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Class 11 Chemistry Notes

Environmental Chemistry Class 11 Notes

Complete, exam-ready notes on environmental chemistry: the composition of the atmosphere, types and sources of air pollution, greenhouse gases and global warming, stratospheric ozone and its depletion, acid rain, photochemical and classical smog, water pollution, and the principles of green chemistry — written for CBSE, JEE and NEET revision.

Class11SubjectChemistryCoversCBSE · JEE · NEET

Written byDeep Narayan· Science & Mathematics EducatorReviewed byPushpanjali

What is environmental chemistry in one line?

Environmental chemistry studies the chemical processes in air, water and soil and how pollutants — natural and man-made — affect them.

Composition of the Atmosphere and Pollutants

  • Clean air: roughly 78% nitrogen, 21% oxygen, 0.93% argon, about 0.03–0.04% carbon dioxide, plus water vapour and traces of other gases.
  • A pollutant is a substance present in harmful excess; a contaminant becomes a pollutant when the amount crossed a danger threshold.
  • Primary pollutants are released directly (CO, SO₂, NO, particulates); secondary pollutants form in the air (O₃, PAN, NO₂).
  • Sources: stationary combustion, motor vehicles, industry and natural events like volcanic eruptions.

Air Pollution and Its Effects

  • Carbon monoxide binds haemoglobin ~300 times stronger than oxygen, causing oxygen starvation.
  • SO₂ and NOx irritate lungs, form acid rain and corrode buildings and stone monuments.
  • Particulates (PM) cause respiratory and lung disease; lead from old fuels damages the nervous system.
  • Fly ash, hydrocarbon vapours, and chlorofluorocarbons (CFCs) complete the list of major air pollutants.
  • Catalytic converters convert CO, NOx and hydrocarbons to CO₂, N₂ and H₂O; electrostatic precipitators trap particulates.

Smog — Classical and Photochemical

Classical and photochemical smog

Smog=smoke+fog\text{Smog} = \text{smoke} + \text{fog}

Classical (London-type) smog results from coal and smoke in humid, foggy conditions with SO₂ and soot. Photochemical (Los Angeles-type) smog forms in sunlight from NOx and hydrocarbons from vehicles — it produces O₃, PAN and aldehydes, which irritate eyes and damage plants and rubber.

  • Photochemical smog needs sunlight, NO₂, hydrocarbons and a warm inversion layer.
  • Ozone near the ground is a harmful pollutant in photochemical smog — unlike the protective layer in the stratosphere.
  • Control: reduce hydrocarbon emissions, use cleaner fuels and catalytic converters.

Greenhouse Effect and Global Warming

Greenhouse gases

CO2,  CH4,  CFCs,  N2O,  O3CO_2,\; CH_4,\; CFCs,\; N_2O,\; O_3

These gases trap infrared radiation re-emitted from Earth's surface, keeping the planet warm. Human activity is raising their concentrations — especially CO₂ — driving global warming and climate change.

  • CO₂ dominates: burning of fossil fuels and deforestation are the main drivers.
  • Methane (livestock, rice, leaks) and nitrous oxide (fertilisers) are potent per-molecule.
  • F-Gases (CFCs, HFCs) also contribute; CFCs additionally destroy ozone.
  • Consequences: rising sea levels, glacier retreat, extreme weather and habitat shifts.

Ozone Layer and Ozone Depletion

Ozone in the stratosphere

O2→UV2O,O+O2→O3O_2 \xrightarrow{UV} 2O,\qquad O + O_2 \to O_3

The stratospheric ozone layer filters out most UV-B radiation. CFCs released at ground level drift upward, release chlorine atoms that catalyse ozone destruction, and thin the layer — the Antarctic ozone hole is the extreme result.

  • Catalytic cycle: CL + O₃ → ClO + O₂; ClO + O → Cl + O₂, regenerating the Cl that destroys more ozone.
  • NOVA compounds (NO) natural cycle plus volcanic particles also deplete ozone.
  • The Montreal Protocol phased out CFCs; ozone levels are now slowly recovering.
  • Ground-level ozone is a pollutant (smog), stratospheric ozone is protective — altitude decides.

