NCERT Solutions for Class 11 Chemistry Chapter 14: Environmental Chemistry

14.1. Define environmental chemistry.

Ans. Environmental chemistry is described as the branch of science that studies the chemical phenomena that occur in the environment, such as the origin, transport, reactions, impacts, and destiny of chemical species in the environment.

14.2. Explain tropospheric pollution in 100 words.

Ans. The increase in the concentration of undesired gases and particulate matter in the troposphere to such an extent that they can produce undesirable effects on human beings and their environment is called tropospheric or air pollution.The tropospheric pollution is due to following two types of polluants :
(i) Gaseous pollutants, e.g., oxides of sulphur (SO2, SO3), oxides of nitrogen (NO, NO2), oxides of carbon (CO, CO2), hydrogen sulphide, hydrocarbons, aldehydes, ketones, etc.
(ii) Particulate matter, e.g., mists, smoke, fumes, dust, carbon particles, lead and cadmium compounds, bacteria, fungi, moulds, etc.
Gaseous pollutants present are oxides of sulphur (SO2 and SO3), nitrogen (NO and NO2), carbon (CO), H2S, hydrocarbons etc. SO2 and SO3 are irritating to respiratory tract. SO2 causes throat and eye irritation. SO2 leads stiffness of flower buds and their fall from plants. They damage building materials. NO2 is a toxic gas and is lung irritant. Plants start shedding their leaves and fruits. Hydrocarbons may cause cancer. Carbon monoxide in blood results into headache, weak eyesight, nervousness and cardiovascular disorder. Participate pollutants include smoke, dust, fly ash etc. which are harmful to animals and human beings. Oxides of nitrogen and sulphur cause acid rain.

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

Ans. Carbon monoxide (CO) is a highly poisonous, colourless and odourless gas which is produced as a result of incomplete combustion of carbon. It is more dangerous to living beings than carbon dioxide gas because of its ability to block the delivery of oxygen to the organs and tissues. It binds to haemoglobin to form carboxyhaemoglobin, which is about 300 times more stable than the oxygen-haemoglobin complex. In blood, when the concentration of carboxyhaemoglobin reaches about 3–4 percent, the oxygen carrying capacity of blood is greatly reduced. This oxygen deficiency, results into headache, weak eyesight, nervousness and cardiovascular disorder. This is the reason why people are advised not to smoke. In pregnant women, who have the habit of smoking the increased CO level in blood may induce premature birth, spontaneous abortions and deformed babies.

14.4. List gases which are responsible for greenhouse effect.

Ans. The major greenhouse gases are :

  1. Carbon dioxide (CO2)
  2. Methane (CH4)
  3. Water (H2O)
  4. Nitrous oxide (NO)
  5. Ozone (O3)
  6. Chlorofluorocarbons (CFCs)

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

Ans. Statues and monuments in India are affected by acid rain because the air around these statues and monuments contains fairly high levels of sulphur and nitrogen oxides. It is mainly due to a large number of industries, power plants around the area and use of poor quality of coal, kerosene and firewood as fuel for domestic purposes.
These oxides undergo oxidation and then react with water vapour to form acids.

$$2\text{SO}_2\text{(g)} + \text{O}_2\text{(g)} + 2\text{H}_2\text{O(l)}\xrightarrow{}2\text{H}_2\text{SO}_4\text{(aq)}$$
$$4\text{NO}_2\text{(g)} + \text{O}_2\text{(g)} + 2\text{H}_2\text{O(l)} \xrightarrow{} 4\text{HNO}_3\text{(aq)}$$

The resulting acid rain causes damage to building and monuments by reacting with the stone or marble of which they are made. For example, In India, acid rain has caused severe damage to Tajmahal by reacting with its marble,(limestone, CaCO3) causing damage to this wonderful monument that has attracted people from around the world.

Acid rain reacts with limestone as :

$$\text{CaCO}_3 + \text{H}_2\text{SO}_4 \xrightarrow{}\text{CaSO}_4 + \text{H}_2\text{O} + \text{CO}_2$$

As a result, the monument is being slowly disfigured and the marble is getting discoloured and lustreless.

