Option E Environmental Chemistry

Describe the main sources of carbon monoxide (CO), oxides of nitrogen (NOx), oxides of sulfur (SOx), particulates and volatile organic compounds (VOCs) in the atmosphere

Methane produced animals as a digestive gas (e.g. cows)

Bacterial decomposition of organic matter in marshes and tropical regions (e.g. rice fields, rivers, lakes)

Natural Sources of Carbon Monoxide

Oxidation of methane

• Methane oxidises to produce the primary pollutant CO and H2O

• 2CH4 (g) + 3O2 (g) 2CO (g) + 4H2O (l)

Man-made sources of Carbon Monoxide

Factory combustion

Power plants

Combustion of Fossil fuels

Vehicular exhausts

Combustion of fossil fuels

• Under the presence of oxygen, combustion of fossil fuels will produce carbon dioxide and water.

CH4 + 2O2 CO2 + 2H2O

• Lack of oxygen will result in incomplete combustion to produce carbon monoxide instead of carbon dioxide.

C7H16 + 7½O2 7CO + 8H2O

Oxides of Nitrogen (NOx)

• Nitrogen gas exists as N2 molecules in the air

• N2 is inert in room temperature and does not produce Oxides of Nitrogen as atoms are share a triple bond

• Reaction occurs at high temperatures

N2 (g) + O2 (g) 2NO (g)

N2 (g) + 2O2 (g) 2NO2 (g)

Natural sources of Nitrogen Oxides

• Bacterial decomposition of nitrogen containing compounds

• High energy of lightning will produce N2O and NO

Man-made sources of Nitrogen Oxides

Exhaust gases from internal combustion engines

Wood and fuels Airplanes

Oxides of Sulfur (SOx)

• Sulphur Dioxide (SO2) is the most abundant in the atmosphere whilst sulphur trioxide (SO3) is only produced in very small amounts

• Atmospheric Sulphur dioxide tend to oxidise into sulphur trioxide over time

• Reaction is speeded up by the presence of heavy metal pollutants and finely divided particulates of ash

• 2SO2 (g) + O2 (g) 2SO3 (g)

Sources of SOx : Coal

• Combustion of coal can produce SOx

• Poor quality coal can contain up to 7% sulphur; although most contain 1 to 5%

• Sulfur present in coal are in the form of elemental sulfur, iron pyrite and organic sulfur compoundsS (s) + O2 (g) SO2 (g)4FeS2 (g) + 11O2 (g) 2Fe2O3 (s) + 8SO2 (g)

Other sources of SOx

• Smelting plants oxidise sulfide ores to produce metal oxides; thus producing SO2

Cu2S (s) + 2O2 (g) 2CuO (s) + SO2 (g)

• Production of Sulfuric acid

• Sulfur dioxide is produced due to lack of oxygen

Natural Sources of SOx

• No major natural primary sources of sulphur dioxide

• Small amounts produced from volcanic and bacterial activity

• Sulphur dioxide is produced in the atmosphere from oxidation of H2S

2H2S (g) + 3O2 (g) 2SO2 (g) + 2H2O (l)

Particulates

These substances are solid particles suspended and carried within the air. They are generally large enough to be seen.

Sources of Particulates

• Naturally produced by sandstorms, volcanic activity and forest fires started by lightning

• Man-made sources include the combustion of fossil fuels used by the industry, vehicular exhaust from transportation, industrial plants which produce smoke, soot, ash and dust.

Volatile Organic Compounds (VOCs)

These substances are primary pollutants directly introduced into the environment. Examples include hydrocarbon gases and volatile organic liquids that can easily convert into gases and escape into the air.

Sources of VOCs

• Methane (CH4) is produced by anaerobic bacterial decomposition of organic matter in bodies of water including swamps and oceans

• Methane is also produce as a digestive gas in many animals (e.g. cows)

• Vegetation produced VOCs called terpenes• Man-made sources include extracting, refining,

transporting petroleum as well as using petroleum products can result in air pollution