Monday, June 18, 2012

Sulphuric Acid & Acid Rain...



Acid rain is an environmental problem that's causing us many problems. Among one of the serious side effects of acid rain is the corrosion of buildings and statues.
Acid rain is rain which has turned acidic because of the presence of Sulphur dioxide (SO2) in the atmosphere. Sulphur dioxide are emitted from volcanoes, sea spra, rotting vegetation,burning of petrol,factories, and plankton. But the large amount of coal and oil burning are the main cause of the pollution. Acidity is measured on a pH scale. A substance with a pH value of less than 7 is acidic. 
Formula:
                        SO2(Sulphur dioxide) + O2(Oxygen) + H2O (Water) =  H2SO(Sulphuric acid)

Sulphur dioxide reacts with water vapour and sunlight to form sulphuric acid. Likewise NO2 form nitric acid in the air. These reactions takes hours, or even days, during which polluted air may move hundreds of kilometres. Thus acid rain can fall far from the source of pollution.
When mist or fog droplets condense they will remove pollutants from the air and can become more strongly acid than acid rain. Even snow can be acidic. 
The figure above shows the sources of acid rain and its effects to the environments and buildings.
About one-fourth of the acidity of rain is accounted for by nitric acid (HNO3). In addition to the natural processes that form small amounts of nitric acid in rainwater, high-temperature air combustion, such as occurs in car engines and power plants, produces large amounts of NO gas. This gas then forms nitric acid via Equations 4 and 5. Thus, a process that occurs naturally at levels tolerable by the environment can harm the environment when human activity causes the process (e.g., formation of nitric acid) to occur to a much greater extent.
What about the other 75% of acidity in rain? Most is accounted for by the presence of sulphuric acid (H2SO4) in rainwater. Although sulphuric acid may be produced naturally in small quantities from biological decay and volcanic activity, it is produced almost entirely by human activity, especially the combustion of sulphur-containing fossil fuels in power plants. When these fossil fuels are burned, the sulphur contained in them reacts with oxygen from the air to form sulphur dioxide (SO2). Combustion of fossil fuels accounts for approximately 80% of the total atmospheric SO2 in the United States. ! Sulphur dioxide, like the oxides of carbon and nitrogen, reacts with water to form sulphuric acid.

Effects of Acid Rain...

Environmental effects of acid rain.
Acid rain triggers a number of inorganic and biochemical reactions with deleterious environmental effects, making this a growing environmental problem worldwide.
  • Many lakes have become so acidic that fish cannot live in them anymore.
  • Degradation of many soil minerals produces metal ions that are then washed away in the runoff, causing several effects:
    • The release of toxic ions, such as Al3+, into the water supply.
    • The loss of important minerals, such as Ca2+, from the soil, killing trees and damaging crops.
  • Atmospheric pollutants are easily moved by wind currents, so acid-rain effects are felt far from where pollutants are generated.
Erosion of stone buildings and monuments.
Marble and limestone have long been preferred materials for constructing durable buildings and monuments.Marble and limestone both consist of calcium carbonate (CaCO3), and differ only in their crystalline structure. Limestone consists of smaller crystals and is more porous than marble; it is used more extensively in buildings. Marble, with its larger crystals and smaller pores, can attain a high polish and is thus preferred for monuments and statues. Although these are recognized as highly durable materials, buildings and outdoor monuments made of marble and limestone are now being gradually eroded away by acid rain.
A chemical reaction (Equation 9) between calcium carbonate and sulfuric acid (the primary acid component of acid rain) results in the dissolution of CaCO3 to give aqueous ions, which in turn are washed away in the water flow.
 
This process occurs at the surface of the buildings or monuments; thus acid rain can easily destroy the details on relief work (e.g., the faces on a statue), but generally does not affect the structural integrity of the building. The degree of damage is determined not only by the acidity of the rainwater, but also by the amount of water flow that a region of the surface receives. Regions exposed to direct downpour of acid rain are highly susceptible to erosion, but regions that are more sheltered from water flow (such as under eaves and overhangs of limestone buildings) are much better preserved.

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