E9+Further+Ozone+Depletion+(HL)

__Further Ozone Depletion (HL ONLY)﻿﻿﻿__
The double bond in an oxygen molecule has an average bond enthalpy of 496kjmol-1

O2 + UV (242 nm) → 2O◦ O3 + UV (242 nm) → O2 + O◦ Ozone is easier to break down than Oxygen because its resonance structure is equivelent to having 1.5 bonds in comparison to Oxygen having the bond order of 2. Therefore it requires less energy to break down ozone than oxygen molecule.

__**//Catalysis of ozone destruction by CFCs and Nitrogen Oxide//**__

When CFC absorbs UV light in the stratosphere, it creates a homolytic fission of the weak C-Cl bond, producing a chlorine radical. These radicals then react with the ozone molecules and regenerate more radicals to break down 100,000 ozone molecules! CCl2F2 (g) → CClF◦(g) + Cl◦ (g) {Initiation stage} Cl◦(g) + O3(g) → ClO◦(g) + O2 (g) ClO◦(g) + O◦(g) → Cl◦ + O2 (g) {propagation stage}
 * CFCs** is responsible for the ozne destruction![[image:http://www.biology.ualberta.ca/taylor.hp/Biology381/topic-13/cfc-d-oz.jpg width="345" height="223" align="right"]]


 * Nitrogen oxide** also catalytially decompose ozone by producing oxygen radical

Nitrogen oxide absorbs UV light also from the stratosphere, which breaks down and produce an oxygen radical NO2(g) → NO◦(g) + O◦ (g) O◦(g) + O3(g) → 2O2(g) NO(g) + O3(g) → NO2 (g) + O2 (g) {regeneration of nitrogen oxide to produce oxygen radical again (and the cycles continue)}

__**Reasons for greater Ozone Depletion in the Polar Regions**__
 * Depletion is seasonal:
 * Winter: low temperatures in the stratosphere above the poles. Small amount of water vapor in the air freezes to form crystals of ice which contains HCl and ClONO2- catalytic reactions produces hypochlorous acid (HClO) and chlorine
 * Spring: Light energy from sun breaks down these molecules forming Cl radicals catalyzing the destruction of ozone

Sunlight can be harmful to the skin, at longer wavelengths it can cause reddening of the skin. At shorter wavelengths however it can damage proteins causing the skin to look wrinkled and leathery, also it can break the bonds in DNA damaging genes and leading to skin cancer.
 * Sun-screening compounds**


 * Commercial Sunscreens such as 4-aminobenzoic acid can absorb light of certain frequencies. They contain conjugated double bonds and delocalized pi electrons. UV light is absorbed by the pi electrons.
 * Sunblocks contain white pgiment such as zinc oxide, which form a barrier to sunlight by reflecting and scattering the light!