15.2- Organohalogen Compounds In The Environment Flashcards
What are organohalogen compounds ?
Molecules that contain at least one halogen atom joined to a carbon chain
What are some uses of organohalogen compounds ?
- general solvents
- dry cleaning solvents
- making polymers
- flame retardants
- refrigerants
What is CHCl3 used for ?
General solvents
Where is the ozone layer ?
Found at the outer edge of the stratosphere at a height that varies from about 10-40 km above the earths surface
What is the ozone layer ?
- absorbs harmful UV-B radiation
- and prevents UV-B from reaching the earth’s surface
What can occur if UV-B reaches the earths surface ?
Leads to genetic damage
What is UV-B commonly associated with ?
Linked to sunburn
What is continually happening in the stratosphere ?
- ozone is continually being formed and broken down by the action of UV radiation
- initially very high energy UV breaks oxygen molecules into oxygen radicals
O2 —-> 2O
What are CFCs ?
Chlorofluorocarbons
What do CFCs do ?
- break down the ozone layer in the atmosphere
- they are molecules that have had all of there hydrogens replace by chlorine and fluorine
What is the ozone layer made or ?
O3 molecules
What are some properties of CFCs ?
They are very stable due to the strength of the carbon- halogen bond
Are CFCs always stable
They’re stable till they reach the stratosphere
What do CFCs do in the stratosphere ?
- they begin to break down forming chlorine radicals
- these chlorine radicals catalyse the breakdown of the ozone layer
What occurs in the initiation stage of this process ?
- bonds break via homolyctic fission
- the C-Cl bond has the lowest bond enthalpy and so if the bond that breaks
- UV breaks the C-Cl bond and produces 2 radicals which will react with the ozone molecule O3
CFCl3 ——> CF3• + Cl•
State the equation to show the break down of CFCl2
CF2Cl2 ——> CF2Cl• + Cl•
- the chlorine radicals formed is a very reactive intermediate
What happens in propagation step 1 ?
Cl• + O3 —-> ClO• + O2
What happens in the propagation step 2 ?
ClO• + O• —-> Cl• + O2
What is the overall equation?
O3 + O• —-> 2O2
How many molecules of ozone can a single CFC molecule break ?
100,000
What other type of radical can also catalyse the breakdown of the ozone layer ?
Nitrogen oxide radicals
When are nitrogen oxide radicals formed naturally ?
- During lighting strikes
- as a result of aircraft travel in the stratosphere
State the equations of nitrogen oxide radicals that cause the breakdown of ozone
NO• + O3 ——> NO2 + O2
NO2• + O• ——> NO• + O2
Overall equation is the same as with the chlorine radicals :
O3 + O• ——> 2O2
What is depletion ?
Gradual thinning of the earths ozone layer in the upper atmosphere caused due to the release of chemical compounds containing gaseous bromine or chlorine from industries or other human activities
