E4: Ozone Depletion (UNMODIFIED)

IMPORTANT NOTE:Ozone Layer + Ozone Depletion = Stratosphere (in this topic)Absorption of UV rays = reason why Earth can shield itself from harmful rays

Natural Formation/ Depletion of O3Ozone = continuously destroyed and formed in the stratosphere (due to the presence of UV light, which has enough energy to break O bonds)O3 vs O2 (UV Light-wise)Ozone (O3) O-O bond = resonance structure (intermediate between single and double bond) Weaker bond Less energy to break O-O bond (BE~393 kJ/mol) Longer wavelength required (330nm required) Oxygen Gas (O2) Double O=O bond Stronger Bond More energy required to break O=O bond (BE~498 kJ/ mol) Shorter wavelength required (242nm required)

The Mechanism of Formation O2(g) --> O•(g) + O•(g) (+ UV rays O•(g) + O2(g) --> O3(g) O•(g) + O2(g) --> O3(g) Therefore: 3 O2(g) --> 2 O3(g)

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The Mechanisms of Depletion O3(g) --> O•(g) + O2(g) O•(g) + O3(g) --> O2(g) + O2(g) Therefore: 2 O3(g) --> 3 O2(g)

Equilibrium Steady state is achieved as rate of depletion and formation = remain the same Therefore: 2 O3(g)  3 O2(g) Ozone layer = "maintained"

Chlorofluorocarbons (CFCs) and NOx Acts as catalysts to ozone depletion Disrupts equilibrium as the rate of depletion > rate of formation Therefore: Ozone decreases overall

CFCProperties Synthetic compounds (with C, Cl, F, and sometimes H) Very stable and unreactive Low toxicity Low flammability Non-polar compounds Inert in troposphere, but reactive in stratosphere Sources:Refrigerants, solvents, etc.Mechanism: CCl3F(g) --> •CCl2F(g) + Cl•(g) (using UV rays) O3(g) + Cl•(g) --> O2(g) + ClO•(g) O•(g) + ClO•(g) --> O2(g) + Cl•(g) Therefore: O3(g) + O•(g) --> 2 O2(g) (using Cl•)

Nitrogen Oxides (NOx)mechanisms = in E9

Basic Properties: Same desirable effects of CFC (non-toxic, low flammability, non-polar) Carry out same functions Not act as Ozone Depleting Substance Not introduce new harm (environmentally, medically) Not persist in the environment No reactive halogens (e.g. Cl) free radicals released No reaction with O3

Hydrocarbons (e.g. propane and methyl-propane)Pros: C-H bonds are stronger than C-Cl Not broken by UV light Readily available Cons: Flammable Still acts as GHG (absorbs IR radiation) Limited use (only alternative to refrigerants) 

FluorocarbonsPros: C-F bonds are stronger than C-Cl Not broken by UV Light Non-flammable, low reactivity, low toxicity Cons:Acts as powerful GHG

HydrocarbonsPros: No C-Cl bond Strong C-F bonds Not broken by UV light Low toxicity Non-Flammable Cons:Acts as GHG

HydrofluorocarbonsPros: Do not decompose as easily than CFCs Do not persist in stratosphere Readily available  20x less destructive Non-flammable Cons: Do reduce ozone due to C-Cl bond Acts as GHG

ImpactsMontreal Protocol (1987) Most successful protocol to this day (to stop the use of CFCs) 197 nations signed treaty

Ozone Cycle

Ozone Depleting Substance

CFC Alternatives