Lesson twenty - Diễn đàn
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Diễn đàn » Hóa học đại học và sau đại học » Tiếng Anh chuyên ngành hóa học (English for Special Purposes) » Lesson twenty (Acid Rain)

Lesson twenty
ProfVietanhNgày: Thứ bảy, 2010-05-08, 12:26 PM | Tin nhắn # 1
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Acid Rain

The problem of acid rain has emerged as one of the more important environmental issues of recent times. Both the causes and the effects of acid rain are well understood. The problem is what to do about it. As the water that has evaporated from oceans and lakes condenses into raindrops, it dissolves small quantities of gases from the atmosphere. Under normal conditions, rain is slightly acidic, with a pH close to 5.6, because of dissolved CO2. In recent decades, however, the acidity of rainwater in many industrialized areas of the world has increased by a factor of over 100, to a pH between 3 and 3.5.

The primary cause of acid rain is industrial and automotive pollution. Each year in industrialized countries, large power plants and smelters that burn sulfur-containing fossil fuels pour millions of tons of sulfur dioxide (SO2) gas into the atmosphere, where some is oxidized by air to produce sulfur trioxide (SO3). Sulfur oxides then dissolve in rain to form dilute sulfurous acid and sulfuric acid:
SO2 (g)+ H2O (l) -> H2SO3 (aq) Sulfurous acid
SO3 (g)+ H2O (l) -> H2SO4 (aq) Sulfuric acid

Nitrogen oxides produced by the high-temperature reaction of N2 with O2 in coal-burning plants and in automobile engines further contribute to the problem. Nitrogen dioxide (NO2) dissolves in water to form dilute nitric acid (HNO3) and nitric oxide (NO):

3NO2 (g) + H2O (l) -> 2 HNO3 (aq) + NO (g)

Oxides of both sulfur and nitrogen have always been present in the atmosphere, produced by such natural sources as volcanoes and lightning bolts, but their amounts have increased dramatically over the last century because of industrialization.

Many processes in nature require such a fine pH balance that they are dramatically upset by the shift that has occurred in the pH of rain. Thousands of lakes in the Adirondack region of upper New York State and in southeastern Canada have become so acidic that all fish life has disappeared. Massive tree die-offs have occurred throughout central and Eastern Europe as acid rain has lowered the pH of the soil and leached nutrients from leaves. Countless marble statues have been slowly dissolved away as their calcium carbonate has been attacked by acid rain.
CaCO3 (s) + 2H+ (aq) -> Ca2+ + H2O (l) + CO2 (g)

Fortunately, acidic emissions from automobiles and power plants have been greatly reduced in recent years. Nitrogen oxide emissions have been lowered by equipping automobiles with catalytic converters, which catalyze the decomposition of nitrogen oxides to N2 and O2 Sulfur dioxide emissions from power plants have been reduced by scrubbing combustion products before they are emitted from plant smoke stacks. In this process an aqueous suspension of lime (CaO) is added to the combustion chamber and the stack. The lime reacts with SO2 to give calcium sulfite (CaSO3):

CaO (s) + SO2 (g) -> CaSO3 (s)

Unfortunately, scrubbers are expensive, and the CaSO3, which has no commercial uses, must be disposed of in land fills. Much more work on methods to control acidic emissions remains to be done because the problem will grow more serious as sources of low-sulfur coal are exhausted and power plants are forced to rely on more abundant sources of high sulfur coal.


Phạm Bá Việt Anh

Department of Analytical Chemistry
Faculty of Chemistry
Hanoi National University of Education
Mobile - Tel: (84) 943 919 789
 
ProfVietanhNgày: Chủ nhật, 2010-05-16, 11:20 PM | Tin nhắn # 2
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Acidity is measured on the pH scale ranges from 0 to 14 with 0 being acid, 7 as neutral, and 14 as alkaline. The pH scale measures the concentration of hydrogen ions in solution, which indicates acidity. The pH scale is also logarithmic, so that a change in one unit represents a tenfold change in acidity, thus a solution of pH 4 is 10 times as acidic as one with pH 5 and 100 times as acidic as pH 6.

"Natural" or unpolluted rainfall is slightly acidic and has a pH of between 5.6 and 5.8. When fossil fuels, such as coal, are combusted, sulfur dioxide (SO2) and nitrogen oxide (NOx) are released into the troposphere. The primary source of acid SO2 is electrical power plants, whereas NOx comes from industrial boilers, mineral smelting plants, and automobiles. Once airborne, these acidic gases mix with precipitation such as rain, sleet, and snow and fall back to earth as “acid” rain.

