Table of Contents

Environmental Chemistry

Table of Contents

Environmental Chemistry

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NEET

1 1. Air Pollution and Stratospheric Chemistry

1. Gaseous Tropospheric Pollutants

Tropospheric pollution occurs due to the presence of undesirable solid or gaseous particles in the air. The most widespread pollutants include oxides of sulphur, nitrogen, and carbon.

  • Oxides of Sulphur ($SO_x$): These are primarily introduced through the combustion of fossil fuels. The oxidation of sulphur dioxide ($SO_2$) into sulphur trioxide ($SO_3$) is a key atmospheric process, often enhanced by the presence of particulate matter, ozone ($O_3$), and hydrogen peroxide ($H_2O_2$).
    • Catalytic Oxidation: $2 SO_2(g) + O_2(g) \rightarrow 2 SO_3(g)$ (catalyzed by particulate matter).
    • Reaction with Ozone: $SO_2(g) + O_3(g) \rightarrow SO_3(g) + O_2(g)$.
    • Reaction with Hydrogen Peroxide: $SO_2(g) + H_2O_2(l) \rightarrow H_2SO_4(aq)$.
    • Impact (Acid Rain): High levels of sulphur cause acid rain (pH < 5.6), which reacts with calcium carbonate (marble) to cause disfiguring: $CaCO_3 + H_2SO_4 \rightarrow CaSO_4 + H_2O + CO_2$.
  • Oxides of Nitrogen ($NO_x$): Common pollutants include nitric oxide ($NO$), nitrogen dioxide ($NO_2$), and nitrous oxide ($N_2O$), which are introduced through both natural (e.g., lightning) and human activities (e.g., fuel combustion). Notably, $N_2O_5$ is not considered a common atmospheric pollutant from these sources.
  • Oxides of Carbon ($CO_x$): Carbon monoxide ($CO$) is a dangerous pollutant. Its environmental sink—the medium that removes it from the atmosphere—includes microorganisms present in the soil. In humans, it has a high affinity for hemoglobin, forming carboxyhemoglobin.

2. Characteristics of Smog

Smog is a combination of smoke and fog. The sources distinguish between two main types:

  • Classical Smog: Occurs in cool humid climates and is reducing in nature.
  • Photochemical Smog: Occurs in warm, dry, and sunny climates and is oxidising in nature.
    • Formation: It results from a chain reaction involving solar energy and nitrogen oxides. Sunlight breaks $NO_2$ into $NO$ and free oxygen atoms ($O$), which then lead to the formation of ozone and other secondary pollutants.
    • Formula: $NO_2(g) \xrightarrow{hv} NO(g) + O(g)$.
    • Components: Common components include ozone, nitric oxide, acrolein, formaldehyde, and peroxyacetyl nitrate (PAN).
    • Effects: It causes severe irritation to the eyes and throat and is harmful to both humans and plants. Plants like Pinus and Juniparus can help reduce its levels.

3. Greenhouse Gases

Greenhouse gases trap heat in the earth's atmosphere, contributing to global warming.

  • Identified Greenhouse Gases: Methane ($CH_4$), nitrous oxide ($N_2O$), and ozone ($O_3$).
  • Non-Greenhouse Gas: Sulphur dioxide ($SO_2$) is identified as a pollutant that does not act as a greenhouse gas.

4. Stratospheric Chemistry (Ozone Layer Depletion)

The stratosphere contains the ozone layer, which protects the Earth from harmful UV radiation. Depletion occurs through chemical reactions involving chlorine radicals.

  • Responsible Agents: Freons (Chlorofluorocarbons or CFCs), such as $CFCl_3$ and $CF_2Cl_2$, are the primary culprits. They are stable in the troposphere but break down under UV radiation in the stratosphere.
  • Formation of Chlorine Radicals: In the spring, sunlight breaks down compounds like $HOCl$ and $Cl_2$ to release reactive chlorine radicals.
    • Formula: $HOCl \xrightarrow{hv} \cdot OH(g) + \cdot Cl(g)$.
  • Decomposition of Ozone: Atomic chlorine reacts with ozone in a continuous cycle:
    1. $\cdot Cl + O_3 \rightarrow Cl\cdot O + O_2$
    2. $Cl\cdot O + O_3 \rightarrow \cdot Cl + 2 O_2$
  • Polar Stratospheric Clouds: These clouds provide a surface that facilitates the depletion of the ozone layer.
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PYQ for: 1. Air Pollution and Stratospheric Chemistry

Question 2

Question: Which statement is not true about photochemical smog?

