--- title: "Acid rain; sources and control of CO and SO₂" subject: "Engineering Chemistry" chapter: 3 key: "semester_2/engineering_chemistry:acid-rain-co-so2" source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/acid-rain-co-so2 site: BCT Engineering --- # Acid rain; sources and control of CO and SO₂ Engineering Chemistry · Chapter 3: Environmental Chemistry - Asked in: 2067 Chaitra, 1+2+2 marks - Video: [Acid Rain and its Effects | What is Acid Rain? | Air Pollution and its effects | Air pollutants](https://www.youtube.com/watch?v=1HXYTuywtDo) — SM-Educate Chemistry - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/acid-rain-co-so2 - Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-3 ## Question Give an account of acid rain? What are the sources of CO and SO₂ pollutants in air? How are they controlled? ## Short answer Acid rain (pH<5.6) results from SO2 and NOx forming H2SO4 and HNO3, damaging lakes, forests, and buildings. CO arises from incomplete combustion; SO2 from coal/oil and smelting. Control: CO—catalytic converters, clean fuels; SO2—low-sulfur fuels, flue gas desulfurization (lime scrubbers). ## Answer #### Acid rain **Acid rain** is rain (or snow, fog or dust) with a pH below 5.6. Normal rain water has a pH of about 5.6; it is slightly acidic because of dissolved CO₂ (CO₂ + H₂O ⇌ H₂CO₃). Acid rain is caused by the oxides of sulphur and nitrogen released into the air. SO₂ comes from burning coal and oil and from smelters; NO and NO₂ come from vehicle engines and power plants (N₂ + O₂ → 2NO at high temperature). In the air they are oxidised and dissolve in cloud water: ```text 2SO₂ + O₂ → 2SO₃ SO₃ + H₂O → H₂SO₄ 2NO + O₂ → 2NO₂ 4NO₂ + O₂ + 2H₂O → 4HNO₃ ``` These strong acids fall as acid rain. Effects: it acidifies lakes and kills fish, damages forests and soil, corrodes buildings and metals (CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂) and harms health. #### Sources of CO and SO₂ - **CO:** incomplete combustion of carbon fuels in vehicle engines (the largest source), burning of wood, coal and charcoal in poorly ventilated stoves, forest fires, industrial processes (steel). - **SO₂:** burning coal and oil containing sulphur, smelting of sulphide ores, thermal power plants, brick kilns, oil refineries; volcanoes naturally. #### Control - **CO:** use catalytic converters (2CO + O₂ → 2CO₂ over Pt/Pd), keep engines well tuned for complete combustion, use cleaner fuels (LPG, CNG, electricity), improve ventilation and stoves, public transport. - **SO₂:** use low-sulphur fuels, remove sulphur from fuels before burning, flue gas desulphurisation (lime scrubbing: SO₂ + CaCO₃ + ½O₂ → CaSO₄ + CO₂), tall chimneys with scrubbers, and renewable energy. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is the limit for acid rain set at pH 5.6 and not 7? A. Normal rain contains dissolved CO2 forming H2CO3, so it is already slightly acidic B. Normal rain contains dissolved SO2 at all times C. Pure water always has pH 5.6 D. Rain dissolves limestone which lowers pH to 5.6 Answer: A. CO2 + H2O gives carbonic acid, so normal rain is about pH 5.6; below this it is acid rain. (Key point: Acid rain pH) 2. Which sequence correctly shows how SO2 becomes acid rain? A. SO2 is reduced to H2S which dissolves in rain B. SO2 is oxidised to SO3, which reacts with water to form H2SO4 C. SO2 reacts with CO2 to form H2CO3 D. SO2 reacts with N2 to form HNO3 Answer: B. 2SO2 + O2 gives 2SO3 and SO3 + H2O gives H2SO4. (Key point: Formation of acids) 3. How does a catalytic converter reduce CO emission? A. It filters CO as solid particles B. It dissolves CO in water C. It oxidises CO to CO2 over Pt/Rh D. It reduces CO to carbon Answer: C. Over Pt/Rh catalysts, 2CO + O2 gives 2CO2. (Key point: Control of CO) 4. What is flue gas desulphurisation by lime scrubbing? A. Burning the flue gas again at higher temperature B. Passing SO2 through CaCO3 so it is converted into CaSO4 C. Adding CO to flue gas to reduce SO2 D. Dispersing SO2 only through taller chimneys without removal Answer: B. SO2 + CaCO3 + 1/2 O2 gives CaSO4 + CO2, removing SO2 from the gas. 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