Ozone+Depletion+(HL)

**__Ozone Depletion HL__**
Terence Leung H2 //__Note: This Wiki Is To Be Read After Ozone Depletion SL__//

//__Course Objectives:__// //__9.1 Explain the dependence of O2 and O3 dissociation on the wavelength of light__// //__9.2 Describe the mechanism in the catalysis of O3 depletion by CFC and NOx__// //__9.3 Outline the reasons for greater ozone depletion in polar regions__//



**Foreword**: Our atmosphere is made up of a layer of gas termed as ozone layer. This layer of gas protects our earth from harmful UV (ultra violet) rays from the sun, and is mainly made up of oxygen. In recent years however, the Ozone layer has been steadily depleting and thinning. My objective with this wiki is to lecture readers about how oxygen in the ozone layer absorbs the UV rays, the chemical equations and formulas behind it. I will also be discussing how our ozone layer is being damaged by CFC (Chloroflurocarbon) and Nitrogen Oxides. To sum up, I will be explaining in relative terms why the extent of ozone depletion is greater in the polar regions.

Points To Remember: 1. Ozone is O 3 __﻿__ 2.Bond Enthalpy at 298K for 0=0 double bond is 496 kJ mol 3. Energy of ultraviolet radiation absorbed by ozone in the atmosphere can be calculated using different bond enthalpies.
 * Ozone Depletion **


 * Ultraviolet Radiation/ Ozone Bond**

Energy of ultraviolet radiation absorbed by ozone is calculated using bond enthalpies At 298K, the oxygen-oxygen double bond enthalpy is 496 KJ mol-1. Hence, we can see that this bond enthalpy can absorb 8.235 x 10 -19 J. We can calculate the wavelength of the ultraviolet radiation that corresponds to this enthalpy value (E) using the equation O 3 has resonance structures, the electrons are delocalized across the oxygen atoms. Hence, its bond order is 1.5. Hence ultraviolet light with a longer wavelength (lower energy) is absorbed in breaking the ozone bond

1. CFC were once mass produced for a variety of purposes. Most CFC contains a tetrahedral carbon, with the fluorine and chlorine differing each other in terms of mass and size.People would use it as refrigerants, propellants, and as a solvent for chemical purposes. Its original purpose was to be used as an anti-fire extinguisher during WWI.However, experiments showed that CFC were extremely harmful to the ozone layer. It is stated that the CFC caused induced the O 3 to break up into O 2, which generated an increase in the presence of O. This allowed more ultraviolet rays to reach the earth, increasing the average temperature and causing skin cancer and other problems.
 * Chloroflurocarbon (CFC)**

// HCCl 3 + 2 HF → HCF 2 Cl + 2 HCl //

Refers to the formation of CFC from chlorinated methanes and ethanes which undergo halogen exchange.

CFC are highly destructive to the ozone layer due to their characteristic of having low reactivity. It was shown from experiments that once in the atmosphere, it has an extremely long lifespan of around 100 years, giving it time to diffuse into the upper stratosphere. There, the suns ultraviolet radiation is extremely powerful and strong, causing the C-Cl bond to undergo homolytic cleavage. This in turns damages the ozone layer. CFC has been banned from production by the Montreal Protocol since 1987, which aims to phase out CFC stores by 2000 in developed countries and 2010 in developing and 3rd world countries.

Alternatively, Nitrogen Monoxide can also react to decompose ozone. The higher the amount of nitrogen dioxide, the more ozone decomposes.

NO(g)+O 3 (g) ->NO 2 (g)+O 2 (g)

Polar Regions such as Antarctica suffer greatly from ozone depletion, which causes harmful UV rays to melt the ice landscape. This process is repeated again and again with no end in sight. This huge ozone layer hole in the Polar Regions is destroying the habitats of the environment there, as temperature is heating up rapidly
 * Polar Region Ozone D****epletion**[[image:poor%20polar%20bear.jpg width="139" height="138" align="right"]]



During the winter time in the stratosphere over these polar regions, the temperature drops to very low. In these extreme conditions, the water vapor that is constant in the air will occasionally freeze to form ice crystals. The crucial point about these ice crystals is that they contain small molecules such as hydrogen chloride (HCL) as well as chlorine nitrate (ClONO2). Some experts deduce that the presence of these molecules give rise to catalytic reactions on the surface area of ice crystal s, which is high as they have a small volume, producing substances such as hypochlorous acid HClO and chlorine gas Cl2. During the coming of spring, the sunlight that occurs will cause these molecules to break down into producing chlorine radicals, which acts as a catalyst in the destruction of the ozone layer above the polar regions. This will give rise to these polar holes, gaps in the ozone layer. When the sun warms the crystals that are formed over the winter, they disperse, taking their chlorine radicals along with them. With the warm winds coming into the region, this allows the ozone layer to slowly increase in concentration again, until the cycle is repeated during winter.
 * Why?**

References: 1. IB Diploma Program Chemistry, Course Companion (2007) Oxford/ Geoffrey Neuss 2. Ground Level Ozone, 2010, US environmental protection agency,[] 3. Ozone Depletion, 2007, Green Nature [] 4. Global Impact, 2010, Ozone Depletion [] 5. The Ozone Depletion Phenomenon, 2009, Beyond Discovery [] 6. Ozone Depletion, 2010, National Geographic, []