Ozone Flashcards
Bond angle in ozone , O3
- Based on a TRIGONAL PLANAR molecule
- The LONE PAIR reduces the bond angle from 120 to 117.5 degrees
Ozone layer
- Found at a height about 20km from the surface of the Earth
- In the outer edge of the STRATOSPHERE
- Only a tiny percentage of the gases in the ozone layer are actually ozone
Between 10km to 40km above the surface
- Ozone filters
- Absorbs UV light of wavelengths < 320nm which damages life
From 7km to about 20km above the surface
- Ozone prevents harmful UV light reaching the Earths surface
- Stabilises temperatures & protects organisms
From ground to 7km above the surface
- Ozone is TOXIC to humans
- Causes PHOTOCHEMICAL smog
UV-A
- 5% ( 320-400nm) is absorbed by ozone layer
- LONG wavelength
- LOW energy
- LESS damaging
UV-B
- 95% (280-320nm) is absorbed by the ozone layer
- SHORT wavelength
- HIGHER in energy
- Causes GENETIC damage
- Increases risk of SKIN CANCER in humans
- A reduction in the level of ozone increases the chance of damage
UV-C
- 100% ( 200-280nm) is absorbed by ozone layer
- SHORT wavelength
- HIGHEST in energy
Making ozone
O2 molecules absorb UV light with a wavelength LESS than 240nm:
O2 + UV (<240nm) ——– 2O (atomic oxygen)
An oxygen atom reacts with O2 to form ozone and heat:
O2 + O — O3 + heat
- The heat produced is ABSORBED by air molecules in the stratosphere.
In the ozone layer
Ozone BREAKS DOWN :
O3 + UV ( 240-310nm) ———— O2 + O
Ozone is then FORMED:
O2 + O ———— O3 + heat
- STEADY amount of ozone in the atmosphere
- Rate of BREAKDOWN of ozone is EQUAL to the FORMATION of ozone ( dynamic equilibrium)
O2 + O REVERSIBLE SIGN O3
REMOVING ozone
- The LOW concentration of ATOMIC oxygen in the stratosphere slowly reacts with ozone to produce OXYGEN:
O + O3 ——— 2O2
Ozone depletion (ODP)
- The natural ozone-oxygen equilibrium can be UPSET by new compounds
ODP ~ the relative amount of breakdown to the ozone layer by the substance.
- RADICALS generated from new compounds in the stratosphere lead to ozone DEPLETION
What are CFCs & HCFCs?
(chlrofluorocarbons & hydrochlorofluorocarbons)
Used until recently as:
- Refrigerants
- Propellants
- In air conditioning units
HIGH STABILITY:
- Strong carbon-halogen bonds within the molecules
- Only remain stable until they reach the stratosphere where they break down to form RADICALS
- These CATALYSE the depletion of ozone
PHOTODISSOCIATION ~ process of using radiation to break down molecules
CFCs
chlorofluorocarbons
LONG RESIDENCE TIME:
- in the troposphere
- can take many years for them to reach the stratosphere
The CFC trichlorofluoromethane , CFCl3 :
- Has an ODP of 1.0
- Causes the GREATEST depletion of ozone
Why are HCFCs less damaging to the ozone layer than CFCs?
- MORE REACTIVE
- Broken down via the breakage of C-H bonds
Mechanism for Ozone depletion with a CFC
INITIATION ~ uv light generates radicals from CFCl3 in the stratosphere via homolytic fission
CFCl3 ——- Cl . + CFCl2
PROPAGATION:
Step 1 ~ Chlorine radical attacks ozone:
Cl . + O3 —— ClO . + O2
Step 2 ~ ClO . radical attacks atomic oxygen
ClO . + O —– Cl . + O2
OVERALL REACTION : O3 + O —– 2O2
Mechanism for ozone depletion with NO
INITIATION ~ NO forms in the atmosphere in the presence of lightning or as a result of aircraft travel:
N2 + O2 —- 2 . NO
PROPAGATION :
Step 1 ~ NO radical attacks ozone:
. NO + O3 —— . NO2 + O2
Step 2 ~ . NO2 radical attacks atomic oxygen:
. NO2 + O ——— . NO + O2
OVERALL REACTION : O3 + O —– 2O2
HCFC’s Vs HFC’s
HYDROCHLOROFLUOROCARBONS:
- break down MORE EASILY
- C-H bonds undergo homolytic fission before reaching the stratosphere
- LOW ODP as fewer molecules reach the stratosphere and make Cl radicals
HYDROFLUOROCARBONS:
- Do not contain CHLORINE
- cannot deplete stratospheric ozone
