NCERT Solutions Class 11 Chemistry Chapter 14

Q.1 Define environmental chemistry.

Ans.

Environmental chemistry is the study of chemical phenomena occurring in the environment, i.e., the origin, reactions and effects of the chemical species on the environment.

Q.2 Explain tropospheric pollution in 100 words.

Ans.

Tropospheric pollution mainly occurs due to the following two factors.

• Gaseous pollutants: These include oxides of nitrogen (NO, NO₂),oxides of sulphur (SO₂, SO₃), hydrocarbons, hydrogen sulphide (H₂S), oxides of carbon (CO, CO₂), ozone (O₃) and other oxidants.

• Particulate pollutants: Mainly two types of particulate matters are present.

1. Viable particulates- The viable particulates are the minute living organisms, such as bacteria, fungi, algae, moulds etc. These are dispersed in air.The fungi causes diseases in plants and allergy in the human beings.
2. Non-viable particulates- There are mainly four types of non-viable particulates in the atmosphere. These are smoke, mist, dust and fumes.

Q.3 List the gases which are responsible for the greenhouse effects.

Ans.

The gases responsible for the greenhouse effects are CO₂, CH₄, O₃, CFCs, nitrous oxide and water vapours. CO₂ has the major contribution to green house effect.

Q.4 Carbon monoxide gas is more dangerous than carbon dioxide gas, why?

Ans.

CO is highly poisonous gas as it can block the supply of oxygen to the organs and tissues. It combines with the haemoglobin of the blood to form carboxyhaemoglobin. If its concentration reaches 3-4 %, it reduces the oxygen carrying capacity of the blood. The deficiency of oxygen in blood can cause headache, weak eyesight, nervousness, cardiovascular disorder, etc.

On the other hand, carbon dioxide gas does not react with haemoglobin of the blood and hence is less harmful as pollutant. It is only the main contributor towards the global warming and greenhouse effect. So carbon monoxide is more dangerous gas than carbon dioxide.

Q.5 Statues and monuments in India are affected by the acid rain, how?

Ans.

There are large number of industries and power plants present in India. For this reason air around the statues and monuments contain high levels of oxides of nitrogen and sulphur. These oxides cause acid rain which attacks the marble of these statues and monuments and gets them damaged.

CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂

As a result, these statues and monuments are being slowly eaten away and marble is getting lustreless and discoloured.

Q.6 What is smog? How is classical smog different from photochemical smogs?

Ans.

Smog is a major air pollutant. It is formed by combination of smoke with tiny particles of fog.

The constituents of classical smog are sulphur oxides and particulate matter from fuel combustion. Sulphur dioxide is a reducing agent, so chemically classical smog is reducing mixture. It causes bronchitis irritation, i.e., problems in lungs.

The constituents of photochemical smog are nitrogen oxides and hydrocarbons. Other minor components are ozone, formaldehyde etc. Since it has high concentration of oxidizing agents, therefore, it is oxidizing in nature. It causes irritation in eyes.

Q.7 Write down the reactions involved during the formation of photochemical smog.

Ans.

When the fossil fuel burns, mainly two types of pollutants are emitted in the air. These pollutants are hydrocarbons & nitric oxide (NO). When their percentage become high in the atmosphere, they interact with the sunlight to give a series of reactions as follows-

1. NO reacts with oxygen in the air and nitrogen dioxide is produced.

2NO\left(g\right)+O_2\to 2N{O}_2\left(g\right)

2. NO₂ absorbs the energy from the sunlight and breaks into NO & free oxygen atom.

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NO₂(g) → NO(g) + O(g)

3. Free atoms are very reactive, so newly formed oxygen atom reacts with O₂ to give ozone (O₃)

O_2\left(g\right)+O\left(g\right)\to O_3\left(g\right)

4. The formed O₃ reacts with NO to regenerate NO₂ gas.

NO\left(g\right)+O_3\left(g\right)\stackrel{}{\to }N{O}_2\left(g\right)+O_2\left(g\right)

5. The unburnt hydrocarbons present in the pollutant react with O₃ and NO₂ to produce organic free radicals which result into a number of chain reactions producing many undesirable compounds such as formaldehyde, acrolein, and PAN (Peroxyacyl nitrates).They constitute photochemical smog.

3C{H}_4+2O_3\to 3HCHO+3H_{2}O

PAN is toxic for plants.

