15.2 Flashcards
Organohalogen compound define:
Molecules that contain at least 1 halogen joined to carbon chain
Are organo halogens found in nature?
Rarely
Where ozone layer found?
Outer edge of stratosphere, height varies from 10-40km about earth’s surface
How much if ozone layer is actually ozone?
Very small fraction, but this is enough to absorb most of biologically damagin UV radiation (called UV-B) from the sun’s rays - only small amount of radiation allowed ro reach earth’s surface
Why is a depleting ozone layer bad for living organisms?
Increased depletion would result in more UV-B reaching earth’s surface, means more increaded genetic damage, and greater risk skin cancer in humans
How ozone formed?
First step:
High energy UV breaks O2 molecule into 2 O radicals:
O2 —> 2O
Second step:
A steady state set up involving O2 and oxygen radicals; in this state ozone forms and breaks down. In this steady state, rate of formation equal to rate of breaking down:
O2 + O <==> O3
What were CFCs and HCFCs used in
Refrigerants
Used in air conditioning units
Aerosol propellants
Why CFCs so stable?
C-halogen bond very strong
Frank sherwood rowland and mario mollina began to look at impact of CFCs on Earth’s atmosphere. Which year? What conclude
1973
Concluded that CFCs stable until reach stratosphere. When reach there, begin to break down, forming Cl radicals, which are thought to catalyse breakdown of ozone layer
Why CFCs have long residence time in troposphere?
Strength of C-Halogen bond means can have long residence time. Can take many years to get to stratosphere.
How do CFCs react in the stratosphere?
1) CFCs reach stratosphere
2) UV radiation provides enough energy to break C-Cl bond (as it has lowest bond enthalpy in molecule). As radiation begins breakdown, this step called photodissociation, shown below
CF2Cl2 —> CF2Cl * + Cl*
3) Cl* is very reactive intermediate, can react with ozone molecule, break down into O2. Done by 2-step process:
Propagation step1:
Cl* + O3 —> ClO* + O2
Propagation step 2
ClO* + O —> Cl* + O2
Overall equation is:
O3 + O —> 2O2
Propagation step 2 regenerates another Cl*, which can attack another molecule of ozone in propagation step 1 - the two propagation steps therefore repeat over and over in chain reaction.
Estimated single CFC molecule can promote breakdown of 100k ozone molecules
Are CFCs the only thing responsible for ozone layer breakdown?
No, NO radicals can break down ozone in mechanism similar ro Cl*. NO formed naturally during lightning strikes, and also as result if aircraft travel in stratosphere
Propagation step 1:
NO* + O3 —> NO2* + O2
Propagation step 2:
NO2* + O —> NO* +O2
Overall equation:
O3 + O —> 2O2