Acid Rain, Water Pollution and Green Chemistry

Acid rain

SO2+H2O→H2SO3,2NO2+H2O→HNO2+HNO3SO_2 + H_2O \to H_2SO_3,\qquad 2NO_2 + H_2O \to HNO_2 + HNO_3

SO₂ and NOx dissolve in rain to give sulphuric and nitric acids, lowering pH well below 5.6 — the natural rain value. Acid rain damages forests, kills aquatic life, and corrodes marble and stone monuments.

  • Water pollution: domestic sewage raises biological oxygen demand (BOD), starving aquatic life; industrial effluents add heavy metals (Hg, Pb, Cd).
  • Eutrophication: excess phosphates and nitrates from fertilisers make algal blooms that deplete oxygen.
  • Biochemical Oxygen Demand measures the oxygen microorganisms need to decompose organic waste.
  • Green chemistry: design products/processes that minimize hazardous substances — the Twelve Principles (prevention, atom economy, safer solvents, ...).

Solved Examples

Example: Distinguish photochemical smog from classical smog.

Solution: Classical smog is reducing, forms in cool humid mornings, and is dominated by SO₂ and soot from coal. Photochemical smog is oxidising, forms in sunny warm conditions from NOx and hydrocarbons, and contains O₃ and PAN.

Example: Why is ground-level ozone harmful while stratospheric ozone is essential?

Solution: In the stratosphere, ozone absorbs harmful UV-B. At ground level (smog), ozone is a powerful oxidant that damages lungs, plants and rubber, so the same molecule is a protectant up high and a pollutant near the surface.

Revision

Key formulas at a glance

Memorise these before attempting numericals — most exam questions hinge on one of them.

Ozone formation

O2→UV2O,  O+O2→O3O_2 \xrightarrow{UV} 2O,\; O + O_2 \to O_3

Ozone destruction cycle

Cl+O3→ClO+O2,  ClO+O→Cl+O2Cl + O_3 \to ClO + O_2,\; ClO + O \to Cl + O_2

Acid rain (S)

SO2+H2O→H2SO3SO_2 + H_2O \to H_2SO_3

Acid rain (N)

2NO2+H2O→HNO2+HNO32NO_2 + H_2O \to HNO_2 + HNO_3

Greenhouse gases

CO2,CH4,CFCs,N2OCO_2, CH_4, CFCs, N_2O

Air composition

N2  78%,  O2  21%,  Ar  0.93%N_2\;78\%,\; O_2\;21\%,\; Ar\;0.93\%

Combustion CO₂

CxHy+O2→CO2+H2OC_xH_y + O_2 \to CO_2 + H_2O

Exam tips

How this chapter is asked

Where this topic appears in CBSE, JEE Main and NEET papers.

  • Primary pollutants released directly; secondary form in the air (O₃, PAN).
  • CO binds haemoglobin ~300× stronger than O₂.
  • Classical smog = coal + fog; photochemical smog = NOx + hydrocarbons + sunlight.
  • Stratospheric ozone protects; ground-level O₃ pollutes.
  • CFCs catalyse ozone destruction (Cl cycle) — Montreal Protocol.
  • Acid rain forms from SO₂ and NOx; natural rain pH ≈ 5.6.
  • Green chemistry minimises hazardous substances at the design stage.

FAQ

Common questions

What is the difference between primary and secondary pollutants?

Primary pollutants are emitted directly into the air — CO, SO₂, NO and particulates. Secondary pollutants form in the atmosphere through reactions, such as ozone, PAN and NO₂ from vehicle emissions in sunlight.

What causes acid rain?

SO₂ and nitrogen oxides from burning fossil fuels dissolve in rain to form sulphuric and nitric acids, lowering the pH below natural 5.6 and damaging forests, aquatic life and stone structures.

How do CFCs destroy the ozone layer?

UV light releases chlorine atoms from CFCs in the stratosphere. Each chlorine atom catalyses the conversion of ozone to oxygen — Cl + O₃ → ClO + O₂, then ClO + O → Cl + O₂ — regenerating chlorine to destroy more ozone.

What is green chemistry?

The design of chemical products and processes that reduce or eliminate hazardous substances — waste prevention, atom economy, safer solvents, renewable feedstocks and energy efficiency are among its guiding principles.

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