Chemistry 2nd Paper · Classes 11-12 · Chapter 1 · 1.13

Acid Rain.

Is even the rain from a clean sky perfectly pure? No, its pH already falls to about 5.6. So when does rain turn into acid rain, and why is the marble of the Taj Mahal eaten away? Two families of gases are guilty, and one gas is innocent.

1. Even normal rain is slightly acidic

In normal rain water the CO₂ of the air dissolves and stays as carbonic acid (H₂CO₃). It is a weak acid, so it ionises very little: CO₂(g) + H₂O(l) ⇌ H₂CO₃(aq) ⇌ H⁺(aq) + HCO₃⁻(aq).

Because of carbonic acid, the lowest pH that rain water can reach is 5.61. The acid has Ka₁ = 4.5 × 10⁻⁷ and Ka₂ = 5.6 × 10⁻¹¹, both tiny, so going lower is not carbonic acid's doing.

Rain that falls from the air with a pH below 5.6 is called acid rain. The cause is man-made air pollution. In industrial areas the pH of acid rain is usually between 5.6 and 3.5.

2. Three acids, two routes

Three acids act behind acid rain: H₂SO₃, H₂SO₄ and HNO₃. They are formed from the primary pollutants SO₂ and the nitrogen oxides (NOx). On every route the oxide comes first and the water after.

The sulphurous acid route: burning high-sulphur coal gives SO₂. In rain water SO₂ forms H₂SO₃, and ozone and H₂O₂ oxidise it to H₂SO₄: H₂O + SO₂ → H₂SO₃; H₂SO₃ + H₂O₂ → H₂SO₄ + H₂O.

The direct route to sulphuric acid: SO₂ is oxidised in the air to SO₃, SO₂ + O₃ → SO₃ + O₂. Then SO₃ + H₂O → H₂SO₄.

The nitric acid route is longer, four steps. In power stations and motor engines N₂ + O₂ → NO. NO becomes NO₂ with the O₂ of the air: 2NO + O₂ → 2NO₂. Then 2NO₂ + O₃ → N₂O₅ + O₂, and last N₂O₅ + H₂O → 2HNO₃. Tap the steps in order.

Now match gases to acids. Careful: one gas only makes normal rain.

3. How low the pH falls

The strong acids H₂SO₄ and HNO₃ can bring the pH of rain water down to between 2.7 and 1.8. A pH of 2.7, like vinegar, was recorded in Sweden. A pH of 1.8, like lemon juice and stomach acid, was recorded in Virginia. Both were in 1984.

Drag the handle to lower the pH and read the name of the zone at each point. You can also pick an event in the top box.

Now place the events in their pH boxes yourself.

4. What acid rain damages

SO₂ from the “Mathura Oil Refinery” of India causes acid rain. It attacks the marble of the “Taj Mahal” and damages bridges and buildings made of metals.

Water animals and plants are attacked. In low-pH water the hatching of fish eggs is blocked. If the acidity is very high the whole ecosystem of a water body can be destroyed and the water body made barren.

Rain forests are specially damaged and seed germination is blocked. Acid rain washes away the phosphorus compounds of calcium, aluminium and zinc in the soil and makes the soil infertile.

5. Remedies

After acid rain, lime or powdered limestone is spread on ponds, lakes and farmland. The extra acid then turns into calcium salts. But the cost of this liming is very high.

So the real job is to keep the troposphere free of SO₂ and NOx. Industry uses FGD plants, and motor cars use catalytic converters, which reduce NOx to N₂.

In an FGD (flue gas desulphurisation) plant the flue gas carrying SO₂ from the chimney is passed through limestone or lime water: CaCO₃ + SO₂ → CaSO₃ + CO₂ (on heating); 2CaSO₃ + O₂ + 4H₂O → 2CaSO₄.2H₂O. The end product is gypsum, used to make dry wall.

In another method CH₄ reduces SO₂ to H₂S. A mixture of H₂S and SO₂ over an Fe₂O₃ catalyst becomes sulphur, and H₂SO₄ is made from that sulphur: 16H₂S + 8SO₂ → 3S₈ + 16H₂O (catalyst Fe₂O₃).

To reduce NOx, the exhaust of automobiles is reduced over V₂O₅ to CO₂, N₂, H₂O and O₂. The NO of a power plant chimney is reduced by ammonia to N₂: 4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O (on heating).

6. Who is guilty, and where the rain falls

CO₂ is not linked with acid rain, because CO₂ gives only the weak H₂CO₃. The gas responsible for acid rain is SO₂.

Sources of SO₂: thermal power stations (coal), automobiles, the extraction of Zn, Cu and Pb from sulphide ores, H₂SO₄ plants and brick kilns. Sources of NOx: automobiles, power stations and HNO₃ plants.

In areas with high-sulphur coal H₂SO₄ plays the larger part. In cities with many vehicles HNO₃ does. An example is Patenga in Chattogram: TSP fertiliser, cement, a petroleum refinery and chemical factories.

Acid rain precipitation depends on the wind, so it can fall far from its source. The acid-rain components in Norway come from the factories of the southern European countries.

When an organic pollutant is completely oxidised it gives CO₂ and H₂O. This CO₂ dissolves in water to give the weak acid H₂CO₃, so the solution is acidic. Burning a hydrocarbon also gives CO₂, but CO₂ does not support burning.

7. Check yourself

Three questions in the exam's style. The first.

The second.

The last.

The whole note on seven cards.

Source: Chemistry 2nd Paper, Classes 11-12 (NCTB), Chapter 1 “Environmental Chemistry”, section 1.13.