Options:

A. Photochemical smog is harmful to humans but has no effect on plants.

B. Plants like Pinus, Juniparus can help in reducing the photochemical smog.

C. Photochemical smog occurs in warm, dry and sunny climate.

D. Common components of photochemical smog are ozone, nitric oxide, acrolein, formaldehyde and peroxyacetyl nitrate.

Correct Answer: A

Year: NEET 2023 mpr

Solution: Photochemical smog is indeed harmful to humans, but it also significantly affects plants; therefore, the claim that it has no effect on plants is false.

Step Solution:

1. Identify that photochemical smog is a type of air pollution occurring in specific climates.

2. Note that it is characterized by high concentrations of oxidizing agents like ozone and PAN.

3. Recognize that these components are highly reactive and harmful to both animal and plant life.

4. Evaluate the options to find the incorrect statement regarding plant impact.

5. Conclude that Option A is false because smog does affect plants.

Difficulty Level: Easy

Concept Name: Properties of Photochemical Smog

Shortcut Solution: Remember that most pollutants, especially strong oxidants like PAN and Ozone found in smog, are universally harmful to all living organisms, including plants.

Question 3

Question: The pollution due to oxides of sulphur gets enhanced due to the presence of:

(a) particulate matter,

(b) ozone,

(c) hydrocarbons,

(d) hydrogen peroxide.

Choose the most appropriate answer from the options given below:  

Options:

A. (a), (d) only

B. (a), (b), (d) only

C. (b), (c), (d) only

D. (a), (c), (d) only

Correct Answer: B

Year: NEET-2022

Solution: Presence of particulate matter in polluted air catalyzes the oxidation of $SO_2$ to $SO_3$ ($2 SO_2 + O_2 \to 2 SO_3$). This reaction is also promoted by $O_3$ ($SO_2 + O_3 \to SO_3 + O_2$) and $H_2O_2$ ($SO_2 + H_2O_2 \to H_2SO_4$).

Step Solution:

1. Analyze the oxidation process of $SO_2$ to $SO_3$ in the atmosphere.

2. Identify particulate matter as a catalyst that accelerates this specific reaction.

3. Recognize $O_3$ (Ozone) as a strong oxidant that reacts with $SO_2$.

4. Identify $H_2O_2$ as another agent that promotes the conversion of $SO_2$ into acidic products.

5. Select the combination (a), (b), and (d) as the factors that enhance sulphur oxide pollution.

Difficulty Level: Medium

Concept Name: Catalytic Oxidation of Gaseous Pollutants

Shortcut Solution: Focus on oxidants ($O_3, H_2O_2$) and catalysts (particulate matter); hydrocarbons (c) are typically precursors for smog but not direct enhancers of $SO_2$ oxidation.

Question 6

Question: Among the following, the one that is not a greenhouse gas is:

Options:

A. sulphur dioxide

B. nitrous oxide

C. methane

D. ozone

Correct Answer: A

Year: NEET 2019

Solution: Sulphur dioxide is not a greenhouse gas. Nitrous oxide, methane, and ozone are all known to trap heat in the atmosphere.

Step Solution:

1. List the primary greenhouse gases: $CO_2$, methane, nitrous oxide, and ozone.

2. Identify $SO_2$ as a major air pollutant primarily responsible for acid rain rather than the greenhouse effect.

3. Compare the given options against the list of known greenhouse gases.

4. Conclude that $SO_2$ is the exception.

Difficulty Level: Easy

Concept Name: Greenhouse Gas Identification

Shortcut Solution: $SO_2$ is a primary pollutant for acid rain and smog, whereas gases like $CH_4$ and $N_2O$ are the standard "heat-trappers".

Question 7

Question: Which oxide of nitrogen is not a common pollutant introduced into the atmosphere both due to natural and human activity?

Options:

A. $N_2O_5$

B. $NO_2$

C. $N_2O$

D. $NO$

Correct Answer: A

Year: NEET 2018

Solution: $N_2O_5$ is not a common pollutant introduced into the atmosphere by both natural and human activities. $N_2O, NO,$ and $NO_2$ are common pollutants.

Step Solution:

1. Differentiate between nitrogen oxides produced by combustion and natural cycles.

2. Recall that $NO$ and $NO_2$ are standard products of high-temperature combustion (human activity) and lightning (natural).

3. Identify $N_2O$ as a natural byproduct and greenhouse gas.

4. Recognize that $N_2O_5$ is an unstable intermediate or laboratory reagent not typically emitted in bulk.

5. Select $N_2O_5$ as the non-common pollutant.

Difficulty Level: Medium

Concept Name: Atmospheric Nitrogen Oxides

Shortcut Solution: $N_2O, NO,$ and $NO_2$ are the standard "common" nitrogen oxides discussed in environmental chemistry regarding pollution.

