Acid Rain PDF

# Acid Rain ## 1. Causes of Acid Rain ### The Term - Acid Rain - The term "Acid Rain" refers to the ways acids formed in the atmosphere condense and fall on Earth. - The two forms of deposition of acid rain are: - Wet deposits - rain, snow, fog, dew - Dry deposits - particles containing s...

# Acid Rain ## 1. Causes of Acid Rain ### The Term - Acid Rain - The term "Acid Rain" refers to the ways acids formed in the atmosphere condense and fall on Earth. - The two forms of deposition of acid rain are: - Wet deposits - rain, snow, fog, dew - Dry deposits - particles containing sulphates and nitrates ### The Pollutants - Responsible for Acid Rain - Oxides of sulfur [SO₂] - Oxides of nitrogen [NO₂] ### Formation of Acid Rain - **Natural Sources:** bacterial decomposition, forest fires, volcanic eruptions. - **Man Made Sources:** power, industrial & smelting plants, and automobile exhausts produce oxides of sulfur and nitrogen, which interact with water vapor in the presence of sunlight in the atmosphere to form sulfuric acid [H₂SO₄] and nitric acid [HNO₃] mist. - The mist remains as vapors at high temperatures and condenses at low temperatures. - The acids mix with rain (or snow, fog, etc.) and fall down on Earth, resulting in acid rain. ## Causes of Acid Rain ### Oxides of Sulfur [SO₂] Produced by - **Burning of Fossil Fuels:** - Fossil fuels are derived from the organic remains of plants and animals. - Principal fossil fuels: Coal, petroleum (oil), natural gas. - Emission of SO₂ from fossil fuels - Electrical power stations burn coal and produce SO₂ from the sulfur impurities in coal. - S + O₂ → SO₂ - Burning of fossil fuels accounts for 70% of man-made SO₂ gas. - **Other Sources** - Bacterial decomposition of organic matter releases H₂S, which is oxidized to SO₂: - 2H₂S + 3O₂ → 2H₂O + 2SO₂ - Smelting plants produce SO₂ when metallic sulphides are roasted in air: - 2ZnS + 3O₂ → 2ZnO + 2SO₂ - Sulphuric acid plants produce SO₂ during production of sulphuric acid. ### Oxides of Nitrogen [NO, NO₂] Produced During Use of - **High Temperatures in Furnaces and Internal Combustion Engines** - Internal Combustion Engine: - Is an engine in which combustion of a fuel (fossil fuel) occurs with an oxidizer (air) in a combustion chamber. - The air drawn from the atmosphere contains about 80% nitrogen. - The nitrogen combines with oxygen at high temperatures producing oxides of nitrogen. - N₂ + O₂ → 2NO (at high temperatures) - Thus exhaust gases of all internal combustion engines release oxides of nitrogen. ## 2. Composition and Formation of Acid Rain ### Natural Rain Water - Natural rain water produced in unpolluted air is only slightly acidic. - The CO₂ present in traces in unpolluted air dissolves in rain water forming weak carbonic acid [H₂CO₃]. - H₂O + CO₂ → H₂CO₃ (carbonic acid) ### pH Scale - A scale which measures acidity or alkalinity of a solution. - pH scale has values from 0 (or 1) (most acidic) to 14 (most basic). - pH of pure water (neutral) is 7. - pH of natural rain water is in the range of 5 to 6. - pH of acid rain is in the range of 5.6 to 3.5. ### Composition of Acid Rain - Acid rain refers to rain (or any precipitation, i.e., snow or fog) which has pH less than 5.6. - Since it is caused by atmospheric pollutants—sulfur dioxide and nitrogen oxides, it is a complex mixture of sulphuric acid [H₂SO₄], nitric acid [HNO₃], (and sulphurous [H₂SO₃] and nitrous [HNO₂] acids). ### Formation of Acid Rain #### Formation of Sulphuric Acid [& sulphurous acid] In Acid Rain - **Electric Power Stations** burn coal (fossil fuel) for power generation. 1. Sulfur (S) impurities in the coal form pollutant sulfur dioxide (SO₂). - S + O₂ → SO₂ 2. Sulfur dioxide (SO₂) reacts with water vapor to form sulphurous acid (H₂SO₃). - H₂O + SO₂ → H₂SO₃ (aq) 3. Sulfur dioxide (SO₂) can also be oxidized to sulfur trioxide (SO₃). - 2SO₂ + O₂ → 2SO₃ - The reaction is catalyzed by dust particles and water droplets. 