Thursday, 18 October 2018

Class 11 - Chemistry - Ch 14 - Environmental Chemistry - Atmospheric Pollution - Concept Points (#cbsenotes)(#eduvictors)

Environmental Chemistry Atmospheric Pollution Concept Points

Class 11 - Chemistry - Ch 14 - Environmental Chemistry - Atmospheric Pollution - Concept Points (#cbsenotes)(#eduvictors)

Environmental Chemistry: 
The branch of chemistry which deals with the study of various chemical changes and reactions occurring in environment is called Environmental chemistry. It includes our surroundings as air, water, soil, forest etc.

Environment constitutes air, water, soil, plants and atmosphere around us.

Environmental Pollution: 
Environmental pollution is the effect of undesirable changes in our surroundings that have harmful effects on plants, animals and human beings.

A substance which causes pollution is known as pollutant. Pollutants can be solid, liquid or gaseous substances, present in higher concentration. It can be produced due to human activities or natural happenings.

Atmospheric Pollution:
The atmosphere that surrounds the earth is not of the same thickness at different heights. Atmospheric pollution is generally studied as tropospheric and stratospheric pollution.

Tropospheric Pollution: 
Tropospheric pollution occurs due to the presence of undesirable solid or gaseous particles in the air. The following are the major gaseous and particulate pollutants present in the troposphere:

Gaseous air pollutants: These are oxides of sulphur, hydrocarbons, ozone and other oxidants.

Particulate pollutants: These are dust, mist, fumes, smoke, smog etc.

A substance which is not present in nature, but released due to human activity and has an adverse effect on environment is called as contaminent.

E.g.The killer gas methly isocyanate (MIC) leaked from union carbide factory in Bhopal.

The medium which is effected by pollution is called receptor.

The medium which reacts with the pollutant and minimises the effect of pollution is called sink.

1) Micro organisms which eat the dead animal or which convert the dry leaves and garbage into fertillizers.

2) Sea water is a big sink for carbondioxide. Plants are also good sink for CO₂.

Gaseous air pollutants: 
These are oxides of sulphide, hydrocarbons, ozone and other oxidants.

Oxides of Sulphur: 
Oxides of sulphur are produced when sulphur containing fossil fuel is burnt. The most common species sulphur dioxide is a gas that is poisonous to both animals and plants.

2SO₂(g) + O₂ (g) --⤍ 2SO₃ (g)
SO₂ (g) + O₂ (g) --⤍ SO₃ (g) + O₂ (g)
SO₂ (g) + H₂O₂ (l) --⤍ H₂SO₄ (aq)

Oxides of Nitrogen: 
NO₂ is oxidised to nitrate ion. When fossil fuel is burnt, dinitrogen and dioxygen combine to yield significant quantities of nitric oxide(NO) and nitrogen dioxide (NO₂).

N₂(g) + O₂ (g) --⤍  2NO(g) [at 1483 K]
2NO(g) + O₂(g) --⤍  2NO₂ (g)
NO(g) + O₃ (g) --⤍  NO₂ (g) + O₂ (g)

Among the hydrocarbons, methane is present in large quantity around the world. Methane is produced by the degradation of organic matter. Hydrocarbons are carcinogenic, i.e., they cause cancer. They harm plants by causing ageing, breakdown of tissues and shedding of leaves, flowers and twigs. Benzopyrene, which causes cancer to human beings is from the diesel engines.Peroxy benzoyl nitrate (PBN) causes irritation to eyes when present in the atmosphere.

Oxides of Carbon (CO and CO₂):

Carbon monoxide: Carbon monoxide is one of the most serious air pollutants. It is highly poisonous to living beings because of its ability to block the delivery of oxygen to the organs and tissues. It binds to haemoglobin to form carboxy haemoglobin, which is about 300 times more stable than the oxygen-haemoglobin complex. This results in headache, weak eyesight, nervousness and cardiovascular disorder. Nearly 80% of carbon monoxide is released from vehicles. In urban area, at peak times the level of the CO gas increases upto 50-100 ppm where as the acceptable level of CO in the air is 9ppm.

Carbon dioxide: Carbon dioxide is released into the atmosphere by respiration, burning of fossil fuels for energy and by decomposition of limestone during the manufacture of cement. The increasing amount of carbon dioxide in the air is mainly responsible for global warming.

22 metals are present in the air in the form of dust. The metals like zinc, copper, magnesium and manganese are less in the air but calcium, silicon, aluminium and Iron are more in the atmosphere. Dust produced by the heavy trraffic and various industries pollutes air.

