Chemistry Class 11 NCERT Solutions: Chapter 14 Environmental Chemistry Part 2

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


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:

While ozone is toxic in nature, both are oxidizing agents. They react with the unburnt hydrocarbons in air to product formaldehyde, PAN, and acrolein.

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


Effects of Photochemical Smog:

Photochemical smog is oxidizing smog owing to the presence of and , causing corrosion of metals, 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 ) 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 and hydrocarbons, which in turn form ozone, PAN, and other chemicals. The use of catalytic converters in automobiles is recommended to prevent the release of and hydrocarbons into the atmosphere.

Plantation of plants such as Pinus, Juniperus, Quercus, Pyrus, and Vitis is also advised as these plants have the capability to metabolize .

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


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



Reaction (ii) indicates the dynamic equilibrium existing between the productions an 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 chlorofluorocarbon compounds . These are non-reactive, non-flammable molecules that are used in refrigerators, air conditioners, plastics, and electronic industries.

Once released mix with atmospheric gases and reach the stratosphere, where are decomposed by UV radiations.


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


The radicals further react with atomic oxygen to produce more chlorine radicals as:


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

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


In Polar regions, stratospheric clouds provide the surface for chlorine nitrate and Hypochlorous acid, which react further to give molecular chlorine. Molecular chlorine and are photolysed to give chlorine-free radicals.

The chlorine – free radicals lead to the decomposition of ozone as:

Hence, a chain reaction is initiated. The chlorine – free radical is continuously regenerated thereby depleting the ozone layer. This phenomenon is known as the ad ‘ozone hole’.

Effects of depletion of ozone layer

The ozone layer protects the Earth from the harmful UV radiations of the sun. With the depletion of the layer, more radiation will enter the Earth's atmosphere. UV 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 fish productivity. Excess exposure may even cause mutation in plants.

Increase in UV radiations, decreases the moisture content of the soil and damages both plants and fibres.