ICSE Class 9 Chemistry Chapter 10 Atmospheric Pollution

Chapter 10

Atmospheric Pollution

Latest Syllabus - Scope of Syllabus - Atmospheric Pollution

Acid rain - composition, cause and its impact: Sulphur in fossil fuels giving oxides of sulphur when burnt. High temperatures in furnaces and internal combustion engines produce oxides of nitrogen. (Equations to be included). Acid rain affects soil chemistry and water bodies.

Global warming: Greenhouse gases - their sources and ways of reducing their presence in the atmosphere. (water vapour, carbon dioxide, methane and oxides of nitrogen)

Ozone depletion: Formation of ozone - relevant equations; Function in the atmosphere; Destruction of the ozone layer - chemicals responsible for this to be named but reactions not required.

Introduction

1. The Term - Atmospheric Pollution

Pollution - the word is derived from the Latin word 'Pollutus' meaning 'to make unclean'.

Atmospheric Pollution - Conditions of air made unclean due to introduction of foreign elements from - i) Natural sources ii) Man-made sources - to the air so as to cause adverse effects on living organisms on earth.

2. Pollutants - In the Atmosphere

Atmospheric Pollutants - resulting in -

Acid Rain - Oxides of sulphur [SO₂] & Oxides of nitrogen [NO₂].

Global Warming - CO₂, Methane [CH₄], Chlorofluorocarbons [CFCs], N₂O, Water vapour, CFBr

Ozone Destruction - CFCs, Methyl chloride [CH₃Cl], CH₃Br, CCl₄, CH₄, Bromofluorocarbons [CFBr]

Teacher's Note

Atmospheric pollution directly affects the air we breathe daily and the quality of life in our communities. Understanding these pollutants helps us make better choices about transportation and energy use.

A. Acid Rain

1. Causes of Acid Rain

The Term - Acid Rain

The term "Acid Rain" refers to the various ways in which the acids - formed in the atmosphere - condense and fall on the earth.

The two forms of deposition of acid rain are -

a) Wet deposits - rain, snow, fog, dew

b) Dry deposits - particles [containing sulphates & nitrates]

The Pollutants - Responsible for acid rain

Oxides of sulphur [SO₂] - Oxides of nitrogen [NO₂]

Formation of Acid Rain

Natural sources - bacterial decomposition, forest fires, volcanic eruptions & Man made sources - power, industrial & smelting plants & automobile exhausts, produce -

oxides of sulphur & nitrogen - which interact with water vapour in the presence of sunlight in the atmosphere to form - sulphuric acid [H₂SO₄] and nitric acid [HNO₃] mist.

The mist remains as vapours at high temperatures & condenses at low temperatures. The acids mix with rain [or snow, fog etc] and fall down on the earth resulting in 'acid rain'

Causes of Acid Rain

Oxides of Sulphur [SO₂] produced by

Burning of Fossil Fuels

Fossil fuels - are derived from the organic remains of plants & animals.

Principal fossil fuels - Coal, petroleum [oil], natural gas.

Emission of SO₂ from fossil fuels -

electrical power stations - burn coal & produce SO₂ from the sulphur impurities in coal

S + O₂ \(\rightarrow\) SO₂

Burning of fossil fuels accounts for - 70% of man-made SO₂ gas.

Other Sources

Bacterial decomposition of organic matter - releases H₂S which is oxidised to SO₂

2H₂S + 3O₂ \(\rightarrow\) 2H₂O + 2SO₂

Smelting plants - produce SO₂ when metallic sulphides are roasted in air.

2ZnS + 3O₂ \(\rightarrow\) 2ZnO + 2SO₂

Sulphuric acid plants - produce SO₂ during production of sulphuric acid.

Oxides of Nitrogen [NO, NO₂] produced during use of

High Temperatures in Furnaces & Internal Combustion Engines

Internal Combustion Engine -

is an engine in which combustion of a fuel [fossil fuel], occurs with an oxidiser [air] in a combustion chamber. The air drawn from the atmosphere contains - about 80% nitrogen.

The nitrogen combines with oxygen at high temperatures producing - oxides of nitrogen

N₂ + O₂ \(\rightarrow\) 2NO [at high temperatures]

Thus exhaust gases of all internal combustion engines release - oxides of nitrogen.

Teacher's Note

Acid rain from fossil fuel combustion is a direct consequence of the energy we use daily in power plants and vehicles. Learning about these chemical reactions helps explain environmental problems linked to industrialization.

2. Composition and Formation of Acid Rain

Natural Rain Water

Natural rain water produced in unpolluted air is only - slightly acidic.

The CO₂ present in traces in unpolluted air dissolves in rain water forming - weak carbonic acid [H₂CO₃]

H₂O + CO₂ \(\rightarrow\) H₂CO₃ [carbonic acid]

pH Scale

A scale which measures - acidity or alkalinity of a solution.

pH scale has values from - 0 (or 1) [most acidic] to 14 [most basic]

pH of pure water [neutral] - is 7.

pH of natural rain water - is in range of 5 to 6.

pH of acid rain - is in range of 5.6 to 3.5.

Composition of Acid Rain

Acid rain refers to rain [or any precipitation i.e. snow or fog] which has pH less than 5.6.

Since it is caused by atmospheric pollutants - sulphur dioxide & nitrogen oxides, it a complex mixture of - sulphuric acid [H₂SO₄] nitric acid [HNO₃] (and sulphurous [H₂SO₃] and nitrous [HNO₂] acids)

Formation of Acid Rain

Formation of Sulphuric Acid [and sulphurous acid] in Acid Rain -

Electric Power Stations - Burn Coal [fossil fuel] for Power Generation.

1. Sulphur [S] impurities in the coal - form pollutant sulphur dioxide [SO₂]

S + O₂ \(\rightarrow\) SO₂

2. Sulphur dioxide [SO₂] reacts with water vapour to form sulphurous acid [H₂SO₃]

H₂O + SO₂ \(\rightarrow\) H₂SO₃ [aq.]

3. Sulphur dioxide [SO₂] can also be oxidised to sulphur trioxide [SO₃]

2SO₂ + O₂ \(\rightarrow\) 2SO₃

[The reaction is catalysed by dust particles & water droplets]

4. Sulphur trioxide [SO₃] reacts with water vapour in air forming sulphuric acid [H₂SO₄]

H₂O + SO₃ \(\rightarrow\) H₂SO₄ [aq.]

Thus the acidity in acid rain is due to - reaction of oxides of sulphur with water vapour - forming dilute acids.

Teacher's Note

The chemical reactions that form acid rain happen invisibly in our atmosphere every day. Understanding these reactions shows how human industrial activities transform pollutants into harmful substances that fall as rain.

This is a preview of the first 3 pages. To get the complete book, click below.