Lecture 6• Recap on where ozone comes from• Catalytic destruction of ozone(We’re following Chapter 17 of the textbook)Coming up:• Antarctic ozone hole (Weds)•Stopping ozone depletion (international agreement)& ozone depletion in the scientific versus anti-scientific debate (Thurs)Oxygenonly - 2Calculate the NET reaction…(1) O2 + UV --> O + O(2) 2O2 + 2O --> 2O3 (3) O3 + UV --> O2 + O(4) O3 + O --> O2 + O2-------------------------------------------------- UV + UV --> heat !• There is no net production or destruction of any chemical species.• This cycle of reactions produces:- heat (warming the stratosphere)- an equilibrium concentration of O3 (also of O)- "dynamic equilibrium"Given solar energy (UV flux) and O2 concentration…• equilibrium concentration of O3 depends on rate of reaction (4).note on MA couple of nuances#1: Strictly speaking, the production reaction should be written:(2) O2 + O + M --> O3 + M The “M” is any molecules but most likely nitrogen, N2, which isthe most abundant component of air (78%)We skipped the "M" for simplicity#2: Note also that photons are sometimes represented inchemical reactions by the symbol “hn”.This is because the energy of a photon is equal to theproduct of h (Planck’s constant) and frequency, n.i.e.photon energy, E = hnCatalyticdefinitionCatalyst definitioncatalystA substance that accelerates the rate of a reaction without itself beingconsumedCatalyticreactionexample(In-class)Example catalytic reaction.Calculate the NET reaction…(a) O3 + X --> XO + O2(b) XO + O --> X + O2 --------------------------------------(net) O3 + O --> O2 + O2• Species X is facilitates the destruction of ozone (and atomicoxygen) but is not itself consumed.• Do you recognize the net reaction? Have you seen it before?CatalyticDestruction - 2Catalytic DestructionA catalyst accelerates the rate of a reaction without itself being consumed.(a) O3 + X --> XO + O2(b) XO + O --> X + O2 --------------------------------------(net) O3 + O --> O2 + O2*the 4th reaction from the oxygen-only cycle is catalyzed by species X.(1) O2 + UV --> O + O(2) O2 + O --> O3 (3) O3 + UV --> O2 + O(4*) O3 + O --> O2 + O2A new model of ozone equilibrium...Result will be a lower equilibrium concentration of O3 (due to moreefficient removal).CatalyticDestruction - 3Catalytic DestructionSpecies X can be: NO (odd-nitrogen)Cl (chlorine)Br (bromine)(a) O3 + Cl --> ClO + O2(b) ClO + O --> Cl + O2 --------------------------------------(net) O3 + O --> O2 + O2Reactions when X = chlorine…NO, Cl, and Br have all been greatly enhanced in thestratosphere due to human activities.Origin ofCFCsRecap on the history of CFCs1929: 100 people are killed in a hospital in Cleveland due to a leak inthe refrigeration system.Refrigeration systems require a "working gas" to transfer heat viacompression and expansion cycle.Traditional working gases were toxic - sulfur dioxide and ammoniaThe invention of chlororfluorocarbons (CFCs) in the 1930s was agreat step forward for public safety. CFCs are "inert" and totally non-toxic.Soon, many other uses were found for CFCs and related brominecompounds known as halons (carbon-fluorine-bromine compounds)Slowly but surely, CFCs accumulated in the atmosphere.Initially no one knew of CFCs’ potential danger."The presence of these compounds constitutes no conceivablehazard" (Jim Lovelock, 1973, in Nature magazine). “This sentence has turned out to be one of my greatest blunders”(Jim Lovelock (1988) “The Ages of Gaia: A biography of our living Earth”, p.165)[Figure source: WMO, Scientific Assessment of Ozone Depletion 2002]Vertical distribution of chlorine speciesThree categories:- source gases (CFCs, etc)- unreactive forms in strato HCl and ClONO2- reactive form in strato: ClOCFC cycle in atmos:- source in tropo; well-mixed- photolyzed in strato;decreases with heightOnly the reactive form is acatalyst and a danger tostratospheric ozoneVertical distribution of CFCsCFC lifetimesAtmospheric lifetimes of CFCsAtmospheric lifetime (or "residence time"):The average length of time a substance spends in theatmosphere.45 yrsCCl3FFreon-11CFC-11100 yrsCCl2F2Freon-12CFC-12atmosphericlifetimechemicalformulatradenamenameobserved CFCdecreases: In-ClassObserved changes in CFCs: in-class activitya) CFC-11b) CFC-12c) carbon tetrachlorided) methyl chloroformEmissions of all these compounds were greatly reduced in about 1990.Which compound has the longest atmospheric lifetime? Explain.t = 45 yrs t = 100 yrst = 5 yrst = 26 yrsRecapBUT then came the OZONE HOLE DISCOVERY (1985)ozone and CFC recap...Understanding in 1970s: - Cl can catalyze reaction (4) - Cl in stratosphere is increasing due to CFCs but-Cl is slowly removed as HCl (relatively unreactive):Cl + CH4 = HCl + CH3 - and most Cl is stratosphere is locked up in unreactiveformsthus, - CFCs should cause only modest loss of ozone(predicted 7% loss by 2100)(1) O2 + UV --> O + O(2) O2 + O --> O3(3) O3 + UV --> O2 + O(4) O3 + O --> O2 + O2Ozone level governed bydynamic equilibrium. Lossrate via step (4) is
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