NCERT Solutions for Class 11 Chemistry Environmental Chemistry [Old Book]

NCERT
SOLUTIONS
CLASS - 11th

aglase .co

Class : 11th
Subject : Chemistry
Chapter : 14
Chapter Name : Environmental chemistry

Q14.1 De ne environmental chemistry.

Answer. Environmental chemistry is the study of chemical and biochemical processes occurring in nature. It
deals with the study of origin, transport, reaction, effects, and fates of various chemical species in the
environment.

Page : 421 , Block Name : Exercise

Q14.2 Explain tropospheric pollution in 100 words.

Answer. Tropospheric pollution arises due to the presence of undesirable substances in the lowest layer of
the atmosphere.
Oxides of sulphur, nitrogen, carbon, and hydrocarbons are the major gaseous pollutants. Oxides of sulphur (
SO md SO,) and nitrogen (NO , NO) are produced as a result of burning of fossil fuels (coal, automobile
2 2

fuel). These oxides react with water in the presence of atmospheric oxygen to form nitric acid (HN03) and
sulphuric acid H SO , which leads to the formation of 'Acid rain'.
2 4

2SO2(g) + O2(g) + 2H2 O(l) ⟶ 2H2 SO4(aq)

4NO2(g) + O2(g) + 2H2 O(l) ⟶ 4HNOy(aq)

Acid rain causes harm to agriculture, plants, and trees, It also leads respiratory ailments. Hydrocarbons are
carbon and hydrogen containing compounds that burn oxides of carbon. Hydrocarbons are carcinogenic md
their products are to various to produce also major pollutants. Carbon monoxide NO is poisonous in nature
as it reacts with the haemoglobin of blood, which can even result in death. Though carbon dioxide CO is2

not toxic in nature, yet it contributes towards global warming by trapping the re ected IR rays. This results
in the heating up of the Earth's atmosphere, thereby leading to the melting of icebergs and glaciers.
Particulates of smoke, dust, mist, and fume are harmful for human health as they are likely to block the
nasal passage of a person, causing respiratory ailments. Smoke and fog combine to produce smog during a
cool, humid day, thereby reducing visibility to a large extent. Photochemical smog is formed due to the
presence of PAN, ozone, formaldehyde, and acrolein. It causes eye irritation, headaches, and chest pain, It
also leads to the cracking of rubber and does damage to plants.

Page : 421 , Block Name : Exercise

Q14.3 Carbon monoxide gas is more dangerous than carbon dioxide gas. Why?

Answer. Carbon dioxide CO and carbon monoxide CO
2

gases are emitted during the combustion Of various fuels. Carbon monoxide is poisonous, whereas carbon-
dioxide is non-toxic in nature. Carbon monoxide is poisonous because it is capable of forming a complex
with haemoglobin (carboxyhemoglobin), which is more stable than the oxygen-haemoglobin complex. The
concentration range of 3—4% of carboxyhemoglobin decreases the oxygen-carrying capacity of blood. This
results in headaches, weak eyesight, nervousness, and cardiovascular disorders. A more increased
concentration may even lead to death. Carbon dioxide is not poisonous. It proves harmful only at very high

concentrations.

Page : 421 , Block Name : Exercise

Q14.4 List gases which are responsible for greenhouse effect.

Answer. The major greenhouse gases are:
1) Carbon dioxide CO 2

2) Methane CH 4

3) Water H O
2

4) Nitrous oxide NO
5) Ozone O 3

6) Chloro uorocarbons (CFCs)

Page : 421 , Block Name : Exercise

Q14.5 Statues and monuments in India are affected by acid rain. How?

