Done by: Amy Huang

Environmental chemistry (Option E):

4. Ozone depletion
Objectives:
4.1.Describe the formation and depletion of ozone in the stratosphere by natural processes
4.1.1.Introduction of ozone
4.1.2.Formation of ozone
4.1.3.Depletion of ozone
4.2.List eh ozone depleting pollutants and thier sources
4.3.Discuss the alternatives to CFCs in terms of their properties
4.1.1.Introduction of ozone


Fig 1. Regions of the atmosphere.
Fig 1. Regions of the atmosphere.

Our Earth's atmosphere is divided into 5 different layers: troposphere, stratosphere, mesosphere, thermosphere and exosphere. The troposphere is located nearest to the Earth and is where ozone formed. Ozone is formed by an interaction between oxygen molecules and ultraviolet light in the troposphere. Most of ozone formed is concentrated in a region located in the stratosphere, called the ozone layer.
4.1.2.Formation of ozoneThe strong bond of O2 molecules is split apart by the high energy ultraviolet rays from the sun into oxygen atoms which are extremely reactive free radicals as they contained unpaired electrons
O2 (g) + Ultraviolet ray → 2O
These oxygen atoms then react with other oxygen molecules, O2 to form ozone, O3:
O2 (g) + O (g) → O3 (g)


Fig 2. Formation of ozone from oxygen atoms
Fig 2. Formation of ozone from oxygen atoms



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Fig 3. Resonance structures of the ozone molecule.


Ozone, O3, is a pale bluish gas with an acrid order. Ozone acts as a powerful oxidizing agent and is chemically reactive which can cause harm on living matters. Although a small amount of ozone can be harmful to living matters, it is essential for lives and plays an important role by its peresent in the ozone layer. Ozone layer protects the lives on Earth from ultraviolet rays sent by the sun. If ozone layer is depleted, more ultraviolet rays will reach the Earth and causing skin cancer, cataracts, damage to plastics and harm plants.
4.2.3.Depletion of Ozone Layer
In the stratosphere, the strong bond of O2 molecules is split apart by the high energy ultraviolet rays from the sun into oxygen atoms which are extremely reactive free radicals as they contained unpaired electrons:
O2 (g) + Ultraviolet ray → 2O
These oxygen atoms then react with other oxygen molecules, O2 to form ozone, O3:
O2 (g) + O (g) → O3 (g)
The forming of ozone is an exothermic reaction and energy is given out during the reaction, causes a rise in the temperature of stratosphere.






Fig.4.2.3. Ozone depletion cycle
Fig.4.2.3. Ozone depletion cycle


The bonds in ozone are weak and therefore the lower energy ultraviolet rays will break these bonds:
O3 (g) + Ultraviolet ray → O (g)+ O2 (g)
When O3 absorbs ultraviolet rays in the upper atmosphere, it is denatured by a particular wavelength of the rays and becomes depleted.
The oxygen broken down by the low energy ultraviolet rays will then react with another ozone molecule to form two other oxygen molecules:
O3 (g) + O (g) → 2O2 (g)
The forming of two oxygen molecules is another exothermic reaction which produces heat and maintains the high temperature of the stratosphere. This cycle of reactions is essential for the survival of life on Earth because dangerous ultraviolet light has been absorbed by the stratosphere causes it to become warmer.
Although ozone is constantly produced and destroyed in a natural cycle as stated above, the amount of ozone will remain stable or unchanged which means that ozone production and destruction are balanced. However, harmful ground-level ozone can be produced by reactions of sunlight and other air pollutants (greenhouse gases) which has harmful effects on the respiratory systems of human and animals. With a large increase in emission of chlorine and bromine, measurement from 1979-1995 shown a clear decline in ozone concentration, and scientists have said the main cause of it is the release of CFCs, chlorofluorocarbons. CFCs are discovered in 1930s by American chemist Thomas Midgley and are now widely used in Industry, refrigerators, home insulation, plastic foam and in food containers.
CFCs is the main cause of ozone depletion. Ozone depletion process caused by CFCs:

Fig 4.1.2 The effects of ozone depletion.
Fig 4.1.2 The effects of ozone depletion.