4. Sulfur trioxide (SO₃) reacts with water vapor in air forming sulfuric acid (H₂SO₄). - H₂O + SO₃ → H₂SO₄ (aq) - Thus the acidity in acid rain is due to the reaction of oxides of sulfur with water vapor—forming dilute acids. #### Formation of Nitric Acid [& nitrous acid] In Acid Rain - **High Temperatures In Furnaces And Internal Combustion Engines** - Nitrogen (N₂) in air at temperatures above 1300^oC combines with oxygen (O₂) to from nitrogen monoxide (nitric oxide - NO). - The high temperatures can be achieved during - operating internal combustion engines or furnaces, or lightening discharge in the atmosphere. 1. Nitrogen (N₂) combines with oxygen (O₂) at high temperatures to give nitric oxide (NO). - N₂ + O₂ → 2NO 2. Nitric oxide (NO) is oxidized to nitrogen dioxide (NO₂). - 2NO + O₂ → 2NO₂ 3. Nitrogen dioxide (NO₂) reacts with water vapor to form nitrous acid (HNO₂) and nitric acid (HNO₃). - H₂O + 2NO₂ → HNO₂ + HNO₃ - The nitrogen dioxide may also dissolve in atmospheric moisture in the presence of oxygen of the air forming nitric acid in the free state. - 2H₂O + 4NO₂ + O₂ → 4HNO₃ - Thus the acidity in the rain is due to the reaction of oxides of nitrogen with water vapor—forming dilute acids. - **A catalytic converter in internal combustion engines helps reduce pollution.** - The reduction catalyst [Pt/Pd] in a converter separates the nitrogen atom from NO and NO₂ molecules, freeing oxygen in the form of O₂ molecule. - 2NO → N₂ + O₂ - 2NO₂ → N₂ + 2O₂ - Thus the oxides of nitrogen, which are pollutants, are converted to free nitrogen. ## 3. Impact of Acid Rain - Acid rains have an adverse effect on the environment, including: - Soil chemistry - Water bodies ### Soil Chemistry - **Causes nutrient leaching:** - The hydrogen ions (H⁺) which are added to the soil—when acid rain falls on the earth—interact chemically with existing soil minerals. - H₂SO₄ → 2H⁺ + SO₄^2- - Minerals like potassium and calcium interact with the H⁺ ions and are displaced from the soil, thus the soil is derived of nutrients. - **Changes the acidity of soil:** - Acid rain is a complex mixture of sulphuric and nitric acid along with sulphurous and nitrous acid. Thus when it falls on the earth, the acidity of the soil increases. - **Affects soil fertility:** - The activity of the nitrogen-fixing bacteria—present in the nodules of leguminous plants, is inhibited by the acid rain, thus the fertility of the soil shows a marked reduction. ### Water Bodies - **Affects marine organisms:** - Toxic metals like mercury, lead, and zinc—present in the soil, get leached by acid rain, thus toxicity enters rivers and streams and destroys aquatic plants and animals. - **Changes the pH of the environment:** - Most aquatic animals cannot survive when pH is less than 4, thus certain species are eliminated at pH less than 6. (Salmons die at pH less than 5.5) ### General Impact of Acid Rain On - - **Plants:** Acid rain affects plants in numerous ways: - Affects transpiration, decolorizes leaf pigments, retards growth of crops and reduces rate of photosynthesis. - **Humans:** Acid rain gets absorbed by plants, animals, and directly or indirectly, toxicity enters the food chain affecting humans. - **Material Damage:** Corrodes metallic surfaces, disintegrates paper and leather, weakens building materials such as statues, sculptures, marble, limestone (CaCO₃) etc.. - CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂ ### Controlling Acid Rain - **Using alternative energy sources:** Alternatives to burning of fossil fuels, such as compressed natural gas (C.N.G.) or use of cleaner energy sources such as hydro power or wind energy—causes almost no pollution. - **Usage of technical devices:** Such as catalytic convertors, which can reduce nitrogen oxide emissions from automobiles, (usage of electrostatic precipitators in industries)