Answer. Acid rain is a byproduct of various human activities that leads to the emission of oxides of sulphur
and nitrogen in the atmosphere. These oxides undergo oxidation and then react with water vapour to form
acids.
2SO 2(g)
+ O + 2H O
2(g) 2 -> 2H SO
(l) 2 4(aq)

4NO2(g) + O2(g) + 2H2 O(l) -> 4HNO 3(aq)

Acid rain causes damage to buildings and structures made of stone md metal. In India, limestone is a major
stone used in the construction of various monuments and statues, including the Taj Mahal.
Acid rain reacts with limestone as:
CaCO3 + H2 SO4 ⟶ CaSO4 + H2 O + CO2

This results in the loss of lustre and colour of monuments, leading to their dis guration.

Page : 421 , Block Name : Exercise

Q14.6 What is smog? How is classical smog different from photochemical smogs?

Answer. Smog is a kind of air pollution. It is the blend of smoke and fog. There are two kinds of smog:
a) Classical smog
b) Photochemical smog
The two smogs can be differentiated as follows:

Page : 421 , Block Name : Exercise

Q14.7 Write down the reactions involved during the formation of photochemical smog.

Answer. Photochemical smog is formed as a result of the reaction of sunlight with hydrocarbons and
nitrogen oxides. Ozone, nitric oxide, acrolein, formaldehyde, and peroxyacetyl nitrate (PAN) are common
components of photochemical smog. The formation of photochemical smog can be summarized as follows:
Burning of fossil fuels leads to the emission of hydrocarbons and nitrogen dioxide in the atmosphere, High
concentrations of these pollutants in air results in their interaction with sunlight as follows:
NO -> NO
2 + O
(g) (g)

O(g) + O2(g) − > O3(g)

O3(g) + NO(g) -> NO 2(g) + O2(g)

While ozone is toxic in nature, both NO and O 2 3(g) are oxidizing agents. They react with the unburnt
hydrocarbons in air to produce formaldehyde, PAN, and acrolein.
3CH4 + 2O3 → 3CH2 = O + 3H2 O

Q14.8 What are the harmful effects of photochemical smog and how can they be controlled?

Answer. EFFECTS OF PHOTOCHEMICAL SMOG:
Photochemical smog is oxidizing smog owing to the presence of N O and O . causing corrosion of metals,
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stones, rubber, and painted surfaces. The other major components of photochemical smog are PAN,
acrolein, and formaldehyde. Both PAN and ozone are eye irritants, while nitric oxide (formed from N02)
causes nose and throat irritation. At higher concentrations, photochemical smog causes chest pain,
headaches, throat dryness, and various respiratory ailments.
CONTROL MEASURES:
Photochemical smog results from the burning of fossil fuels and automobile fuels that emit NO and 2

hydrocarbons, which in turn form ozone, PAN, and other chemicals. The use of catalytic converters in
automobiles is recommended to prevent the release of N02 and hydrocarbons into the atmosphere.
Plantation of plants 9.uch as Pinus, Juniparur, Quercus, Pyrus, and Vitis is also advised as these plants have
the capability to metabolizeNO 2

Q14.9 What are the reactions involved for ozone layer depletion in the stratosphere?

Answer. In the stratosphere, ozone is a product of the action of UV radiations on dioxygen as:
uv

(i) O 2(g) ⟶ O(g) + O(g)

(ii) O 2(g)
+ O(g) -> O 3(g)

Reaction (ii) indicates the dynamic equilibrium existing between the production and decomposition of
ozone molecules. Any factor that disturbs the equilibrium may cause depletion of ozone layer by its
decomposition. One such factor is the release of chloro uorocarbon compounds (CFCs). These are non-
reactive, non- ammable molecules that are used in refrigerators, air conditioners, plastics, and electronic
industries. Once released CFCs mix with atmospheric gases and reach the stratosphere, where they are
decomposed by UV radiations.
uv

(iii)CF Cl 2 2(g)
˙ ˙
⟶ Cl(x) + CF2 Cl(g)

The chlorine free radical produced in reaction (iii) reacts with ozone as:

(iv) Cl
˙
(g
˙
+ O3(g) ⟶ ClO(g) + O2(g)

The Cl
˙
radicals further react with atomic oxygen to produce more chlorine radicals as:
(v) ClO + O ⟶ Cl
˙
(g) (x)
˙
+ O (x) 2(x)

The regeneration ˙
Cl of causes a continuous breakdown of ozone present in the stratosphere, damaging the
ozone layer.

