Class 11 Environmental Chemistry – Unit 13: Air (FBISE)
This section provides complete, exam-oriented notes for Class 11 Environmental Chemistry – Unit 13: Air according to the Federal Board (FBISE) syllabus. Students will gain a clear understanding of the key concepts related to air pollution and environmental chemistry.
Key topics include smog, acid rain, and air quality parameters. The notes explain the causes, effects, and prevention strategies, supported with diagrams, examples, and key points important for exams.
Students can also access video lectures, MCQs, test series, and live classes for this unit on our official YouTube channel and stay updated through our WhatsApp channel.
Notes
1. Definition of Smog
- Smog is a type of air pollution resulting from the combination of fog and smoke (or other atmospheric pollutants).
- It appears as a hazy, often yellowish veil that significantly reduces visibility.
- It forms when pollutants from vehicles, industries, and natural sources react in the presence of sunlight.
2. Types of Smog
| Type | Common Name | Primary Causes / Composition | Environment |
|---|---|---|---|
| Photochemical Smog | - | Nitrogen oxides ($NO_x$), Volatile Organic Compounds ($VOCs$), and Ozone ($O_3$). | Urban areas with high vehicular emissions and bright sunlight. |
| Industrial Smog | "Black Smog" | Particulate matter and Sulphur dioxide ($SO_2$) from burning coal or fossil fuels. | Areas where coal is the primary energy source. |
3. Mechanism of Photochemical Smog Formation
The process involves complex chemical reactions triggered by sunlight:
- $NO_2 + \text{sunlight} \rightarrow NO + O$
- $O_2 + O \rightarrow O_3$ (Formation of Ozone)
- $NO + O_3 \rightarrow NO_2 + O_2$
- $NO_2 + R \rightarrow \text{Products such as PAN}$ (where $R$ is an organic fragment from $VOCs$)
- $NO + ROX \rightarrow NO_2 + \text{other products}$
Note: PAN stands for Peroxyacetyl nitrate.
4. Hazardous Effects
- Human Health:
- Respiratory: Exacerbates asthma, bronchitis, and pharyngitis; causes coughing and breathing difficulty.
- Cardiovascular: Linked to heart attacks and arrhythmia.
- Eyes: Causes redness, itching, and watering.
- Safety & Visibility: Increases accident risks in road, air, and maritime travel.
- Environmental: Damages crops and forests; $O_3$ harms plant growth and reduces yields.
- Global Warming: Components like methane act as greenhouse gases.
- Aesthetic: Creates persistent haze that spoils scenic views.
Relevant Questions & Answers
Q1: What are the primary chemical constituents of photochemical smog?
A: Photochemical smog is primarily composed of Nitrogen oxides ($NO_x$), Volatile Organic Compounds ($VOCs$), and Ozone ($O_3$).
Q2: Differentiate between Photochemical Smog and Industrial Smog.
A: Photochemical smog is sunlight-dependent and forms from vehicular emissions ($NO_x$ and $VOCs$). Industrial smog, or "black smog," results from burning fossil fuels like coal and contains high levels of $SO_2$ and particulate matter.
Q3: How is Ozone ($O_3$) formed in the general mechanism of smog?
A: Sunlight breaks down Nitrogen dioxide into Nitric oxide and atomic oxygen:$$NO_2 + \text{sunlight} \rightarrow NO + O$$The atomic oxygen then reacts with molecular oxygen to form ozone:$$O_2 + O \rightarrow O_3$$
Q4: What is PAN and how is it produced?
A: PAN stands for Peroxyacetyl nitrate. It is produced when Nitrogen dioxide ($NO_2$) reacts with organic fragments ($R$) derived from Volatile Organic Compounds ($VOCs$).
Q5: What are the environmental impacts of smog beyond human health?
A: Smog reduces visibility leading to travel accidents, damages aquatic ecosystems, inhibits plant growth leading to lower crop yields, and contributes to global warming through components like methane.
Notes: Acid Rain and Air Quality Parameters
1. Acid Rain (Section 13.3)
- Definition: Rainfall or precipitation with a pH level lower than normal (less than $5.6$).
- Natural Acidity: Normal rainwater has a pH of approximately $5.6$ due to dissolved $CO_{2}$ forming weak carbonic acid:
- $CO_{2} + H_{2}O \rightarrow H_{2}CO_{3}$
- $H_{2}CO_{3} + H_{2}O \rightarrow H_{3}O^{+}_{(aq)} + HCO_{3(aq)}^{-}$
2. Causes and Reactions (Section 13.3.1)
- Primary Causes: Emissions of Sulphur dioxide ($SO_{2}$) and Nitrogen oxides ($NO_{x}$) from burning fossil fuels (power plants, vehicles) and industrial processes (mining, manufacturing).
- Sulphuric Acid Formation:
- $2SO_{2} + O_{2} + 2H_{2}O \rightarrow 2H_{2}SO_{4}$
- $H_{2}SO_{4} + H_{2}O \rightarrow H_{3}O^{+}_{(aq)} + SO_{4(aq)}^{2-}$
- Nitric Acid Formation:
- $NO_{2} + H_{2}O \rightarrow HNO_{3}$
- $HNO_{3} + H_{2}O \rightarrow H_{3}O^{+}_{(aq)} + NO_{3(aq)}^{-}$
3. Impacts of Acid Rain (Section 13.3.2)
- Environmental: Damages plant foliage and alters pH of soil and water, threatening aquatic life.
- Corrosion: Accelerates damage to buildings and statues (e.g., calcium carbonate structures):
- $CaCO_{3} + H_{2}SO_{4} \rightarrow CaSO_{4(aq)} + H_{2}O + CO_{2}$
- Loss of Biodiversity: Lower pH levels in water can cause the formation of Aluminum-based compounds that clog fish gills.
- Global Implications: Pollutants can travel long distances, leading to trans-boundary pollution.
4. Air Quality Parameters (Section 13.4)
- Air Quality Index (AQI): A standardized measure quantifying air quality based on pollutants like PM2.5, PM10, $O_{3}$, $CO$, $SO_{2}$, and $NO_{2}$.
- Particulate Matter (PM) Monitoring: Measures fine (PM2.5) and coarse (PM10) particles.
- Gas Monitoring: Uses instruments like gas chromatographs to measure concentrations of $CO_{2}$, $CO$, $SO_{2}$, $NO_{2}$, and $O_{3}$.
- Remote Sensing: Satellite-based sensors provide data over large geographic areas.
- Health Impact Assessment: Quantifies public health impact by analyzing hospital admissions and respiratory illnesses.
Questions and Answers
Q1: Why is "normal" rainwater slightly acidic even if it is not considered acid rain?A: Normal rainwater is slightly acidic because carbon dioxide in the atmosphere reacts with water to form carbonic acid ($H_{2}CO_{3}$), which dissociates to release hydronium ions ($H_{3}O^{+}$), typically resulting in a pH of about $5.6$.
Q2: Write the chemical equation for the corrosion of limestone (calcium carbonate) by acid rain.A: $CaCO_{3} + H_{2}SO_{4} \rightarrow CaSO_{4(aq)} + H_{2}O + CO_{2}$
Q3: What is the difference between PM2.5 and PM10?A: PM2.5 refers to fine particles with a diameter of $2.5$ micrometers or smaller, while PM10 refers to coarser particles with a diameter of $10$ micrometers or smaller. Both are monitored due to their significant health impacts.
Q4: How does acid rain specifically affect aquatic biodiversity?A: Acid rain lowers the pH level of water bodies. This acidity can lead to the formation of Aluminum-based gelatinous compounds that clog the gills of fish and amphibians, leading to a decline in sensitive species.
Q5: What modern method is used to measure air quality over large geographic areas?A: Remote Sensing is used, where satellites equipped with specialized sensors measure various pollutants and aerosols in the atmosphere across broad regions.