14.6. What is smog? How is classical smog different from photochemical smogs?

Ans. Smog is a type of air pollution which is a combination of smoke and fog.

Difference between classical and photochemical smog

Classical smog Photochemical smog
At first, the classical smog is observed in London in 1952. At first, the photochemical smog is observed in Los angeles in 1950.
The classical smog is the H2SO4 fog deposit on the particulates. H2SO4 fog is a mixture of SO2 and humidity. The NO2 and hydro-carbons present in air reacts photochemically to form photochemical smog.
It is a combination of smoke and fog. Photochemical smog do not involve any smoke or fog.
Majorly, classical smog is formed during winter season. It is formed during summer season during afternoon when there is bright sunlight.
It can cause irritation to lungs. It can cause irritation to eyes.
It can cause irritation to lungs. It can cause irritation to eyes.
It is reducing in character. It is oxidising in character.

14.7. Write down the reactions involved during the formation of photochemical smog.

Ans. Photochemical smog occurs in warm, dry and sunny climate. The main components of the photochemical smog result from the action of sunlight on unsaturated hydrocarbons and nitrogen oxides produced by automobiles and factories. Photochemical smog is called as oxidising smog because it has high concentration of oxidising agent. The formation of photochemical smong can be summarised as follows:

$$\text{NO}_2\text{(g)}\xrightarrow{hv} \text{NO(g)} + \text{O(g) ...(i)}$$

Oxygen atoms are very reactive and combine with the O2 in air to produce ozone.

$$\text{O(g)} + \text{O}_2\text{(g)} \rightleftharpoons \text{O}_3\text{(g) ...(ii)}$$

The ozone formed in the above reaction (ii) reacts rapidly with the NO(g) formed in the reaction (i) to regenerate NO2.NO2 is a brown gas and at sufficiently high levels can contribute to haze.

$$\text{NO(g)} + \text{O}_3 \text{(g)} \xrightarrow{} \text{NO}_2 \text{(g)} + \text{O}_2 \text{(g) ...(iii)}$$

While ozone is toxic in nature, both NO2 and O3 are oxidising agents. They react with the unburnt hydrocarbons in air to produce formaldehyde, PAN, and acrolein.

$$3\text{CH}_4 + 2\text{O}_3 \xrightarrow{} 3\text{CH}_2 = \text{O} + 3\text{H}_2\text{O}\\ \space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\text{Formaldehyde}$$

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

Ans. Harmful effects of photochemical smog:

(i) Their high concentration causes headache, chest pain and dryness of the throat.

(ii) Ozone and PAN act as powerful eye irritants.

(iii) Photochemical smog leads to cracking of rubber and extensive damage to plant liife.

(iv) It causes corrosion of metals, stones, building materials, and painted surface etc.
(i) Use of catalytic converter in automobiles prevents the release of nitrogen dioxide and hydrocarbons to the atmosphere.

(ii) Pinus, juniparus, quercus, pyrus etc. can metabolise nitrogen dioxide. Thus, their plantation could help to some extent.

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

Ans. The main reason of ozone layer depletion is believed to be the release of chlorofluorocarbon compounds (CFCs), also known as freons. These compounds are non-reactive, non-flammable, non-toxic organic molecules and therefore used in refrigerators, air conditioners, plastic and electronic industries.
However, once CFCs are released in the atmosphere, they mix with the normal atmospheric gases and eventually reach the stratosphere. In stratosphere, they get broken down by powerful UV radiations, releasing chlorine free radical.

$$\text{CF}_2\text{Cl}_2\text{(g)} \xrightarrow{\text{UV}} \text{Cl(g)} + \text{CF}_2\text{Cl(g) ...(i)} $$

The chlorine radical then react with stratospheric ozone to form chlorine monoxide radicals and molecular oxygen.

$$\text{Cl(g)} + \text{O}_3\text{(g)} \xrightarrow{} \text{ClO(g)} + \text{O}_2 \text{ ...(ii)}$$

Reaction of chlorine monoxide radical with atomic oxygen produces more chlorine radicals.

$$\text{ClO(g)} + \text{O(g)} \xrightarrow{} \text{Cl(g)} + \text{O}_2 \text{...(iii)}$$

The chlorine radicals are continuously regenerated and cause the breakdown of ozone. Thus, CFCs are transporting agents for continuously generating chlorine radicals into the stratosphere and damaging the ozone layer.

