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The Chemistry of Acid Rain

Key Concepts

Rain from an unpolluted atmosphere has a pH close to 6.0 (slightly acidic).

This acidity is due to the reaction of water vapour and non-metal oxides in the atmosphere, such as carbon dioxide and nitrogen oxide, forming dilute acids.

  • carbon dioxide reacts with water to form carbonic acid:

    CO2(g) + H2O(l) H2CO3(aq)

    Since carbonic acid is a weak acid it partially dissociates:

    CO2(g) + H2O(l) H+(aq) + HCO3-(aq)

  • nitrogen dioxide reacts with water to form a mixture of nitrous acid and nitric acid:

    2NO2(g) + H2O(l) HNO2(aq) + HNO3(aq)

Acid rain has a pH below 5.6 due mainly to the reaction of water vapour with sulfur dioxide and the oxides of nitrogen.

  • Sulfur dioxide reacts with water to form sulfurous acid (H2SO3):
    SO2(g) + H2O(l) H2SO3(aq)

    Sulfur dioxide (SO2) can be oxidised gradually to sulfur trioxide (SO3):
    2SO2(g) + O2(g) → 2SO3(g)

    Sulfur trioxide (SO3) reacts with water to form sulfuric acid (H2SO4):
    SO3(g) + H2O(l) → H2SO4(aq)

  • Oxides of nitrogen, particularly nitrogen dioxide (NO2) react with water to form nitrous acid (HNO2) and nitric acid (HNO3):
    2NO2(g) + H2O(l) → HNO2(aq) + HNO3(aq)

Sources of the acids in clean and polluted air

Acid Clean air Polluted air
Carbonic acid

(H2CO3)

natural carbon dioxide (CO2)
produced during plant and animal respiration
produced during decomposition of organic matter
Carbon dioxide (CO2) released from the combustion of fuels
The complete combustion of coal:
C(s) + O2(g) → CO2(g)
The complete combustion of petrol (for example octane, C8H18(l)):
2C8H18(l)+25O2(g) → 16CO2(g)+18H2O(g)
Complete combustion of ethanol (ethyl alcohol):
C2H5OH(l)+3O2(g) → 2CO2(g)+3H2O(g)

Formic acid (Methanoic acid)

(HCOOH)

Oxidation of natural methane (CH4) formed during the anaerobic decomposition of organic matter Increased oxidation

Sulfuric acid (Sulphuric acid)

(H2SO4)

Natural decay of organic matter releases hydrogen sulfide gas (H2S) which can be oxidised to sulfur dioxide (SO2):
2H2S(g)+3O2(g) → 2SO2(g)+2H2O(l)
Sulfur dioxide can be oxidised to sulfur trioxide (SO3):
2SO2(g)+O2(g) → 2SO3(g)
Sulfur trioxide then reacts with water to form sulfuric acid:
SO3(g)+H2O(l) → H2SO4(aq)
Volcanoes emit sulfur dioxide which can be oxidised to sulfur trioxide which then reacts with water forming sulfuric acid.
Ocean algae release sulfur gases such as dimethyl sulfide which is oxidised to form sulfuric acid.
Combustion of coal and other fossil fuels account for about 80% of the man-made sulfur dioxide in the atmosphere (sulfur is present in the proteins of the original living matter that has been fossilised to produce the fossil fuel such as coal, oil or petroleum). Most of this is from coal-fired power stations, motor vehicle emmissions account for only about 1% of the sulfur dioxide present.
Sulfur dioxide is also produced when sulfur ores are roasted:
2ZnS(s)+3O2(g) → 2ZnO(s)+2SO2(g)
Sulfur dioxide is also produced in the manufacture of sulfuric acid by the contact process, in petroleum refining and in the manufacture of coke from coal

Nitric acid

(HNO3)

Lightning flashes lead to a reaction between atmospheric nitrogen and oxygen in the presence of water vapour which forms nitric acid Combustion of fossil fuels
Nitric oxide (nitrogen monoxide, NO) is produced in internal combustion engines as a result of the reaction between oxygen and nitrogen at high temperatures:
N2(g)+O2(g) → 2NO(g)
Nitrogen monoxide is readily oxidised to nitrogen dioxide (NO2):
2NO(g)+O2(g) → 2NO2(g)
Nitrogen dioxide reacts with water to form nitrous acid (HNO2) and nitric acid (HNO3):
2NO2(g)+H2O(l) → HNO2(aq)+HNO3(aq)

Methanesulfonic acid

(Methanesulphonic acid)

Ocean algae emit dimethyl sulfide which oxidises in air to produce methanesulfonic acid. methanesulfonic acid is only produced naturally

Effects of Acid Rain

Acid rain is a form of environmental pollution that damages buildings and marble statues by reacting with the calcium carbonate to form soluble calcium hydrogen carbonate (calcium bicarbonate, Ca(HCO3)2)

CaCO3 + acid rain → Ca(HCO3)2(aq)

Acid rain can leach aluminium from the soil into ground water, lakes and rivers, poisoning fish and plant roots.

The sulfates and hydrogen sulfates in acid rain can can leach essential plant nutrients such as calcium and magnesium, from the soil.

Acid rain disrupts the process of photosynthesis resulting in damage to plant life.

At low concentrations it retards the production of chlorophyll and at high concentrations it forms sulfuric acid which kills the plant.

Some organisms are sensitive to changes of acidity in water which can affect their ability to reproduce and in some cases may kill them.


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