HSC CHEMISTRY 2015​.

Evaluating Concerns          

 

Concerns about Acidic Oxides

 

Sulfur and Nitrogen oxides originate from a range of sources both natural and industrial.

Sulfur Dioxide, a toxic colourless gas naturally originates from the combustion of organic matter; in bush fires for example, or from the combustion of sulfur itself in volcanoes (S(s) + O2(g) --> SO2(g)). Industrially it is emitted from; burning and processing fossil fuels, metal smelters when separating metals from sulfides (2ZnS(s)+3O2(g)-->2ZnO(s)+2SO2(g)), garbage incineration and sewage treatment.

 

Nitrogen Oxides, including Nitrogen Dioxide (NO2), Nitric Oxide (NO) and Nitrous Oxide (N2O) is produced by lightning (O2(g) + N2(g) --> 2NO(g)), sunlight, NO & O2 (O2(g) + NO(g) --> 2NO2(g)) and nitrogen fixing bacteria in soil. However industrially the originate from Combustion of fuels (O2(g) + N2(g) --> 2NO(g)), (O2(g) + NO(g) --> 2NO2(g)), and produced as a sedative/laughing gas (N2O(g))

 

When in the atmosphere SO2 and NO2 react with water to form soluble sulfate and nitrate ions respectively:

Sulfur dioxide reacts with water to form sulfurous acid:SO2(g) + H2O(l) --> H2SO3(aq). Impurities in air then catalyse sulfurous acid into sulfuric acid: 2H2SO3(aq) + O2(g) --> 2H2SO4(aq) (with catalyst). H2SO4 is soluble in water thus splits into sulfate ions.

Nitrogen dioxide reacts with water to form both nitrous and nitric acids: 2NO2(g) + H2O(l) --> HNO2(aq) + HNO3(aq) .Then the nitrous acid (aq) is catalytically (from air) oxidised to nitric acid: 2HNO2(aq) + O2(g) --> 2HNO3(aq) (with catalyst) H2NO3 is soluble in water thus splits into nitrate ions. Additionally Nitric Oxide (NO) reacts with O2 to form NO2 thus also contributes to the formation of nitrate ions.

When H2SO4 and HNO3 dissolve in water they split into their ions. The higher concentration of hydrogen ions in the rain 

increases the acidity and above a concentration of 10-5 mol/L it is considered acid rain.

 

Acid Rain has many effects on the environment and man-made structures. Its acidic qualities lead to erosion of buildings where carbonated react readily with acids forming gypsum, a crumbly mineral. The hydrogen ions oxidise iron in steel structures corroding them. The acid rain also leads to defoliation of forests and decreases the nutrients in soils by reacting sulfate with essential minerals such as calcium and magnesium as well as releasing harmful aluminium ions into the soil. Lakes and aquatic animals are also effected by increasing the acidity of the water affecting plant growth and animal reproduction decreasing populations, and also allows soil to release heavy metals poisoning many of the organisms.

 

Measuring SO2 and NOx concentrations is more difficult than that of CO2 due to the sulfate and nitrate ions formed by SO2 & NOx are mostly soluble in water, unlike carbonate ions. These ions are not precipitated out and remain in the biosphere and hydrosphere. Thus measurements are less valid as large amounts of the gasses are dissolved. Additionally SO2 & NOx exist in much smaller concentrations (≈0.001ppm) as opposed to CO2 (≈380ppm) and as instruments able to measure such small concentrations were not available before the 1970’s, our reliable data is limited to only 40 years.

Despite this, evidence of acid rain damage surrounding industrial areas, ice core samples measuring increasing SO2 and NOx atmospheric concentrations at times of high industrialisation, and decreases at times of emissions control regulations, as well as high concentrations measured in industrial areas than that of un-industrialised area through the use of the Ozone Measuring Instrument orbiting the earth , signify that anthropogenic sources of Sulfur and Nitrogen oxides are the main cause of related problems.

 

The effects of sulfur and nitrogen oxides are largely damaging many aspects of our planet; plants, soil, water ways and aquatic animals, and if their presence in our atmosphere continues to increase the damage will increase having some detrimental effects on the planet such as massive deforestation, organism extinction and declining soil fertility. This will in turn will have damaging effects on other aspect of life on earth. There are methods of decreasing SO2 and NOx emissions that can dramatically increase mankind’s sustainability and quality of life. Such as emission collection of SO2, switching to renewable/low emission sources of energy, for example wind power, solar power, ethanol fuel and hydroelectricity, reduce acidic emissions through “scrubbing” (passing through a base slurry), catalytic converters in car exhausts.

 

The effects of acidic oxides on our planet and our ability to influence their severity is great cause for concern and seeing that many of the issues originate from anthropogenic sources, humans have a responsibility to do what we can to decrease our contribution of acidic oxide emissions.