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

Ans. The term ‘ozone hole’ refers to the depletion of the protective ozone layer in the upper atmosphere (stratosphere) over Earth’s polar region.
Ozone Depletion’s Consequences :
(i) With the depletion of ozone layer, more UV radiation filters into troposphere. UV radiations lead to ageing of skin, cataract, sunburn and skin cancer.

(ii) UV radiation also kills many of the phytoplanktons thereby, damaging the fish productivity.

(iii) Plants exposed to ultraviolet light have an unfavourable effect on their proteins, resulting in chlorophyll degradation and harmful mutation of cells.

(iv) UV radiation also increases evaporation of surface water through the stomata of the leaves and decreases the moisture content of the soil.

(v) Increase in UV radiations damage paints and fibres, causing them to fade faster.

14.11. What are the major causes of water pollution? Explain.

Ans. Water pollution arises as a result of several human activities, which leads to the presence of several undesirable substances in water. The following are the major causes of water pollution:

(i) Pathogens: The most serious water pollutants are the disease causing agents called pathogens. Pathogens include bacteria and other organisms that enter water from domestic sewage and animal excreta. Human excreta contain bacteria such as Escherichia coli and Streptococcus faecalis which cause gastrointestinal diseases.

(ii) Organic wastes : Organic matter such as discharge from food processing factories, leaves, grass, etc. pollute water. These wastes are biodegradable. The large population of bacteria decomposes organic matter present in water. These bacteria consume oxygen dissolved in water. If too much of organic matter is added to water, all the available oxygen is used up. This causes oxygen dependent aquatic life to die. Anaerobic bacteria also decompose the organic wastes and produce chemicals that have a foul smell and are harmful to human health.

(iii) Chemical pollutants : Water is an excellent solvent. It dissolves inorganic chemicals that include heavy metals such as cadmium, mercury, nickel which are harmful to humans because our body cannot excrete them. These metals can damage kidneys, central nervous system, liver, etc. Organic chemicals present in the petroleum products pollute many sources of water, e.g., major oil spills in oceans. Various industrial chemicals such as cleansing solvent, detergents and fertilizers also pollute water. Eutrophication is another source of water pollution. It is the process that results when large quantities of phosphates and nitrates are released into aquatic ecosystems. This polution causes the amount of oxygen in water to decrease and many organisms die because there is not enough oxygen present for respiration.

14.12. Have you are observed any water pollution in yur area? What measures would you suggest to control it.

Ans. Yes, I have observed polluted water flowing in open sewage drains in my area.
We can control if it by the following measures :

  1. Industrial waste discharge from paper, fertilisers, pesticides, detergents, drugs industries and refineries should not be allowed to get mixed in water bodies such as river, lakes, etc.
  2. Non-biodegradable detergents should be avoided and only biodegradable detergents should be used for cleansing of clothes.
  3. The pH of water should be checked.
  4. Excessive use of fertilisers should be prevented.
  5. Oil spills should be avoided as much as possible.
  6. Domestic waste water should be properly discharged and treated.
  7. Avoid the use of DDT, malathion at home.
  8. Waste water should be treated in sewage treatment plant.
  9. Treatment of polluted water, lime etc. also helps in its purification.

14.13. 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 a sample of water, is called as the Biochemical Oxygen Demand (BOD). The amount of BOD in the water is a measure of the amount of organic material in the water, in terms of how much oxygen will be required to break it down biologically. Clean water would have BOD value of less than 5 ppm whereas highly polluted water could have a BOD value of 17 ppm or more.