Q14.10 What do you mean by ozone hole? What are its consequences?

Answer. In Polar regions, stratospheric clouds provide the surface for chlorine nitrate and hypochlorous
acid, which react further to give molecular chlorine. Molecular chlorine and HOCI are photolysed to give
chlorine-free radicals.
ClONO2(g) + H2 O(g) ⟶ HOCl(g) + HNO3(g)

ClONO2(g) + HCl(g) ⟶ Cl2(g) + HNO3(g)
m
˙ ˙
HOCl(g) ⟶ OH(g) + Cl(g)

hv
˙
Cl2(x) ⟶ Cl(g) + C1(g)

The chlorine-free radicals lead to the decomposition of ozone as:
˙
Cl(g) + O3(g) ⟶ ClO(g) + O2(g)

Hence, a chain reaction is initiated. The chlorine-free radical is continuously regenerated, thereby depleting
the ozone layer. This phenomenon is known as the as 'ozone hole'.
Effects of depletion of ozone layer
The ozone layer protects the Earth from the harmful LJV radiations of the sun. With the depletion of the
layer, more radiation will enter the Earth's atmosphere. t_JV radiations are harmful because they lead to the
ageing of skin, cataract, skin cancer, and sunburns. They cause death of many phytoplanktons, which leads
to a decrease of sh productivity. Excess exposure may even cause mutation in plants. Increase in I-JV
radiations, decreases the moisture content of the soil and damages both plants and bres.

Q14.11 What are the major causes of water pollution? Explain.

Answer. Water pollution arises as a result of several human activities, which leads to the presence of several
undesirable substances in water.
Major water pollutants with their sources have been tabulated as follows:

Roles played by major pollutants are:
1. Pathogens: These water pollutants include bacteria and other organisms. They enter water from animal
excreta and domestic sewage. Bacteria present in human excreta (for example, Escherichia coli and
Streptococcus faecalis) cause gastrointestinal diseases.
2. Organic wastes: These are biodegradable wastes that pollute water as a result of run off. The presence of
excess organic wastes in water decreases the amount of oxygen .held by water. This decrease in the amount
of dissolved oxygen inhibits aquatic life.
3. Chemical pollutants: These are water soluble chemicals like heavy metals such as cadmium, mercury,
nickel, etc. The presence of these chemicals (above the tolerance limit) can damage the kidneys, central
nervous system, and liver.

Q14.12 Have you ever observed any water pollution in your area? What measures would you suggest to
control it?

Answer. Water pollution arises as a result of various human activities. This includes discharges from
wastewater treatment plants, run-off from agricultural elds, storm-water drainage, etc. Pollutants from
these sources enter the water bodies, thereby contaminating the water and rendering it impure.
Industries and chemical factories discharge toxic, heavy metals such as Fe, Mn, A1, etc., along with organic
wastes into water. Domestic sewage and animal excreta are also responsible for pathogenic contamination
of water, These pollutants make water un t for drinking.
Therefore, all industrial and chemical discharges should be made free from toxic metals before allowing
them to enter a water body. The concentration of these pollutants should be checked regularly. Compost
should be preferred over chemical fertilizers in gardens and agricultural elds to avoid harmful chemIcals

from entering groundwater.

Q14.13 What do you mean by Biochemical Oxygen Demand (BOD)?

Answer. Biochemical oxygen demand is the amount Of oxygen required by bacteria to decompose organic
matter in a certain volume of sample of water. Clean water would have a BOD value of less than 5 ppm,
whereas highly polluted water has a BOD value of 17 ppm or more.

Q14.14 Do you observe any soil pollution in your neighbourhood? What efforts will you make for controlling
the soil pollution?

