[Music] hi and welcome back to free science lessons by the end of this video you should be able to describe the role of the ozone layer you should then be able to describe how chlorofluorocarbons are linked to ozone destruction and this is for the ocr and aqa specs students following the ocr spec also need to describe the role of nitrogen oxide radicals in ozone depletion we're going to start by looking at the role of the ozone layer and how ozone is formed we're then going to look at the reactions which can lead to the destruction of ozone and i'd recommend that you learn all of the equations that i show in this video okay now the surface of the earth is constantly bombarded with ultraviolet radiation in sunlight uv radiation can damage dna so too much exposure to uv can be harmful to humans and is linked to skin cancer now in the atmosphere there's a region called the ozone layer this is found in the stratosphere around 20 to 40 kilometers above the earth's surface the ozone layer contains a relatively high concentration of the chemical ozone ozone has the symbol the ozone layer absorbs a great deal of the ultraviolet radiation from the sun before it can reach the earth's surface so the ozone layer protects living organisms from excessive uv exposure you need to be able to describe how the ozone layer does this in the stratosphere uv radiation causes the double bond in oxygen molecules to break like this in this reaction we form two oxygen radicals and remember that a radical is a species with an unpaired electron you'll notice that i'm not showing the unpaired electron in the radicals but you need to remember that they are still radicals we can also show this reaction in a simpler form like this now an oxygen radical reacts with an oxygen molecule to form a molecule of ozone now if this was the only reaction taking place then ozone levels would continuously increase however an ozone molecule can absorb ultraviolet light turning back to an oxygen radical and an oxygen molecule and this absorption of uv radiation is how the ozone layer protects us now because ozone's formed and broken down at the same rate the amount of ozone in the ozone layer should remain constant however certain chemicals can cause ozone in the ozone layer to be broken down these chemicals are called chlorofluorocarbons or cfcs and i'm showing an example here cfcs are haloalkanes now cfcs are very stable molecules and that's because of the high bond enthalpy of the carbon to halogen bonds cfcs are also relatively non-toxic in the past cfcs were produced in vast quantities and were used in a range of applications for example in fridges and freezers however scientists later discovered that cfcs could cause the destruction of the ozone layer cfcs make their way up to the stratosphere once in the stratosphere ultraviolet radiation causes a carbon to chlorine bond to break scientists call this process photodissociation this is homolytic fission and produces a chlorine radical which is highly reactive the next stage involves two steps these are called propagation step one and propagation step 2. in propagation step 1 a chlorine radical reacts with the molecule of ozone this produces an oxygen molecule plus a highly reactive chlorine monoxide radical in propagation step two the chlorine monoxide radical reacts with an oxygen radical as we saw earlier oxygen radicals are constantly formed in the stratosphere this produces a chlorine radical plus an oxygen molecule now the chlorine radical from step two can go back and trigger step one again so in effect the chlorine radicals are acting as a catalyst for this reaction alternatively the chlorine monoxide radical can react with an ozone molecule instead but again a chlorine radical is produced and the cycle continues because of this cycle one cfc molecule can lead to the destruction of many thousands of ozone molecules now many scientific organizations provided evidence that cfcs were leading to the destruction of the ozone layer and this led to cfcs being phased out unless harmful alternatives developed now ozone can also be destroyed by nitrogen oxide radicals these are formed by lightning and from aircraft and if you're following the ocr spec then you need to learn these reactions the nitrogen monoxide radical reacts with ozone to produce the nitrogen dioxide radical plus an oxygen molecule the nitrogen dioxide radical then reacts with an oxygen radical this forms another nitrogen monoxide radical plus an oxygen molecule as you can see this takes place in a cycle similar to the one we saw before okay so hopefully now you can describe the link between radicals and ozone destruction