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

Ans. Yes, we see soil pollution in our neighbourhood as a result of the dumping of non-biodegradable waste , biodegradable waste, industrial waste and agricultural pollutants such as fertilisers, pesticides, insecticides etc. The best way to manage household waste is to keep two garbage cans, one for biodegradable waste and one for non- biodegradable waste. Many combustible wastes can be burned and the ashes used as landfill. However, toxic gases that can be released during combustion must be avoided.
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.

14.15. What are pesticides and herbicides ? Explain giving examples.

Ans. Pesticides are synthetic chemical compounds being toxic in nature they are used in agriculture to protect the crops and plants from the pests like insects, rodents weeds and various crop diseases. However, they are basically synthetic toxic chemicals with ecological repercussions. The repeated use of the same or similar pesticides give rise to pests that are resistant to that group of pesticides thus making the pesticides ineffective. For example, DDT was used as pesticide in the beginning. The insect developed resistance towards DDT gradually over the years and therefore, other organic toxins such as Aldrin and Dieldrin were inroduced for use as pesticide. But these were non-biodegradable and slowly transferred to human beings through food chain causing metabolic and physiological disorders. Consequently, a new series of biodegradable pesticides, organophosphates and carbamates have been introduced. However, they are severe nereve toxins and hence more harmful to humans.

Herbicides : These are the chemical used to control weeds. Inorganic compounds such as sodium chlorate (NaClO3) and sodium aresenite (Na3AsO3) were used in the beginning but aresenic compounds, being toxic to mammals, are no longer preferred. Instead, organic compounds such as triazines, are now considered as better herbicides, expecially for the cornfields.

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

Ans. Green chemistry is a way of thinking and is about utilising the existing knowledge and principles of chemistry and other sciences to reduce the adverse impact on environment. It is a production process that would bring about minimum pollution or deterioration to the environment. Green chemistry help decrease in environmental pollution in following ways:

(i) 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, H2O2 has replaced tetrachlorethane and chlorine gas in drying and bleaching of paper.

(ii) Automobile engines have been fitted with catalytic converters which prevent the release of the vapours of hydrocarbons and oxides of nitrogen into acrolein and peroxyacetyl nitrate.

(iii) CO2 has replaced CFCs as blowing agents in the manufacture of polystyrene foam sheets.

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

Ans. Earth’s most abundant greenhouse gases are CO2, CH4, O3, CFCs, and water vapour. These 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 uninhabitable. As a result, life on Earth would be impossible.

14.18. 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. Fishes can die either due to the presence of some poisonous substances or due to lack of oxygen. Since in this case there is no evidence of toxic dumping, so it can be only scarcity of oxygen. Excess amount oif phytoplankton present in water are biodegradable and a large population of bacteria deacomposes them in water. During the process they consume dissolved O2 in water and level of dissolved O2 falls below 5 ppm and then fish can not survive. Hence, they die and float dead on the lake.

14.19. How can domestic waste be used as manure?

Ans. Domestic waste is divided into two categories, biodegradable waste such as leaves, rotten food, etc. and non-biodegradable waste such as plastics, glass, metal scrap, etc., Non-biodegradable waste is recycled in the industry whereas biodegradable waste can be converted to manure through sanitary landfills, i.e., where the wet biodegradable waste is buried in layers and covered with several layers of soil. The compacted soil layer is to prevent odours and windblown debris. Through biodegradation the waste is converted to manure.

14.20. 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 producing pit should be set up at a suitable place to protect ourselves from bad odour and flies. Biodegradable domestic wastes, e.g., used tea leaves, vegetable and fruits waste are put in the compost pit and it is covered with a little sand. After some time, it is converted into compost by the action of heat and bacteria. Compost pit should be kept covered so that flies cannot make entry into it and the foul odour is minimised.

Non-biodegradable domestic wastes such as plastic, glass, metal scraps, polythene bags, etc., are sent for recycling. The process of recycling converts waste into wealth.