Position of non-metals that form acidic oxides & their relationship in the Periodic Table
The aforementioned non-metals that react with oxygen to produce acidic oxides (acid anhydrides) lie to the right hand side of the Periodic Table, pertaining to the non-metals region. In general, the oxides of the elements to the left side of the periodic table (metals) form basic oxides while those on the right (non-metals) form acidic oxides, such as the halogens in group 7 (diagram below). However, the inert gases in group 0 of the Periodic Table do not form oxides. Most of the transition metals that border the metals and nonmetals in the Periodic table form amphoteric oxides which have both acidic and basic properties.
The electropositive behaviour of the oxide's atom determines if the oxide will be acidic or basic. Electropositive behaviour increases from right to left across the periodic table and increases down the column. As the electropositivity of the atom increases, so too the basicity of the oxide. The more electronegative the atom, the more acidic the oxide.
Sulfur dioxide & oxides of Nitrogen
The oxides of nitrogen form naturally when lightning strikes cause nitrogen and oxygen in the air to combine. Thus the nitrogen oxides become oxidised to nitric acid, nitrates and nitrites.
N2 + O2 2NO
Nitrogen monoxide or nitrogen oxide can also be formed (industrially) in internal combustion engines or high temperature combustion reactions in furnaces:
N2 + O2 2NO
This nitrogen monoxide gas can further react with oxygen in the air to form brown acidic nitrogen dioxide.
2NO + O2 2NO2
Nitrogen oxides are soluble in water and have known contributions to pollutants (PAN) in photochemical smog as well as its detrimental effects to animal respiratory systems. There are a number of these such oxides including: nitrogen dioxide (NO2), nitrous oxide (N2O) and nitrogen monoxide (NO).
Sulfur dioxide and the oxides of nitrogen profoundly contribute to the acidity of the atmosphere and ultimately, the production of acid rain.
Sulfur dioxide (SO2) is an acidic oxide that is toxic and pungently scented colourless gas. It has known detrimental effects to living things as it irritates the eyes and the respiratory system.
Natural sources of sulfur oxides
The natural decay of organic material releases hydrogen sulfide gas (H2S) which can be oxidised to form sulfur dioxide (SO2):
Sulfur dioxide can also be oxidised to produce sulfur trioxide (SO3):
Ocean algae also release sulfur gases such as dimethyl sulfide. When this is oxidised, it forms sulfuric acid.
When the sulfur dioxide originating from volcanic eruptions is oxidised, sulfur trioxide is produced, which then reacts with water to form sulfuric acid.
Industrial sources of sulfur oxides
The combustion of coal and other fossil fuels (mostly from coal-fired power stations) contribute to 80% of the sulfur dioxide levels in the atmosphere while motor vehicle emissions contribute a small part. As a result of heavy industry, sulfur dioxide is produced in the refining of petroleum, the manufacture of sulfuric acid, the manufacture of coke from coal and when roasting sulfur ores as given by the below equation:
Through the burning of petroleum, natural gas and coal:
Acid Rain
Acid rain is rain that has a higher than normal hydrogen ion content because of the amount of dissolved carbon dioxide it contains (which forms carbonic acid). The reaction of SO2, nitrogen oxides, NOx gases and their particulate matter derivatives with water in the atmosphere forms a mixture of sulfuric acid, nitric acid and nitrous acid. The dilute solutions of these acids result in rain water with a higher than normal pH, also known as acid rain. Acid rain is used to describe several ways that acids fall out of the atmosphere. This type of acid deposition has two parts: wet and dry.
Wet deposition refers to acidic rain, fog, and snow. As this acidic water flows over and through the ground, it affects a variety of plants and animals. The strength of the effects depend on many factors, including how acidic the water is, the chemistry and of the soils involved, and the types of fish, trees, and other living things that rely on the water.
Dry deposition refers to acidic gases and particles. About half of the acidity in the atmosphere falls back to earth through dry deposition. The wind blows these acidic particles and gases onto buildings, cars, homes, and trees. Dry deposited gases and particles can also be washed from trees and other surfaces by rainstorms. When that happens, the runoff water adds those acids to the acid rain, making the combination more acidic than the falling rain alone.
Diagram: production and release of the gases that contribute to acid rain.
Acid rain causes acidification of lakes and streams and contributes to damage of trees at high elevations and many sensitive forest soils. In addition, acid rain accelerates the erosion of limestone and marble as well as the decay of building materials and paints, including irreplaceable buildings, statues, and sculptures that are part of our nation's cultural heritage. The sulfate particles in the atmosphere are known to cause reduced visibility.
