e 4. ozone depletion in stratosphere describe the formation and depletion of ozone in the...
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E 4. Ozone depletion in stratosphere
Describe the formation and depletion of ozone in the stratosphere by natural processes.
List the ozone-depleting pollutants and their sources.
Discuss the alternatives to CFCs in terms of their properties.
Ozone Oxygen is present in two forms, O2 and O3 protect life on the Earth’s surface from
harmful ultraviolet (UV) radiation.
OzoneOzone O3
very pale bluish gas very powerful oxidising agent pungent smelling odor absorbs UV light detection: [O3] in a sample of air can be
measured using UV spectroscopy; the more UV is absorbed the higher [O3]
in upper stratosphere; 15 to 45 km
The bonds in oxygen and ozone are broken by UV of different wavelengths The bond in oxygen and ozone are both
broken when they absorb UV radiation of sufficient energy.
The double bond in O2 is stronger than the 1.5 bond in ozone and so is broken by radiation of shorter wavelengths.
Worked Example The bond energy in ozone is 363 kJ mol-1. Calculate the
wavelength of UV radiation needed to break the bond.
Ozone depletionOzone depletionTwo functions absorbs UV – 290 – 320 nm; UV
causes sunburn, skin cancer, eye cataracts (=clouding of the eye – can lead to blindness)
reduces plant growth as O3 destroys apparatus for photosynthesis
can cause genetic mutations causes loss of plankton
Ozone production releases energy which produces an increase in temperature in stratosphere which gives it stability
The natural formation of ozone The temperature of the atmosphere
generally decreases with height But at 12km above the Earth’s surface,
temperature starts to rise because the ultraviolet radiation is absorbed in a number of photo chemical reactions.
This part of the atmosphere is called the stratosphere.
Ozone:Ozone: natural cycle (stratosphere) formation of ozone: O2 + uv O + O (λ = 242 nm)
O2 + O O3
More energy needed to break double bond.
natural depletion of ozone O3 + O 2O2
O3 + uv O2 + O (λ = 290 – 320 nm)Less energy needed to break ‘1.5 bond’ (delocalized pi bond).
rate of formation = equal to rate of depletion = steady state both types of reactions are slow
OzoneOzone: man-made depletion nitrogen oxides: sources: combustion, airplanes, nitrogenous
fertilizers
Ozone depletionOzone depletion: equations catalytic depletion:
NO + O3 NO2 + O2
NO2 + O NO + O2
When added:
O3 + O 2O2
Chlorofluorocarbons CFCs = chlorofluorocarbons end up in stratosphere as they are not broken
down C-Cl bond is weakest; easily broken by UV: Cl free radical produced by uv -
photodissociation Cl acts as catalyst in ozone depletion – catalytic
depletion
ChloroFluoroCarbons:ChloroFluoroCarbons: useful compounds chemically stable; long atmospheric life-time low toxicity low cost to manufacture volatile liquids good solvents Insulating fire-suppressant coolant in ACs and fridges dry-cleaning agent
Ozone depletionOzone depletion: equations photodissociation: C- Cl is weakest bond
CCl2F2 CClF2 + Cl catalytic depletion:
Cl + O3 ClO + O2
ClO + O Cl + O2
Ozone depletion:Ozone depletion: alternatives to CFCs
hydrocarbons such as propane and 2- methyl propane as refrigerant coolants: no halogens
fluorocarbons: stronger C-F bonds don’t break hydrochlorofluorocarbons: hydrogen makes it
more stable; fewer halogen free radicals released hydrofluorocarbons: stronger C-F bonds don’t
break
Ozone depletion:Ozone depletion: alternatives to CFCs propane and 2- methyl propane as refrigerant
coolants: greenhouse gases/flammable; highly hazardous
fluorocarbons: greenhouse gases but not flammable; non-hazardous
Hydrofluorocarbons: greenhouse gas, non-flammable, low toxicity
hydrochlorofluorocarbons: still some depletion as has Cl, and also greenhouse gases; hazardous
Ozone:Ozone: evidence for depletion
Antartica, autumn 2003
ozone hole = area having less than 220 Dobson units
(if 100 DU of ozone were brought to the Earth's surface, it would form a layer 1 millimeter thick)
Ozone depletion at the poles Special mechanism causes esp high ozone
depletion around poles During winter, poles become very cold, with
circular winds preventing warm air from entering Forms ice in clouds, which act as heterogeneous
catalysts for ozone depletion HCl and ClNO3 combine to form Cl2, which then
disassociates in summer to form Cl*