Photochemical Smog:
Smoke and fog is called smog. Photochemical smog was first identified in Los Angles. The smog which contains oxidising agents is called oxidising smog formed in summer. The smog which contains reducing agents like SO 2 and carbon is called reducing smog formed in winter.

Due to air pollution, the possible hazards will occur
a) acid rains
b) depletion in ozone layer
c) green house effect or global warming.

Global Warming and Greenhouse Effect:
Some gases like carbon dioxide, methane, ozone, water vapours, CFCs have the capacity to trap some of the heat radiations that are released from the earth or from sun. These gases are known as greenhouse gases and the effect is called greenhouse effect. This leads to global warming. A 50% increase in carbon dioxide level increases the surface temperature of the earth by 3°C.

Consequences of Global Warming:

(i) It leads to melting of polar ice caps and flooding of low lying areas all over the earth.

(ii) Global rise in temperature increases the incidence of infectious diseases like dengue, malaria, yellow fever, sleeping sickness etc.

Acid Rain:
When the pH of the rain water drops below 5.6, it is called acid rain. Oxides of nitrogen and sulphur released as a result of combustion of fossil fuels dissolve in water to form nitric acid and sulphuric acid.
2SO₂ (g) + O₂ (g) + H₂O(l) --⤍  2H₂SO₄ (aq)
4NO₂ (g) + O₂(g) + H₂O(l) --⤍  4HNO₃ (aq)

Harmful Effects of Acid Rain:

(i) It has harmful effects on trees and plants as it dissolves and washes away nutrients needed for their growth.

(ii) It has very bad effect on aquatic ecosystem.

(iii) Acid rain damages buildings and other structures made of stone or metal. Taj Mahal in India has been affected
by acid rain.

Particulate Pollutants: 
Particulate pollutants are the minute solid particles or liquid droplets in air. Particulates in the atmosphere may be viable or non-viable.

Viable Particulates: 
They are minute living organisms that are dispersed in the atmosphere. e.g., bacteria, fungi, moulds, algae etc. They cause plant diseases.

Non-Viable Particulates: 
It can be classified according to their size and nature as follows:

(i) Smoke: It is the mixture of solid and liquid particles formed during combustion of organic matter. Example: cigarette smoke, smoke from burning of fossil fuel.

(ii) Dust: It is composed of fine solid particles (over 2 gm in diameter). It is produced during crushing, grinding and attribution of solid particles.

(iii) Mist: These are produced due to the spray of liquids like herbicides and pesticides over the plants. They travel through air and form mist.

(iv) Fumes: They are generally released to the atmosphere by the metallurgical operations and also by several chemical reactions.

Harmful Effects of Particulate Pollutants:

(i) Fine particles less than 5 microns penetrate into the lungs. Inhalation of such particles can lead to serious lung diseases including lung cancer.

(ii) Suspended particles of bigger size can hinder the sun rays from reaching the earth surface. This can lower the temperature of earth and make the weather foggy.

This is the common form of air pollution which is combination of smoke and fog. Smog exists in two types:

(i) Classical Smog: It occurs in cool humid climate. It contains smoke, fog and sulphur dioxide. It is also called as reducing smog.

(ii) Photochemical Smog: This type of smog results from the action of sunlight on unsaturated hydrocarbons and nitrogen oxides released by the vehicles and industries. It has high concentration of oxidising agents and is therefore, called as oxidising smog.

Harmful effects of Photochemical Smog:

(i)  They damage metals, stones, building materials etc.
(ii) They produce irritation in the eyes and respiratory system.

Stratospheric Pollution:

Formation of Ozone: Ozone in the stratosphere is produced by UV radiations. When UV – radiations act on dioxygen (O 2 ) molecules, ozone is produced.

Ozone is thermodynamically unstable and decomposes to molecular oxygen. Thus, there exists equilibrium between production and decomposition of ozone molecules.

Depletion of Ozone layer: 
Ozone blanket in the upper atmosphere prevents the harmful UV radiations from reaching earth. But in recent years, there have been reports of depletion of this layer due to presence of certain chemicals in the stratosphere. Chlorofluorocarbons (CFCs), nitrogen oxides, CCl 4 etc. are the chemicals responsible for depletion of ozone layer.

Effects of the Depletion of Ozone Layer:
(i) This leads to many diseases like skin cancer, sunburn, ageing of skin, cataract etc.
(ii) UV radiations can kill many phytoplanktons, damage the fish productivity.
(iii)It can decrease moisture content of the soil by increasing the evaporation of surface water.
(iv) UV radiations can damage paints and fibres, causing them to fade faster.