The ozone story- Haloalkanes and ozone
What is bond fission?
Breaking a covalent bond
What is homolytic fission? - What is formed
- Where each bonding atom receives one electron from the shared pair. - Two electrically uncharged '*radicals*' are formed. Radicals are particles that have unpaired electrons, and because of the unpaired electron, these radicals are *very reactive*.
How is UV radiation good/bad for you? - How does the ozone layer protect us from it
- *UVB* radiation can damage the DNA in cells and cause *skin cancer* and is the main cause of *sunburn*. *UVA* can also lead to skin cancer. However, we need it to produce *vitamin D*, and when the skins exposed to UV is *tans* protecting deeper tissue from the radiation. - When ozone breaks down it absorbs *high energy UV radiation* so the ozone layer removes all the high energy UVC radiation and about 90% of UVB.
What are CFCs? - How are CFCs breaking down ozone? - Why does this mostly happen in the stasophere
- CFCs are haloalkanes that have had *all* their hydrogen atoms replaced by a *chlorine or fluorine atoms* - The C-CL bond in CFCs is broken down by *high energy UV radiation* in the *stratosphere* and forms *chlorine radicals.* They act as catalysts in the breakdown of *ozone*. - Further down in *troposphere* only a few CFCs get broken down from by UV as it has mostly been absorbed in by the ozone layer, this means ozone is less likely to be broken down in the troposphere as theirs less *suitable catalysts*.
How can ozone occur in the troposphere (at ground level)? - What does this cause?
- Sunlight can react with *nitrogen dioxide* and *hydrocarbons*, these can occur *naturally* but vehicle engines and power stations produce large amounts. - Ozone can mix with *particulates* and create an air pollutant called *photochemical smog* Photochemical smog can cause *respiratory problems* and is *toxic* to humans, it can also be dangerous for animals and plants and materials too.
What is ozone? - How is it formed (equation)
- The ozone layer is in a layer of the atmosphere called the *stratosphere*, it contains most of the atmosphere's ozone molecules (O₃). - Ozone is formed when *UV radiation* from the sun hits oxygen molecules.
What is heterolytic fission? - What is formed
- Where one atom in the bonding pair receives both electrons from the shared pair. - *Two different substances are formed*- a positively charged cation (X+) and a negatively charged anion (Y-), Y has electron pair.
Three main stages in a radical chain reaction
1.) *Initiation* reactions: free radicals are *produced*. 2.) *Propagation* reactions: Free radicals react with molecules and produce *new radicals*, these go on to react with more molecules, producing *even more* radicals. This is the *chain* part of the reaction. 3.) *Termination* reactions: two radicals react with each other to form a *stable molecule*.
Example of some radicals - General equation for a radical reacting with ozone
HO, NO, other halogen free radicals