NCERT Solution Class 11th Chemistry Chapter – 14 Environmental Chemistry Question & Answer

NCERT Solution Class 11th Chemistry Chapter – 14 Environmental Chemistry

NCERT Solution Class 11th Chemistry Chapter – 14 Environmental Chemistry

?Chapter – 14?

✍Environmental Chemistry✍

?Question & Answer?

Q1) Define environmental chemistry.

Answer: Environmental chemistry refers to the study of biochemical and chemical processes that occur in our nature. It also delves into the reaction, origin, effects and transport of the chemical species in our earth.

Q2) Explain tropospheric pollution in 100 words.

Answer: The presence of unwanted substances in the troposphere’s lowest layer cause tropospheric pollution

The major pollutants include oxides of nitrogen, sulphur, carbon and hydrocarbons.

• Oxides of nitrogen (NO₂, NO) and sulphur (SO₂ and SO₃) which are formed due to the burning of coal, automobile fuel and other fossil fuels, form nitric acid (HNO₃) and sulphuric acid (H₂SO₄) by reacting with water in the presence of atmospheric oxygen. This results in ‘Acid rain’.

2SO_2(g) + O_2(g) + 2H₂O_(l) $\to$ 2H₂SO_4(aq)

4NO_(g) + O_2(g) + 2H₂O_(l) $\to$  4HNO_3(aq)

Acid rain harms plants, agriculture and trees. It also causes respiratory problems in humans.

• Hydrocarbons are compounds that contain carbon and hydrogen in them. They burn to form oxides of carbon. They are carcinogenic in nature and are also regarded as major pollutants.
• Carbon monoxide (CO) reacts with the haemoglobin in our blood and is poisonous in nature and can even be fatal. Even though carbon dioxide (CO₂)is not toxic by itself, it contributes to the increase in global warming by trapping extra sun rays. This results in a heating effect upon the earth thus increasing the earth’s temperature.
• Particulates like dust, smoke, fume and mist block our nasal passage and are considered harmful for our health.
• Smog is caused due to the combination of smoke and fog which reduces the visibility of traffic.

Photochemical smog is formed as a result of the presence of ozone, PAN, acrolein and formaldehyde. It causes headaches, eye irritation and chest pain in humans. It cracks rubber tires and also damages the plants.

Q3) Carbon monoxide gas is more dangerous than carbon dioxide gas. Why?

Answer: • Both carbon dioxide (CO₂) and carbon monoxide (CO) are emitted during the combustion of various fuels. Carbon monoxide is toxic in nature whereas carbon-dioxide is non-poisonous in nature.

• Carbon monoxide is deemed toxic since it forms a complex with haemoglobin (carboxyhemoglobin), which is more stable than the oxygen-haemoglobin complex. A range of 3–4% of carboxyhemoglobin can decrease the oxygen-carrying capacity of our blood. This can result in weak eyesight, headaches, cardiovascular disorders and nervousness. A little increase over the above concentration can even be fatal.

• Carbon dioxide is harmful only at very high concentrations.

Q4) List gases which are responsible for greenhouse effect.

Answer: The major gases that cause greenhouse effect are:

1) Chlorofluorocarbons (CFCs)

2) Methane (CH₄)

3) Carbon dioxide (CO₂)

4) Nitrous oxide (NO)

5) Water (H₂O)

6) Ozone (O₃)

Q5) Statues and monuments in India are affected by acid rain. How?

Answer:  Oxides of nitrogen (NO₂, NO) and sulphur (SO₂ and SO₃) which are formed due to the burning of coal, automobile fuel and other fossil fuels, form nitric acid (HNO₃) and sulphuric acid (H₂SO₄) by reacting with water in the presence of atmospheric oxygen. This results in ‘Acid rain’.

2SO_2(g) + O_2(g) + 2H₂O_(l) $\to$ 2H₂SO_4(aq)

4NO_(g) + O_2(g) + 2H₂O_(l) $\to$  4HNO_3(aq)

Acid rain harms plants, agriculture and trees. It also causes respiratory problems in humans.

Acid rain damages buildings and structures made of metal and stone. Limestone is a major constituent used in various monuments and statues like Taj Mahal in India.

Acid rain when reacting with limestone causes loss of colour and lustre of monuments, resulting in their disfiguration.

The reaction is as follows:

CaCO₃ + H₂SO₄  $\to$  CaSO₄ + H₂O + CO₂

Q6) What is smog? How is classical smog different from photochemical smogs?