Q.8 What are the harmful effects of photochemical smog and how can they be controlled?

Ans.

The harmful effects of photochemical smog:

The constituents of this smog are NO₂, O₃, and organic matters, i.e., PAN, formaldehyde, acrolein. These materials cause serious health hazards.

1. Aldehydes and PAN cause severe eye-irritation.
2. Ozone and nitric oxide can cause nose and throat irritation.
3. PAN is toxic for plants. Due to this compound, the tender leaves of tree become grayish.

Control of photochemical smog:

1. If the primary precursors (such as NO₂ and hydrocarbons) can be controlled, then the secondary precursors (such as O₃ and PAN) will be automatically controlled.
2. The efficient catalytic converters are used in automobiles to reduce the formation of nitrogen oxides and hydrocarbons in the atmosphere.
3. Certain plants (Pinus, Juniparus, Quercus, Pyrus and Vitis) help to absorb nitrogen oxides, so their plantation could help to control the photochemical smog.

Q.9 What are the reactions involved for ozone layer depletion in the stratosphere?

Ans.

The main reason for ozone layer depletion is the release of chlorofluorocarbons (CFCs), also called freons. When they are released in air, they reach the stratosphere, where they are decomposed by UV radiations to produce chlorine free radical.

C{F}_{2}C{l}_2\stackrel{UV}{\to }\underset{\text{Free radical}}{\stackrel{•}{C}l\left(g\right)}+C{F}_{2}Cl\left(g\right)

This chlorine free radical reacts with ozone to produce chlorine monoxide free radical and oxygen.

\stackrel{•}{C}l\left(g\right)+O_3\left(g\right)\to Cl\stackrel{•}{O}\left(g\right)+O_2\left(g\right)

Reaction of ClO radicals with atomic oxygen produces more Cl radicals.

Cl\stackrel{•}{O}+O\to \stackrel{•}{C}l+O_2

This is a chain reaction, the chlorine free radicals can destroy the ozone layer by this type of reaction. This results in the depletion of ozone layer. It has been calculated that one molecule of CFC can destroy thousand molecules of ozone.

Q.10 What do you mean by ozone hole? What are its consequences?

Ans.

Due to the free radical reaction of chlorine, ozone layer is depleting. This leads to the formation of ozone hole. Ozone hole is a region where the layer is extremely thin.

The ozone layer protects us from the UV radiation, but due to the depletion of ozone layer, UV rays penetrate through it and enter the troposphere. These radiations are harmful to our life. The ill effects are as follows:

1. Skin cancer, swelling of skin, burning sensation on skin.
2. The genetic material,DNA changes its characteristics which are irreparable.
3. They may harm the marine plants, marine animals and fish.
4. They damage paints and fibres leading to their fading faster.
5. They increase the evaporation of surface water of leaves & reduce the moisture content of soil.

Q.11 What are the major causes of water pollution? Explain.

Ans.

The major causes of water pollution are as follows:

1. Organic wastes: The examples of organic wastes are domestic sewage, animal manures, decaying plants and animals. These are the major biodegradable water pollutants.

2. Chemical pollutants Inorganic compounds like salts of heavy metals (Cd, Hg, and Ni) are important water pollutants.

3. Other water pollutants are petroleum products, organic chemicals, pesticides, and industrial chemicals like polychlorinated biphenyls (PCBs).

4. Pathogens: Pathogens are disease causing bacteria and micro-organisms present in the water. They enter in water through domestic sewage and animal excreta. These pathogens cause gastronomical problems in human beings.

5. Municipal and domestic wastes: Water contains human and animal wastes, kitchen waste, organic matters which are the source for the water born diseases like cholera, typhoid, dysentery etc.

6. Thermal pollutants: The main source of thermal pollutants is thermal power plants. These units discharge their unused heat into the nearby flowing water.

7. Radioactive discharges: These are mainly nuclear wastes which are generated from the nuclear power plants. The radiations emitted by them are highly hazardous.

Q.12 Have you ever observed any water pollution in your area? What measures would you suggest to control it?

Ans.

Yes, I have observed water pollution in my area. I have seen that sewage water enters the ponds and canals in our locality. People use that water for cleaning clothes, bathing, washing dishes.

Water for domestic use needs to be free from harmful chemicals and the organisms causing diseases.

