Question 27
How is the 'ozone oxygen cycle' maintained in the stratosphere and state what disturbs the balance, resulting in ozone depletion or destruction, Give balanced equation for the same.
Ozone is formed in the stratosphere — Highly energetic ultraviolet radiations from the sun breaks the oxygen molecule [O2] into two atoms. This reaction is photodissociation or photolysis.
Highly reactive oxygen atom [O] can then react with oxygen molecule [O2] to form ozone [O3]
O2 [O] + [O]
O2 + [O] ⟶ O3 [ozone]
Ozone is also broken down in the stratosphere — In unpolluted air containing minimal chemical compounds, unstable ozone molecule, in presence of solar radiations may split into molecular oxygen [O2] and an atom of atomic oxygen [O].
The atomic oxygen then combines with another ozone molecule to form two molecules of molecular oxygen.
O3 O2 + [O]
O3 + [O] ⟶ 2O2
Thus a continuing process called 'ozone oxygen cycle' is maintained between the amount of ozone [O3] being produced and being destroyed. As a result, the total concentration of ozone remains almost the same.
The balance is disturbed by release of chemicals — such as chlorofluorocarbons containing atomic chlorine and bromine and other free radicals which breaks the ozone molecule causing ozone depletion.
Chapter Overview: Atmospheric Pollution
This chapter examines the causes, effects, and prevention of atmospheric pollution. The atmosphere is composed of nitrogen (78%), oxygen (21%), argon (0.93%), carbon dioxide (0.04%), and trace amounts of other gases and water vapour. Air pollution occurs when harmful substances (pollutants) contaminate the atmosphere. Major air pollutants include carbon monoxide (CO), sulphur dioxide (SO2), nitrogen oxides (NOx), particulate matter, and chlorofluorocarbons (CFCs). Acid rain forms when SO2 and NOx dissolve in rainwater to form sulphuric acid and nitric acid. The greenhouse effect is the trapping of heat by gases like CO2, CH4, and N2O, leading to global warming. Ozone depletion is caused by CFCs breaking down the ozone (O3) layer in the stratosphere. Students must understand the sources of each pollutant, its harmful effects on health and environment, and measures to control pollution. This chapter connects chemistry concepts to real-world environmental issues.
Key Concepts
| Pollutant / Issue | Details |
|---|---|
| CO (Carbon Monoxide) | From incomplete combustion; binds to haemoglobin, reducing O2 transport |
| SO2 | From burning fossil fuels with sulphur; causes acid rain and respiratory problems |
| Acid Rain | SO2 + H2O → H2SO3; 2NO2 + H2O → HNO3 + HNO2; pH < 5.6 |
| Greenhouse Effect | CO2, CH4, N2O trap infrared radiation, warming Earth's surface |
| Ozone Depletion | CFCs release Cl atoms that destroy O3: Cl + O3 → ClO + O2 |
| Global Warming | Rise in Earth's average temperature due to enhanced greenhouse effect |
| Smog | Smoke + fog; photochemical smog contains O3, NO2, and organic compounds |
Must-Know Concepts
- Composition of clean air: N2 (78%), O2 (21%), Ar (0.93%), CO2 (0.04%)
- CO is a silent killer - colourless, odourless, and highly toxic
- Acid rain (pH < 5.6) damages buildings (especially marble: CaCO3 + H2SO4 → CaSO4 + H2O + CO2), kills aquatic life, and harms vegetation
- Greenhouse gases in order of impact: CO2 > CH4 > N2O > CFCs
- Montreal Protocol (1987) aims to phase out CFCs to protect the ozone layer
- Control measures: catalytic converters in vehicles, using clean fuels, afforestation, emission regulations
Greenhouse Effect vs Ozone Depletion
| Feature | Greenhouse Effect | Ozone Depletion |
|---|---|---|
| Cause | Excess CO2, CH4, N2O | CFCs and halons |
| Location | Lower atmosphere (troposphere) | Upper atmosphere (stratosphere) |
| Effect | Global warming, sea level rise | More UV radiation, skin cancer |
| Prevention | Reduce fossil fuel use, afforestation | Ban CFCs (Montreal Protocol) |
Important Diagrams to Practice
- Greenhouse effect diagram showing incoming solar radiation and trapped infrared radiation
- Acid rain formation cycle (from factory emissions to acid precipitation)
- Ozone layer depletion mechanism (CFC breakdown and ozone destruction cycle)
Common Mistakes
- Confusing greenhouse effect (lower atmosphere, warming) with ozone depletion (upper atmosphere, UV radiation)
- Saying the greenhouse effect is entirely bad (it is natural and essential; the problem is the enhanced effect)
- Calling CO2 a pollutant in all contexts (it is a natural atmospheric component; excess is the problem)
- Confusing ozone at ground level (harmful pollutant in smog) with stratospheric ozone (protective)
Scoring Tips
- For essay-type questions, organise into: causes → effects → prevention
- Include chemical equations wherever possible (acid rain formation, ozone destruction)
- Mention specific international agreements: Montreal Protocol, Kyoto Protocol, Paris Agreement
- Use specific data: normal rain pH = 5.6, acid rain pH < 5.6; CO2 level now ~420 ppm
Frequently Asked Questions
Is the greenhouse effect good or bad?
The natural greenhouse effect is essential - without it, Earth's average temperature would be about −18°C instead of the current +15°C. The problem is the enhanced greenhouse effect caused by human activities increasing CO2 and other greenhouse gases, leading to global warming.
Why is the ozone hole mainly over Antarctica?
The extreme cold of the Antarctic winter creates polar stratospheric clouds where CFCs accumulate. When spring sunlight returns, UV light breaks CFC molecules, releasing chlorine atoms that rapidly destroy ozone. One chlorine atom can destroy up to 100,000 ozone molecules.
How does acid rain damage buildings?
Acid rain reacts with marble and limestone (CaCO3) in buildings: CaCO3 + H2SO4 → CaSO4 + H2O + CO2. This dissolves the stone surface, causing the "stone cancer" effect. The Taj Mahal is a famous example of acid rain damage.