Answer: Smog is a combination of smoke and fog which causes air pollution. There are two types of smog:

a) Photochemical smog

b) Classical smog

They can be differentiated as follows:

Q7) Write down the reactions involved during the formation of photochemical smog.

Answer: Photochemical smog is formed due to the reaction of sunlight with nitrogen oxides and hydrocarbons. Formaldehyde, nitric oxide, ozone, PAN, acrolein are the main components of photochemical smog. The steps involved in the formation of photochemical smog are:

Hydrocarbons and nitrogen dioxide are released into the atmosphere as a result of the burning of fossil fuels. Due to high concentrations of these pollutants, they react with sunlight. The reactions proceed as shown below:

NO₂ \overset{h\gamma }{\rightarrow}→hγ  NO + O

(Nitrogen dioxide)            (Nitric oxide)

O          +          O₂   $\leftrightarrow$   O₃

O₃       +          NO   $\to$     NO₂ + O₂

Since NO₂ and O₃ are oxidizing agents, they react with the unburnt hydrocarbons to produce acrolein, PAN and formaldehyde.

Q8) What are the harmful effects of photochemical smog and how can they be controlled?

Answer: Photochemical smog is oxidizing in nature since it is composed of NO₂ and O₃, which are responsible for corrosion of rubber, stones, metals and painted surfaces. Photochemical smog also contains formaldehyde, PAN and acrolein. Both ozone and PAN and cause eye irritation while nitric oxide (derived from NO₂) is responsible for throat and nose irritation. At higher levels, this type of smog can also cause headaches, throat dryness, chest pain and other respiratory ailments.

CONTROL MEASURES:

Use of catalytic converters can be used to prevent the release of NO₂ into the atmosphere.

Plants such as Pyrus, Pinus, Quercus, Juniparur, and Vitis can planted as these have the capability to metabolize NO₂.

Q9) What are the reactions involved for ozone layer depletion in the stratosphere?

Answer: In the stratosphere, ozone is actually formed by the action of UV rays on Dioxygen molecules(O₂).

(i)  O_2(g)  \overset{UV}{\rightarrow}→UV  O_(g) +O_(g)

(ii)  O_2(g) + O_(g)  \overset{UV}{\leftrightarrow}↔UV  O_3(g)

The second reaction shows that there is a dynamic equilibrium between the decomposition and production of ozone molecules. Any factor that disturbs this equilibrium may lead to the ozone layer getting depleted due to its decomposition. One such example of these factors is the chlorofluorocarbon compounds (CFCs). These are non-flammable and non-reactive molecules that find usage in air conditioners, refrigerators, plastics, and electronic industries.

Once it is released into the atmosphere, these CFCs get mixed with atmospheric gases and then, reach the stratosphere, where they get decomposed due to reactions with UV radiations.

(iii) CF₂Cl_2(g) \overset{UV}{\rightarrow}→UV  \dot{C}C˙l_(g) + \dot{C}C˙F₂Cl_(g)

The chlorine free radical which is produced in the third reaction reacts with ozone as shown below:

(iv) \dot{C}C˙l_(g) + O_3(g) $\to$ Cl\dot{O}O˙ _(g) + O_2(g)

And, Cl\dot{O}O˙ _(g) + O_(g) $\to$  \dot{C}C˙l_(g) + O_2(g)

The radicals, on further reaction with atomic oxygen, resulting in the production of more chlorine radicals.

This regeneration of chlorine-free radicals results in a continuous breakdown of ozone in the stratosphere, thus almost permanently damaging the ozone layer.

Q10) What do you mean by ozone hole? What are its consequences?

Answer: Stratospheric clouds present in Polar Regions provide a surface for the reaction of hypochlorous acid and chlorine nitrate which on further reaction, gives molecular chlorine. HOCl and Molecular chlorine get photolysed to provide chlorine-free radicals.

• The radicals, on reaction with atomic oxygen, resulting in the production of more chlorine radicals.
• This regeneration of chlorine-free radicals results in a continuous breakdown of ozone in the stratosphere, thus almost permanently damaging the ozone layer.
• This phenomenon is known as the ‘ozone hole’.

Effects of ozone layer depletion:

The Earth is shielded from the harmful UV radiations of the sun by the ozone layer. When this layer gets depleted, a greater amount of radiations will enter the earth’s atmosphere. They are deemed harmful since they lead to skin cancer, cataract, ageing of the skin and also cause sunburns. They also prove fatal to phytoplankton, thus resulting in a decline in fish productivity. Excess exposure to these rays may, in some cases, also lead to mutation in plants. The moisture content of the soil decreases with an increase in exposure to UV radiations.