Answer. Major sources of soil pollution are industrial wastes and agricultural pollutants such as pesticides,
fertilizers, etc. It is very important to maintain the quality and fertility of soil to ensure and sustain the
growth of plants and food crops. Insecticides like DDT are not soluble in water, For this reason, they remain
in soil for a long time, contaminating the root crops. Pesticides like Aldrin and Dieldrin are non-
biodegradable and highly toxic in nature. They can enter the higher trophic levels through food chains,
causing metabolic and physiological disorders. The same is true for industrial wastes that comprises of
several toxic metals like Pb, As, Hg, Cd, etc.
Hence, the best way to check soil pollution is to avoid direct addition of pollutants to the soil. Also, wastes
should undergo proper treatment. They should be recycled and only then, allowed to be dumped.

Q14.15 What are pesticides and herbicides? Explain giving examples.

Answer. Pesticides are a mixture of two or more substances. They are used for killing pests. Pests include
insects, plant pathogens, weeds, mollusks, etc., that destroy the plant crop and spread diseases. Aldrin and
dieldrin are the names of some common pesticides Herbicides are pesticides specially meant for killing
weeds. For example, sodium chlorate .
NAClO , sodium arsenite Na ASO , etc.
3 3 3

Q14.16 What do you mean by green chemistry? How will it help decrease environmental pollution?

Answer. Green chemistry is a production process that aims at using the existing knowledge and principles of
chemistry for developing and implementing chemical products and processes to reduce the use and
generation of substances hazardous to the environment The release of different harmful chemicals
(particulates, gases, organic and inorganic wastes) causes environmental pollution. In green chemistry, the
reactants to be used in chemical reactions are chosen in such a way that the yield of the end products is up
to 100%. This prevents or limits chemical pollutants from being introduced into the environment. Through
the efforts of green chemists,H O has replaced tetrachloroethane and chlorine gas in drying and bleaching
2 2

of paper.

Q14.17 What would have happened if the greenhouse gases were totally missing in the earth’s atmosphere?

Discuss.

Answer. Earth's most abundant greenhouse gases are CO , CH , O etc. CFCs, and water vapour. These
2 4 3

gases are present near the Earth's surface. They absorb solar energy that is radiated back from the surface of
the Earth. The absorption of radiation results in the heating up of the atmosphere. Hence, greenhouse gases
are essential for maintaining the temperature of the Earth for the sustenance of life. In the absence of
greenhouse gases, the average temperature of the Earth will decrease drastically, making it uninhabited. As
a result, life on Earth would be impossible.

Q14.18 A large number of sh are suddenly found oating dead on a lake. There is no evidence of toxic
dumping but you nd an abundance of phytoplankton. Suggest a reason for the sh kill.

Answer. The amount Of dissolved oxygen present in water is limited. The abundance Of phytoplanktons
causes depletion of this dissolved oxygen. This is because, phytoplanktons are degraded by bacteria present
in water. For their decomposition, they require a large amount Of oxygen. Hence, they consume the oxygen
dissolved in water. As a result, the BOD level of water drops below 6 ppm, inhibiting the growth of sh and
causing excessive sh-kill.

Q14.19 How can domestic waste be used as manure?

Answer. Depending upon the nature of the waste, domestic waste can be segregated into two categories i.e.,
biodegradable and non—biodegradable. Biodegradable waste such as leaves, rotten food, etc. should be
deposited in land lls, where they get decomposed aerobically and anaerobically into manure. Non-
biodegradable waste (which cannot be degraded) such as plastic, glass, metal scraps etc. should be sent for
recycling.

Q14.20 For your agricultural eld or garden you have developed a compost producing pit. Discuss the
process in the light of bad odour, ies and recycling of wastes for a good produce.

Answer. It is essential to take proper care of the compost producing pit in order to protect ourselves from
bad odour and ies. It should be kept covered to minimize bad odour and prevent ies from entering it. The
recyclable waste should not be dumped in the compost producing pit. It should be sent to the industries
through vendors for recycling.