Ozone

Question 1

Bond angle in ozone , O3

Answer 1

• Based on a TRIGONAL PLANAR molecule
• The LONE PAIR reduces the bond angle from 120 to 117.5 degrees

Question 2

Ozone layer

Answer 2

• 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

Question 3

Between 10km to 40km above the surface

Answer 3

• Ozone filters
• Absorbs UV light of wavelengths < 320nm which damages life

Question 4

From 7km to about 20km above the surface

Answer 4

• Ozone prevents harmful UV light reaching the Earths surface
• Stabilises temperatures & protects organisms

Question 5

From ground to 7km above the surface

Answer 5

• Ozone is TOXIC to humans
• Causes PHOTOCHEMICAL smog

Question 6

UV-A

Answer 6

• 5% ( 320-400nm) is absorbed by ozone layer
• LONG wavelength
• LOW energy
• LESS damaging

Question 7

UV-B

Answer 7

• 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

Question 8

UV-C

Answer 8

• 100% ( 200-280nm) is absorbed by ozone layer
• SHORT wavelength
• HIGHEST in energy

Question 9

Making ozone

Answer 9

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.

Question 10

In the ozone layer

Answer 10

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

Question 11

REMOVING ozone

Answer 11

• The LOW concentration of ATOMIC oxygen in the stratosphere slowly reacts with ozone to produce OXYGEN:

O + O3 ——— 2O2

Question 12

Ozone depletion (ODP)

Answer 12

• 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

Question 13

What are CFCs & HCFCs?
(chlrofluorocarbons & hydrochlorofluorocarbons)

Answer 13

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

Question 14

CFCs
chlorofluorocarbons

Answer 14

• LONG RESIDENCE TIM in the troposphere
• can take many years for them to reach the stratosphere

Trichlorofluoromethane, CFCl3:

• ODP of 1.0
• Causes the GREATEST depletion of ozone

Question 15

Why are HCFCs less damaging to the ozone layer than CFCs?

Answer 15

• MORE REACTIVE
• Broken down via the breakage of C-H bonds

Question 16

Mechanism for Ozone depletion with a CFC

Answer 16

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

Question 17

Mechanism for ozone depletion with NO

Answer 17

INITIATION ~ NO forms in the atmosphere in the presence of lightning or as a result of aircraft travel:

N2 + O2 —- 2NO.

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

Question 18

HCFC’s Vs HFC’s

Answer 18

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