The ecological effects of acid rain are most clearly seen in aquatic environments such as streams and lakes. Acid rain flows to streams and lakes (directly or from runoff) and into aquatic habitats where it increases the acidity of natural waters, making it detrimental to life populations. Current research has shown that acid rain causes slower growth, injury, or death of forests. Acid rain has been implicated in forest and soil degradation in many areas, particularly high elevation forests.
The pollutants that cause acid rain (sulfur dioxide (SO2) and nitrogen oxides (NOx)) also impact on public health. These gases interact in the atmosphere to form fine sulfate and nitrate particles that can be transported long distances by winds and inhaled by animals. Fine particles can also penetrate indoors. Many scientific studies have identified a relationship between elevated levels of fine particles and increased illness and premature death from heart and lung disorders, such as asthma and bronchitis.
Increases in atmospheric concentration of oxides of sulfur and nitrogen
Although the availability of chemical instruments capable of measuring low levels of atmospheric concentrations have only become accessible since the 1970’s, there is sufficient evidence from reliable statistical sources to suggest an overall increase in the atmospheric concentration of sulfur and nitrogen oxides. The graph below verifies the steady increase of nitrous oxide over time - since the industrial revolution (on a global scale).
Table 1 below also provides evidence to back up the claim that greenhouse gases are increasing, according to the US energy information administration. In particular, it can be seen that nitrous oxide and sulfur hexafluoride levels have increased since the industrial revolution of the 1750’s. The table also shows these steadily increasing levels with relevance to the average annual change, although there are now many government initiatives established to reduce the amounts of greenhouse gases produced. Analysis of ice core samples from the Antarctic region by the CSIRO has found increases in carbon dioxide from 280ppm before industrialisation to 360ppm recently, a 10% increase in the concentration of nitrous oxide and a 700ppb to over 1700ppb increase in methane content in the past 200 years.
The graph illustrated right also indicate a steady increase in the atmospheric concentrations of sulfur dioxide and NOx over time. This graph provides a reliable estimate of the global sulfur dioxide and NOx levels during the listed time period, since the production of these gases were closely mirrored by other developed countries (eg. Australia). All statistical evidence point to increased atmospheric concentrations of sulfur and nitrogen oxides since the industrial revolution, despite current strategies to minimise their production.
Industrial Origins of sulfur dioxide and oxides of Nitrogen
The industrial origins of the aforementioned gases contribute to global dilemmas such as global warming, the greenhouse effect and acid rain.
Nitrogen (IV) oxide or nitrogen dioxide (NO2) is produced in the:
- combustion of fuel in motor vehicles and power stations
- manufacture of sulfuric acid, fertilizer and explosives
Nitrogen (I) oxide or dinitrogen monoxide or nitrous oxide (N2O) is manufactured as a fuel for racing cars and for use as a sedative such as laughing gas.
Nitrogen (II) oxide or nitrogen monoxide or nitric acid (NO) is commonly produced in the:
- Burning of biomass
- Combustion of fuel in motor vehicles and power stations
Sulfur dioxide (SO2) is produced in:
- The combustion of fossil fuels
- Metal smelters which extract lead, zinc and copper from sulfides
- The incineration of garbage
- Petroleum refineries
- Industries using sulfur dioxide for the production of materials.
The concerns leading to the use and production of these acidic oxides revolves around the global need to reduce sources of air pollution. Although the atmospheric concentrations of sulfur dioxide and the oxides of nitrogen are likely to continue to rise into the future, many governments and institutions worldwide have implemented strategies to resolve this dilemma.
The release of Nitrogen oxides such as NO and NO2 prompts public concern due to its ecological detriments to Nature and public health. Nitrogen oxides may encourage heart and lung problems as well as the spread of cancer. Nitrogen dioxide may decrease our resistance to diseases and suppress plant growth.
The concern revolving around sulfur dioxide emissions encompasses public health issues, making it a problem for people with asthma and respiratory diseases. Sulfur dioxide is also capable of dissolving in water to form sulfuric acid. This has detrimental effects on human drinking water as well as aquatic communities where it interferes with osmoregulation in freshwater fish and endangers aquatic flora and fauna species.
Bibliography
- The World Almanac and book of facts 2002, 2002, Ken Park, USA
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Schell Marilyn, Haantjens Rein, Getsmart Chemistry, 2004, Science Press, Marrickville
- The industrial revolution 1800-1850/ text by Pier Paolo Poggio, Carlo Simoni ; illustrated by Giorgio Bacchin; Philadelphia : Chelsea House Publishers, 2002.
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Date accessed: 9/11/04
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Date accessed: 9/11/04
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Date accessed: 9/11/04
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Date accessed: 9/11/04
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Date accessed: 9/11/04
