reflects the delicate balance between creation & destruction http://www.theozonehole.com / Sheets Page 8 Lecture 25
Chlorofluorocarbons (CFCs) examples: CFCl 3 CF 2 Cl 2 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 CF 2 Cl 2 + hν CF 2 Cl + Cl (λ < 240 nm) 2Cl + 2O 3 2ClO + 2O 2 ClO + ClO ClOOCl ClOOCl + hν ClOO + Cl ClOO + hν Cl + O 2 NET: 2O 3 3O 2 Cl atom from CFC catalyzes O 3 destruction (speeds up reaction but is not consumed or used up during the reaction); 1 Cl atom destroys > 100,000 O 3 molecules!?!! Sheets Page 9 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 O 3 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 sunblock Sheets Page 10 Lecture 25
Ozone hole (cont.) http://www.atm.ch.cam.ac.uk/tour/part2.html http://www.nasa.gov/vision/earth/environment/ozone_resource_page.html Ozone hole on 7 Oct 2008 Sheets Page 11 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 effects on crop yields and??? EPA estimates that ozone levels will return to normal by 2050 if full compliance with protocol is achieved. Sheets Page 12 Lecture 25
Our effect on the troposphere greenhouse gases & global warming acid rain photochemical smog Sheets Page 13 Lecture 25
Greenhouse gases & global warming http://www.al.noaa.gov/w WWHD/pubdocs/mission. Earth emits IR radiation, some of which is trapped (or ) by greenhouse gases the good: keeps Earthʼs temperature ~15 C (59 F) instead of 18 C (0 F); evens out day/night temperature variation http://www.markstivers.com/cartoons/stivers%206-10- 02%20Bush%20and%20global%20warming.gif Swimwear www.funnyhub.com/pictures/pages/global-warming-swimwear.html Sheets Page 14 Lecture 25
http://www.gcrio.org/ocp96/figs/fig20.gif http://atschool.eduweb.co.uk/kingworc/departments/geography/nottingham/atmosphere/pages/gfx/ greenhouseeffect.gif Sheets Page 15 Lecture 25
Greenhouse gases & global warming (cont.) the bad: greenhouse gas levels keep increasing so global temperature keeps increasing CO 2 fossil fuel, biomass combustion, volcanic eruptions (but plants help take up excess CO 2 ) CH 4 animal gas [stinky!], swamp & natural gas venting, incomplete combustion, coal mining H 2 O vapor evaporation/condensation www.carbuyersnotebook.com/archives/cow.jpg http://en.wikipedia.org/wiki/image:global_warming_map.jpg Sheets Page 16 Lecture 25
Acid rain: sulfur in atmosphere sources of SO 2 : bacterial decay of organic matter volcanic gases forest fires fossil fuel combustion industrial process in atmosphere 2SO 2 (g) + O 2 (g) SO 3 (g) + H 2 O(l) http://media.allrefer.com/s4/l/p0013033-acid-rain.gif 2SO 3 (g) H 2 SO 4 (aq) http://www.epa.gov/airmarkets/cmap/mapgallery/mg_so2.html Sheets Page 17 Lecture 25
Acid rain: sulfur in atmosphere (cont.) natural rain: ph 6 acid rain: ph of 4 to 4.5 affects ph of soil and water corrodes metals (Fe) dissolves stone (marble, limestone) CaCO 3 (s)+h 2 SO 4 (aq) CaSO 4 (aq)+co 2 (g)+h 2 O( ) http://upload.wikimedia.org/wikipedia/commons/thumb/5/54/-_acid_rain_damaged_gargoyle_-.jpg/800px--_acid_rain_damaged_gargoyle_-.jpg Sheets Page 18 Lecture 25
Photochemical smog primary hν secondary pollutants pollutants NO, NO 2, CO O 3 hydrocarbons in auto engine: N 2 +O 2 2NO ΔH =181 kj http://en.wikipedia.org/wiki/smog in air: 2NO + O 2 2NO 2 oxidation NO 2 + hν 2NO + O photodissociation λ = 400 nm O + O 2 + M O 3 + M (M=another molecule) NO 2 + H 2 O HNO 3 acid rain, burns eyes ozone: good in the stratosphere, but not in the troposphere, where it diminishes respiratory capabilities; reacts with NO to form NO 2 and O 2 ; photodissociates to form reactive oxygen radicals, which then react with hydrocarbons, etc. Sheets Page 19 Lecture 25
Before next class: Read: BLB 5.5; 11.4 HW: BLB 5:48,49,51; 11:33,37,39 Know: phase changes heat capacity calorimetry Answers: p. 3: P Ne = 1.79 10 5 atm Sheets Page 20 Lecture 25