Madeleine Prell
HSC CHEMISTRY 2015​.
Acid Rain
Acid Rain: Rain that has a higher hydrogen concentration than normal – higher that about 10-5 mol/L
Water pH
Pure water: pH 7
Natural water: pH 6-6.5 (contains dissolved gasses such as CO2 ^ pH)
Acid rain: pH 4-5
SO2 in rain
Sulfur dioxide reacts with water to form sulfurous acid:
SO2(g) + H2O(l) Ã H2SO3(aq)
Impurities in air catalyse sulfurous acid into sulfuric acid
2H2SO3(aq) + O2(g) Ã 2H2SO4(aq) (with catalyst)
H2SO4 is soluble in water thus splits to ions in rain.
NO2 in rain
Nitrogen dioxide reacts with water to form both nitrous and nitric acids:
2NO2(g) + H2O(l) Ã HNO2(aq) HNO3(aq)
The nitrous acid (aq) is catalytically (from air) oxidised to nitric acid:
2HNO2(aq) + O2(g) Ã 2HNO3(aq) (with catalyst)
H2NO3 is soluble in water thus splits to ions in rain.
NB: Nitric Oxide (NO) reacts with O2 to form NO2 thus also contributes to acid rain.
Effects of Acid Rain
Limestone: EROSION
Contains carbonates (particularly calcium carbonate) which react easily with acid forming Calcium sulfate which when wet crystallises into gypsum a porous and crumbly mineral.
CaCO3(s) + H2SO4(aq) Ã CaSO4(s) + H2O(l) + CO2(g)
Steel Structures: CORROSION
The iron is oxidised by the H+ ions in the acid chemically weathering the metal.
Fe(s) + H2SO4(aq) Ã FeSO4(aq) + H2(g)
Plants and Forests: DEFOLIATION & STUNTED GROWTH
Strips pine needles of their wax coating, thus they go brown and the tree loses its foliage.
Soils: LESS NUTRIENTS & HARMFUL MINERALS
The sulfate ions in the water react with essential minerals such as calcium, magnesium and potassium leaching (removing) them out of the soil. Compounds such as aluminium sulfate which is only soluble in acidified water releases harmful aluminium ions which interfere with nutrient uptake of plants. Acidic rain also affects nitrogen fixing bacteria.
Lakes and Aquatic Animals: INCREASES ACIDITY & DECREASES POPULATION
Acid rain draining into lakes increases the acidity of the water. H^+ ions in the water affects the CO2/carbonate balance. This affects plant growth and many animals can’t reproduce in acidic environments thus the population significantly decreases and species die out. High acidity also leads to a higher amount of heavy metal ions (Aluminium) released from soils, thus population decreases can also be attributed to heavy metal poisoning.
These issues commonly occur around areas of heavy industry due to the higher emissions of SO2 and NOx resulting in higher chances of acid rain.