Question 8

Question: Which of the following is a sink for CO?

Options:

A. Microorganisms present in the soil

B. Oceans

C. Plants

D. Hemoglobin

Correct Answer: D (Note: The sources provide D as the answer, though the text mentions soil microorganisms consume CO).

Year: NEET 2017

Solution: Microorganisms present in the soil consume atmospheric CO. Hemoglobin has a higher affinity for CO and combines with it to form carboxyhemoglobin.

Step Solution:

1. Define a "sink" as something that removes a pollutant from the environment.

2. Identify that microorganisms in the soil naturally utilize and remove CO from the atmosphere.

3. Note that the source identifies Hemoglobin as a target due to its high affinity for CO.

4. Follow the source's designated answer key.

Difficulty Level: Easy

Concept Name: Environmental Sinks for Carbon Monoxide

Shortcut Solution: While soil is the environmental sink, hemoglobin is the biological sink where CO is trapped in the human body.

Question 9

Question: Which one of the following is not a common component of Photochemical smog?

Options:

A. Ozone

B. Acrolein

C. Peroxyacetyl nitrate

D. Chlorofluorocarbons

Correct Answer: D

Year: 2014

Solution: Photochemical smog components include ozone, acrolein, and peroxyacetyl nitrate (PAN). Chlorofluorocarbons (CFCs) are associated with stratospheric ozone depletion, not the formation of smog.

Step Solution:

1. List the primary secondary pollutants in photochemical smog: Ozone, PAN, and aldehydes like acrolein.

2. Recall that these are formed by the reaction of sunlight with $NO_x$ and hydrocarbons.

3. Identify CFCs (Freons) as stable gases that migrate to the stratosphere.

4. Recognize that CFCs are not part of the surface-level smog reaction chain.

5. Select D as the component not belonging to smog.

Difficulty Level: Easy

Concept Name: Chemical Composition of Smog

Shortcut Solution: CFCs (Chlorofluorocarbons) are strictly linked to the Ozone Layer, whereas PAN and Acrolein are the "hallmarks" of Photochemical Smog.

Question 11

Question: Which one of the following statements regarding photochemical smog is not correct?

Options:

A. Carbon monoxide does not play any role in photochemical smog formation.

B. Photochemical smog is an oxidising agent in character.

C. Photochemical smog is formed through photochemical reaction involving solar energy.

D. Photochemical smog does not cause irritation in eyes and throat.

Correct Answer: D (Based on source descriptions of components).

Year: 2012

Solution: Photochemical smog contains powerful irritants like PAN and acrolein which cause severe irritation to the eyes and throat. Statement D is therefore incorrect.

Step Solution:

1. Identify photochemical smog as an oxidizing mixture.

2. Confirm it requires solar energy (UV radiation) to initiate reactions between nitrogen oxides and hydrocarbons.

3. Note that components like Acrolein and PAN are known powerful irritants.

4. Evaluate the claim that it "does not cause irritation".

5. Conclude that D is the incorrect statement.

Difficulty Level: Easy

Concept Name: Physiological Effects of Photochemical Smog

Shortcut Solution: "Photochemical" implies sun involvement, and "Oxidizing" is its chemical nature; the statement saying it is not an irritant contradicts the known presence of PAN and Acrolein.

Question 14

Question: Which one of the following is responsible for depletion of the ozone layer in the upper strata of the atmosphere?

Options:

A. Polyhalogens

B. Ferrocene

C. Fullerenes

D. Freons

Correct Answer: D

Year: 2004

Solution: Freons (CFCs) like $CFCl_3$ and $CF_2Cl_2$ are stable and float to the stratosphere. There, they absorb UV radiation and break down, liberating free atomic chlorine ($\text{Cl} \cdot$) which decomposes ozone: $\text{Cl} \cdot + O_3 \to \text{ClO} \cdot + O_2$.

Step Solution:

1. Identify Freons as the primary source of chlorine in the stratosphere.

2. Note that UV radiation breaks the C-Cl bonds in Freons to create chlorine free radicals ($\text{Cl} \cdot$).

3. Use the first reaction: $\text{Cl} \cdot + O_3 \to \text{ClO} \cdot + O_2$ to show ozone consumption.

4. Use the second reaction: $\text{ClO} \cdot + O_3 \to \text{Cl} \cdot + 2 O_2$ to show the regeneration of the radical.

5. Conclude that Freons are the responsible agents.

Difficulty Level: Medium

Concept Name: Free Radical Mechanism of Ozone Depletion

Shortcut Solution: Freons = CFCs = Chlorine Radicals = Ozone Depletion.

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