Q11) What are the major causes of water pollution? Explain.

Answer: Water pollution occurs when undesirable and unwanted substances are added to the water bodies thus choking the aquatic life in those water bodies.

The major sources of water pollutants are as given below:

The major pollutants are:

1. Pathogens: Bacteria and other micro-organisms come under this category. They get into water bodies through domestic sewage and animal excreta. Bacteria like Escherichia coli and Streptococcus faecalis which are found in human excreta are known vectors of gastrointestinal diseases.

2. Organic wastes: They are generally bio-degradable wastes that pollute the water bodies by mixing with them during their runoff phase. An excess amount of organic matter in water bodies suck up the dissolved oxygen in water thus inhibiting aquatic life in these water bodies.

3. Chemical pollutants: Heavy metals like mercury, cadmium and nickel come under this category. These chemicals when present above the allowable limit can result in damage of the central nervous system, kidneys and liver.

Q12) Have you ever observed any water pollution in your area? What measures would you suggest to control it?

Answer: Human activities like storm-water drainage, run-off from agricultural fields, discharges from wastewater treatment plants etc, result in water pollution.

• Toxic metals like Fe, Al and Mn are released into water bodies by industries and factories. So, it should be made sure that the water released from these industries are free of this type of toxicity. Their discharges should be frequently checked for traces of these metals and then released into water bodies.
• Instead of using fertilizers, manure and compost can be used effectively, so that these fertilizers don’t get into water bodies and pollute them.

Q13) 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 a sample of water. Clean water will have a BOD of below 5 ppm whereas a highly polluted water source will have a BOD of more than 17 ppm.

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

Answer: Pesticides and fertilizers are the major pollutants that cause soil pollution. When insecticides like DDT are used, they get stuck in the soil for a longer time since they are not soluble in water and thus contaminating the crops and soil. Moreover, pesticides and insecticides are not biodegradable in nature and when they enter the food chain, they go till the highest trophic levels and thus affect the whole biodiversity of an area.

• Thus, soil pollution can be controlled by controlling the addition of pesticides and fertilizers and instead use manures and compost.

Q15) What are pesticides and herbicides? Explain giving examples.

Answer: A mixture of two or more substances that are used to kill pests is called a Pesticide. Pests that need to be killed are plant pathogens, weeds, insects, mollusks, etc. which affect the plants and lead to their death. Some common pesticides include Dieldrin and Aldrin.

• Herbicides are similar to pesticides but are used to kill weeds. Examples include sodium arsenite (Na₃AsO₃), sodium chlorate (NaClO₃).

Q16) What do you mean by green chemistry? How will it help decrease environmental pollution?

Answer: The production process uses the existing knowledge that we have on the principles of chemistry to create, develop and implement chemical compounds and products to decrease the number of hazardous substances in the environment.

• Green chemistry aims to make sure that the end products yield is 100% so that there are no harmful substances released into the environment.
• For example, we now use H₂O₂ instead of tetrachloroethane and chlorine gas in the process of bleaching and drying of paper.

Q17) What would have happened if the greenhouse gases were totally missing in the earth’s atmosphere? Discuss.

Answer: The greenhouses gases that are present in our atmosphere trap the UV rays coming from Sun and heat up the earth.

In the absence of greenhouse gases, the earth won’t be able to retain any heat which is essential for the survival for most of the species on earth.

Without greenhouse gases, the earth’s average temperature will decline thus making this planet uninhabitable.

Q18) 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.

Answer: Phytoplankton is consumed by bacteria which require dissolved oxygen for this process. Thus, the higher the amount of Phytoplankton, the larger is the use of dissolved oxygen by bacteria to consume Phytoplankton. Thus, all the oxygen in the lake is used up and BOD drops below 6 ppm, suffocating the fish.

Q19) How can domestic waste be used as manure?

Answer: Firstly, the waste has to be segregated into biodegradable and non-biodegradable wastes. Those that can be degraded by bacteria such as leaves, food wastes are biodegradable and dumped in landfills along with micro-organisms that decompose it. The decomposed matter called Humus can be used as manure. The rest of the wastes, which are non-biodegradable have to be recycled.

Q20) 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 product.

Answer: The compost pit should be covered to prevent bad odour and flies.

• Non-biodegradable wastes should not be dropped into the compost pit so as not interfere in the decomposition of the wastes. Instead, they should be sent for recycling.