E4+Ozone+Depletion

__**E4**__ __**Ozone Depletion**__
__E4.1__ __Natural formation and destruction of ozone__
 * Ozone (O3) is an allotrope of oxygen (O2).
 * Ozone gas protects us by preventing most UV rays from reaching the Earth.
 * Formed when the double bond of an oxygen molecule is broken due to UV rays which have high penetrating energy.
 * O2(g) -(UV)-> 2O*(g)**
 * O*(g) + O2(g) --> O3(g) (can decompose to O2 + O*)**


 * High energy radiation can split the oxygen into two atoms with free radicals, which is very unstable
 * The free radical will react with more oxygen to give an ozone molecule.
 * The ozone molecule is unstable and will decompose to steady state (O2). Due to the resonance structure of O3, which has bonds between single and double bonds, the molecule can easily break up by absorbing more UV.
 * O3(g) --> O*(g) + O2(g) (this can decompose to O3 again)**
 * O*(g) + O3(g) --> 2O2(g)**
 * Both processes happen at equal rates so the amount of natural ozone should stay constant, a steady state.

__E4.2__ __Ozone-depleting pollutants__
 * 1) Chlorofluorocarbons (CFC's) - These react with stratospheric ozone and destroys it. They can be found in substances such as refrigerants, propellants, and cleaning solvents. CFC's, such as CCl2F2, have a weak C-CL bond that can be easily broken by UV rays, which effectively produces highly reactive chlorine free radicals. As a result, the CFC acts as a catalyst - destroying the existing O3 and preventing the formation of O3 without being consumed.
 * 2) Nitrogen Oxides - Nitric oxides react with ozone to form O2 gas, which interferes with the natural process of the formation of ozone. Nitrogen oxides are produced when lightning strikes, in engines where nitrogen is sucked in at high temperatures and from combustion of hydrocarbons.

__E4.3__ __CFC Alternatives__
 * 1) Hydrochlorofluorocarbons (HCFC's such as CHClF2) - These are used as an alternative to CFC's because before they reach the stratosphere, they are largely destroyed in the lower region of the atmosphere. Therefore, they have no effects on the ozone layer.
 * 2) Hydroflorocarbons (HFC's) - These molecules are used instead because they contain fluorine atoms rather than any chlorine atoms. This means that there is less radicalization taking place, hence there will be no radicals to react with the ozone layer. It takes more energy to break the bonds of fluorine, so there is a less chance of radicalization.