I would suggest the following measures which can control the water pollution-

1. Sewage treatment plants remove suspended particles by adding alum or polyelectrolyte (water soluble charged polymers with high molecular weight).
2. By chlorination or ozonation, germs and other oxidisable matters can be removed.
3. Biological treatment is similar to aerobic chemical oxidation, dissolves organic matter.

Q.13 What do you mean by Biochemical Oxygen Demand (BOD)?

Ans.

The amount of oxygen required by bacteria to break down the organic matter present in a certain volume of sample is called biochemical oxygen demand or BOD.

Q.14 Do you observe any soil pollution in your neighbourhood? What efforts will you make for controlling the soil pollution?

Ans.

Yes, I observe soil pollution in our neighbourhood. The substances which are responsible for decreasing the productivity of soil are called soil pollutants. I have seen a place in my neighbourhood, where domestic refuse and industrial wastes are dumped on the land. Probable procedure of waste disposal, I may suggest is collection and disposal of domestic waste.

1. The domestic wastes are collected in small bins.
2. The small bins are transferred to the community bins by the private or municipal workers.
3. Then the community bins are collected and sent to disposal site.
4. In the disposal site, the wastes are separated in two parts, biodegradable and non-bio degradable wastes.
5. Biodegradable ones are stored in landfill and are converted to compost.
6. Non bio-degradable wastes are kept for recycling. For example plastics, metal scraps, glasses.

Q.15 What are pesticides and herbicides? Explain giving examples.

Ans.

Pesticides are the substances which are used in agriculture to kill unwanted organisms or to restrict their reproductive processes. These are mainly used for protection of the crops. Examples of pesticides are DDT (dichlorodiphenyltrichloroethane) and BHC (beta-hexachlorocyclohexane).

Herbicides are used to control the pests. They are toxic to mammals. Some examples of herbicides are sodium chlorate (NaClO₃) and sodium arsenite (Na₃AsO₃) which are mainly used to control weeds.

Q.16 What do you mean by green chemistry? How will it help decrease environmental pollution?

Ans.

Green chemistry is involved in the design, development, and implementation of chemical products and processes to reduce or eliminate the use and production of hazardous materials.

Green chemistry involves the following concepts such as waste minimization, solvent selection, maximum utilization, intensive processing and alternative synthetic method from renewable resources. All these processes help to reduce the environmental pollution.

Q.17 What would have happened if the greenhouse gases were totally missing in the Earth’s atmosphere? Discuss.

Ans.

In the cold places, the flowers, plants and vegetables are grown in glass covered areas and this is called green house. Human beings also live in covered blanket of air, which acts as greenhouse. The green house gases trap the sun’s heat inside and keep the earth’s surface warm and help in the growth of plants and existence of life on the earth. This is called natural green house effect. Earth cannot trap the solar energy in the form of heat. In absence of green house gases, there would have no vegetation on earth and hence no life on the earth.

Q.18 A large number of fish are suddenly found floating dead on a lake. There is no evidence of toxic dumping but you find an abundance of phytoplankton. Suggest a reason for the fish kill.

Ans.

Phytoplankton (organic pollutants such as leaves, grass etc.)present in water is biodegradable. A large number of bacteria decompose these organic matters by consuming the dissolved oxygen in water. Therefore, the concentration of dissolved oxygen is lowered. Water has limited concentration of dissolved oxygen and it is 10 ppm. When it falls below 6 ppm, the fish can’t survive. Thus, the presence of large number of phytoplankton in water reduces the amount of oxygen, so fishes die and float dead on the lake.

Q.19 How can domestic waste be used as manure?

Ans.

Domestic waste consist of two types of materials ;biodegradable like leaves,rotten food etc. and non-biodegradable like plastics, glass, etc. The non-biodegradable wastes are sent to industry for recycling while biodegradable wastes are collected and deposited in the landfill. Bacteria and other microorganisms decompose these wastes into nitrogen and phosphorous compounds, which increase the fertility of the soil and are used as the manure for the soil.

Q.20 For your agricultural field or garden you have developed a compost producing pit. Discuss the process in the light of bad odour, flies and recycling of wastes for a good produce.

Ans.

The compost is very important and useful for agriculture, but the compost producing pits may give bad odour and flies. Hence, the pit should be set up in a suitable place and properly covered to protect from bad odour and flies. Covering is required so that flies cannot enter and bad odour is minimized. If there is any recyclable material like plastics, glass, newspapers etc., it should be separated and transferred to the industries for recycling.