Notations:
1:atmosphere (before)
2.above Earth's surface (before)
3.ozone depleting substances
4.stratosphere
5.above Earth's surface (after)
6.atmosphere (after)
1. CFCs and other ozone depleting substances (HCFs, carbon tetrachloride, methyl chloroform, methyl bromide and other substances release chlorine atoms) are emitted into atmosphere and milted with troposphere. The remaining gases are distributed evenly to other levels.
2. As CFCs are extremely stable, thus do not dissolve in rainwater. After a long period of time, these ozone depleting substances reach the stratosphere which has the highest concentration of ozone.
3. Strong ultraviolet light breaks down ozone depleting substances and chlorine is removed from CFCs. Once chlorine is removed from the molecules, it is free to move around and then it attracts one of the three oxygen atoms in the ozone molecule. It is estimated that one chlorine atom can destroy over 100,000 ozone molecules before it is removed from the stratosphere.
Second cause of ozone depletion would be the presence of nitrogen oxides (NO and NO2). Nitrogen oxides cannot be removed from the atmosphere easily after been released and will stay in the atmosphere for at least 22 to 111 years, damaging many ozone molecules.
4.2.Ozone depleting pollutants and their sources
Ozone in the ozone layer is being depleted mainly by two depleting gases:
1. CFCs, chlorofluorocarbons
souces:
  • Spray cans
  • Older refrigerators
  • Air conditions
  • Foaming agent in chemical fire extiguishers
  • balloon-
Impacts:
  • Because CFCs have little or no ability to react (inert), they do not decompose immediately after been released but will float slowly in the atmosphere into stratosphere where ozone is contained in. After they reached the unfiltered ultraviolet rays of the sun, they turned into extremely reactive chlorine atoms with an unpaired electron, and is known as chlorine free radicals:
CCl2F2 + Ultraviolet ray → CClF2 + Cl
2. nitric oxide
Sources:
  • Aircraft engines: The N2 and O2 in aircraft engines react with ozone forming nitric acid:
NO + O3→ NO2 + O2
  • agriculure waste
  • veheciles
Impacts:
  • The reaction between N2 and O2 in aircraft engines and ozone causes a decrease in ozone concentration in the stratosphere. Due to the decrease in ozone concentration, more ultraviolet rays can reach the Earth surface, thus increasing cases of skin cancer, eye cataracts, sunburn and dama ges in animals and plants lives (suppression of growth, genetic mutations, global warming).
4.3.Alternatives to CFCs and their properties
1. Propane:
Use of propane, C3H8 and 2-methylpropane hydrocarbons: refrigerant coolants. These propane do not lead to ozone depletion, but they are flammable as well as being greenhouse gases as they are able to absorb infrared radiation. This will then further lead to global warming.
2. Fluorocarbons:
They are not toxic or flammable and the very strong C-F bond makes them stable to ultraviolet radiation so they won’t catalyze ozone depletion. However, fluorocarbons are greenhouse gases and will lead to global warming.
3. Hydrochlorofluorocarbons (HCIFs):
HCIFs molecules contain hydrogen, chlorine, fluorine and carbon atoms. The presence of a hydrogen atom makes the compound harder to decompose since the C-H bond is stronger (412 KJmol-1) than the C-Cl bond. The properties of the bonds reduce the ozone layer because of the presence of a weak C-Cl bond in the molecule that is only considered as a temporary solution.
4. Hydrofuorocarbons (HFCs):
Uses of HCFs:
Residential Uses
Commercial and Industrial Uses
  • Window air-conditioning units
  • Dehumidifiers
  • Central air-conditioners
  • Air-to-air heat pumps
  • Ground-source heat pumps
  • Ductless air-conditioners
  • Chest or upright freezers
  • Packaged air-conditioners and heat pumps
  • Chillers
  • Retail food refrigeration
  • Cold storage warehouses
  • Industrial process refrigeration
  • Transport refrigeration

HCFs are considered good alternative to CFCs as no chlorine atoms exist, and are primarily responsible for ozone depletion. One example of HFCs is CF3CH2F, 1,1,1,2-tetrafluoroethane.
Summary:

CFC alternative

Hydrocarbons
Fluorocarbons (perfluorocarbons)
Hydrofluorocarbons
Examples
Propane, C3H8
Butane, C4H10
Octafluoropropane, C3F8
Perfluorohexane, C6H14
HCF-23, CHF3
HCF-152a, CH3CHF2
Chemical formula contains
C and H only
C and F only
C, H and F only
Toxicity
Dependent on does and route of ingestion; hydrocarbon abuse due to sniffing, bagging or huffing
Very low toxicity
Low acute toxicity
Flammability
Quite flammable
Low flammability
Low flammability
Damage to the ozone layer
No damage to ozone layer
F does not catalyse ozone destruction so therefore no damage
F does not catalyse ozone destruction so therefore no damage
Ability to absorb infrared radiation and therefore contribute act as a greenhouse gas
Can absorb
Can absorb
Can absorb
Appendix:
Environmental impact of ozone depletion:
Ozone layer is essential for lives as it protects the surface of the Earth from dangerous and harmful high-energy ultraviolet rays from the sun. when ozone layer is destroyed, it causes more ultraviolet rays to reach the Earth surface. This high-energy ultraviolet radiation is harmful to lives because it can excite electrons and break bonds in biologically important molecules such as DNA and change their properties. The mutation in genetics induces skin cancer, eye cataracts and blindness. The harmful ultraviolet radiation can cause harm to plants too by damaging plant cells. Once the plant cells are been damaged, their growth and photosynthesis rates are affected and are more susceptible to diseases. Aquatic lives can also be harmed, especially an organism called plankton which have no natural defenses and locates near to the surface. As plankton is one of the bases of the oceanic food chain, organisms placed at the bottom food chain will be affected too. It has been predicted that by the use of CFC today, there will be another 100 years of ozone depletion. With the continuous ozone depletion, there will be lesser food for lives, more diseases will occur and the whole ecosystem will be affected and destroyed.
Summary:
Effects of Human health:
  1. Causes nonmelanoma skin cancer
  2. Malignant melanoa development
  3. Damage eye contaracts

Effects on Plants:
  1. Physiological and development processes of plants are affected
  2. Nutrients are not distributed equally within the plants
  3. Implications for plant competitive balance, herbivory, plant diseases and biogeochemical cycles

Effects on Marine Ecosystems:
  1. Phytoplankton productivity is limited to the euphotic zone
2. Motility in phytoplankton, resulting in reduced survival rates for these organisms as it is the fundation of aquatic food chains
3. Causes damage to early developmental stages of fish, shrimp, crab, amphibians and other animals

Effects on biological ecosystems:
  1. Affects terrestrial and aquatic biogeochemical cycles
Solutions:
1. First global solution discussed in 1987 named the Montreal Protocol suggested to freeze CFC production 1986 levels and a 50% reduction by 1999
2. A Kyoto Protocol is signed in 1997 by multiple countries, having agree to reduce their emissions of carbon dioxide and five other greenhouse gases (methane, nitrous oxide, sulphur hexafluoride, CFCs and perfluocarbons)
3. Use some alternatives such as perfluorocarbons, PCFs and hydroflouorocarbons, HFCs to replace the CFC molecule in its typicall uses.
4. Reduce the usage of inorganic fertilizers which release methane and nitrous oxide
5. Reduce the rate of deforestation
1. Draw the lewis diagrams for the two resonance forms of ozone.
2. Describe the natural formation and depletion of ozone in the stratosphere with chemical equations included.
3.List down ozone-depleting pollutants and their sources.
4. What are some impacts of ozone depletion?
Discuss question:
Discuss why CFCs can harm the ozone layer but the CFC alternatives are classified as having zero ozone depletion capability.
References:
BOOKS:
  • Neuss, G. (2007). IB DIPLOMA PROGRAMME Chemistry Course Companion. Oxford, OX;
Oxford University Press.
  • John,G & Sadru,D. (1999). Chemistry, 2nd edition. Oxford, OX. IBID press.
  • Brown, C. Ford, M. (2008). Standard level chem.istry. Heineman International.
Websites: