Chemistry Inorganic - How to Solve: Environmental Chemistry (CFCs, Ozone Depletion, Acid Rain, BOD/COD, Green Chemistry) – IIT JEE Guide

How to Solve: Environmental Chemistry (CFCs, Ozone Depletion, Acid Rain, BOD/COD, Green Chemistry) – IIT JEE Guide

Introduction

Mastering Environmental Chemistry unlocks 8-10 marks in IIT JEE (Main + Advanced) every year—enough to push you into the top 1%. From CFC-induced ozone holes to acid rain corroding monuments, this topic bridges real-world impact with high-yield exam questions. If you can predict ozone depletion reactions or calculate BOD/COD ratios, you’re not just solving problems—you’re scoring guaranteed marks.

WHAT YOU NEED TO KNOW FIRST

Before diving in, ensure you understand:
1. Basic chemical kinetics (rate laws, catalysts, chain reactions).
2. pH and acid-base equilibria (strong vs. weak acids, pH calculations).
3. Redox reactions (oxidation states, balancing redox equations).

If any of these are shaky, pause and review—this topic builds on them.

KEY TERMS & FORMULAS

1. Ozone Depletion (CFCs & O₃ Layer)

2. Acid Rain

3. BOD & COD (Water Pollution)

4. Green Chemistry (12 Principles)

STEP-BY-STEP METHOD

How to Solve Ozone Depletion Problems

Step 1: Identify the source of Cl radicals (usually CFCs like CCl₂F₂). Step 2: Write the initiation reaction (UV light breaks C-Cl bond): CCl₂F₂ → Cl• + CClF₂• Step 3: Write the propagation steps (Cl• destroys O₃):
1. Cl• + O₃ → ClO• + O₂
2. ClO• + O → Cl• + O₂ (regenerates Cl•) Step 4: Calculate total O₃ destroyed (1 Cl• destroys ~100,000 O₃ molecules). Step 5: If asked for Dobson Units (DU), convert using: 1 DU = 0.01 mm ozone at STP

How to Solve Acid Rain Problems

Step 1: Identify the primary pollutant (SO₂ or NO₂). Step 2: Write the acid formation reactions: - For SO₂: SO₂ + H₂O → H₂SO₃ 2SO₂ + O₂ → 2SO₃ → H₂SO₄ - For NO₂: 2NO₂ + H₂O → HNO₃ Step 3: Calculate pH (if given concentrations, use pH = -log[H⁺]). Step 4: Compare with normal rain (pH ~5.6) to confirm acid rain.

How to Solve BOD/COD Problems

Step 1: For BOD, use: BOD = (D₁ - D₂) × P - D₁ = Initial dissolved oxygen (DO) - D₂ = DO after 5 days - P = Dilution factor (if sample is diluted) Step 2: For COD, remember: - COD > BOD (always) - If given COD/BOD ratio, use it to find missing values. Step 3: If asked about water quality, recall: - BOD < 1 mg/L → Clean water - BOD > 5 mg/L → Polluted water

How to Solve Green Chemistry Problems

Step 1: Identify the principle being tested (e.g., atom economy, safer solvents). Step 2: For atom economy, use: % Atom Economy = (Molar mass of desired product / Molar mass of all reactants) × 100 Step 3: Compare green vs. non-green methods (e.g., CO₂ vs. phosgene for polycarbonate synthesis).

WORKED EXAMPLES

Example 1 – Basic (Ozone Depletion)

Question: How many ozone molecules can a single Cl• radical destroy before being removed from the stratosphere?

Solution:
1. Initiation: CFCs release Cl• under UV light.
2. Propagation: - Cl• + O₃ → ClO• + O₂ (1 O₃ destroyed) - ClO• + O → Cl• + O₂ (Cl• regenerated)
3. Cycle repeats ~100,000 times before Cl• is removed. Answer: 100,000 ozone molecules

What we did and why: - We used the catalytic cycle of Cl• to show how one radical destroys thousands of O₃ molecules. - This is a common exam question—memorise the cycle!

Example 2 – Medium (Acid Rain pH Calculation)

Question: If rainwater contains 1 × 10⁻⁵ M H₂SO₄, what is its pH? (Assume complete dissociation)

Solution:
1. H₂SO₄ is a strong acid → Fully dissociates: H₂SO₄ → 2H⁺ + SO₄²⁻
2. [H⁺] = 2 × [H₂SO₄] = 2 × 10⁻⁵ M
3. pH = -log[H⁺] = -log(2 × 10⁻⁵) ≈ 4.7
4. pH < 5.6 → Acid rain confirmed.

Answer: pH ≈ 4.7

What we did and why: - We doubled [H⁺] because H₂SO₄ gives 2 H⁺ ions. - pH < 5.6 is the key indicator of acid rain.

Example 3 – Exam-Style (BOD Calculation)

Question: A 5 mL water sample is diluted to 50 mL. Initial DO = 8 mg/L, DO after 5 days = 4 mg/L. What is the BOD?

Solution:
1. Dilution factor (P) = Final volume / Initial volume = 50 mL / 5 mL = 10
2. BOD = (D₁ - D₂) × P = (8 - 4) × 10 = 40 mg/L
3. BOD > 5 mg/L → Polluted water.

Answer: 40 mg/L

What we did and why: - We applied the BOD formula with the dilution factor. - BOD > 5 mg/L is a red flag for pollution—examiners love this!

COMMON MISTAKES

EXAM TRAPS

1-MINUTE RECAP (Night Before Exam)

"Listen up—this is your 8-10 mark guarantee in IIT JEE Environmental Chemistry.

1. Ozone Depletion:
2. CFCs → Cl• → Destroys O₃ in a catalytic cycle.
3. 1 Cl• = 100,000 O₃ molecules gone.

4. Dobson Units (DU): 1 DU = 0.01 mm ozone.

5. Acid Rain:

6. SO₂ → H₂SO₄, NO₂ → HNO₃.
7. pH < 5.6 = acid rain.

8. Double [H⁺] for H₂SO₄!

9. BOD/COD:

10. BOD = (D₁ - D₂) × P (dilution factor matters!).
11. COD > BOD (always).

12. BOD > 5 mg/L = polluted water.

13. Green Chemistry:

14. 12 principles—memorise atom economy, safer solvents, prevention.
15. % Atom Economy = (Desired product mass / All reactants mass) × 100.

Common traps? - pH 7 is NOT normal rain (it’s 5.6). - CFCs don’t directly destroy O₃ (Cl• does). - COD is always higher than BOD.

You’ve got this. Go crush it!