Acid rain can affect both terrestrial and aquatic ecosystems, but the effects can vary depending on local conditions. The American Midwest, for example, has naturally alkaline soils that can buffer acid fallout. Likewise, some lakes lie on limestone, sandstone, or other alkaline formations that help neutralize acidity. On the other hand, some regions where lakes and soils lie on granite or glacial tills have low pH values to begin with and thus are greatly affected by acid rain.

Forests can be greatly affected by acid rain. Extensive stretches of forests in Switzerland and Germany have been damaged or destroyed by acid rain. Extensive vegetation dieback and soil erosion have occurred in the Canadian province of Ontario and acid rain has been implicated in tree damage reaching all the way south along the Appalachian Mountains to Georgia.

Aquatic effects from acid rain result from increasing acidity of rivers, lakes, and streams. Aquatic ecosystems have a preferred and tolerated range for acidity and if levels exceed the tolerated range plants, insects, and fish can disappear. In Scandinavia, as well as the eastern seaboard, many waterways have been affected. Some have been declared technically "dead" or placed on the critical list.

Materials and structures are also affected by acid rain. The most common problem is corrosion of buildings and statues made of marble and limestone. Steel structures are also susceptible to corrosion resulting from acid rain. In Poland, where there are few or no emissions controls on smokestacks, and where they burn large quantities of high sulfur coal, acid deposition is beginning to erode railroad tracks.

One of the difficulties involved in dealing with acidic deposition is that it often falls in different areas or countries from where it originated. Sulfur dioxide emissions can travel up to 2,000 kilometers in a few days. Most of the acidic deposition in the eastern United States, for example, is the result of coal burning power plants in the Midwest. Acidic deposition has become a significant transboundary issue. Both Sweden and Norway claim that most of the SO2 they receive comes from other countries, most notably Poland.

A range of specific control technologies exists for both sulfur dioxide and nitrogen oxides. Since sulfur dioxide is produced primarily from the combustion of coal, there are a number of technologies designed to burn coal cleanlier. These include more efficient boilers, cleaning technologies, and fluidized combustion beds. Other technologies designed to reduce SO2 and NOx emissions include limestone injection burners, reburners, flue gas desulphurizers, in-duct sprayers, and low NOx burners.

In addition, technologies exist for both wet and dry deposition, monitoring and measurement as well as materials protection. Monitoring and measurement technologies include rain gauges and pH analyzers. Materials protection technologies include waxes, special coatings, and paints.


Phạm Bá Việt Anh

Department of Analytical Chemistry
Faculty of Chemistry
Hanoi National University of Education
Mobile - Tel: (84) 943 919 789
 
ProfVietanhNgày: Thứ tư, 2010-06-02, 12:20 PM | Tin nhắn # 3
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What is Acid Rain?
Acid rain is rain or any other form of precipitation that is unusually acidic, i.e. elevated levels of hydrogen ions (low pH). It has harmful effects on plants, aquatic animals, and infrastructure. Acid rain is mostly caused by emissions of compounds of sulfur, nitrogen, and carbon which react with the water molecules in the atmosphere to produce acids. However, it can also be caused naturally by the splitting of nitrogen compounds by the energy produced by lightning strikes, or the release of sulfur dioxide into the atmosphere by phenomena of volcano eruptions.

How acidic is acid rain?
Very strong acids will burn if they touch your skin and can even destroy metals. Acid rain is much, much weaker than this, never acidic enough to burn your skin.

Rain is always slightly acidic because it mixes with naturally occurring oxides in the air. Unpolluted rain would have a pH value of between 5 and 6. When the air becomes more polluted with nitrogen oxides and sulphur dioxide the acidity can increase to a pH value of 4. Some rain has even been recorded as being pH 2.

Vinegar has a pH value of 2.2 and lemon juice has a pH value of 2.3. Even the strongest recorded acid rain is only about as acidic as lemon juice or vinegar and we know that these don't harm us - so why do we worry about acid rain?

Where is it coming from?
For many years, there was considerable debate and disagreement over what caused acid rain.The primary causes of acid rain are sulfur dioxide and nitrogen oxides. These chemicals are released by certain industrial processes, and as a result, the more industrialized nations of Europe as well as the US suffer severely from acid rain.

Most sulfur dioxide comes from power plants that use coal as their fuel. These plants emit 100 million tons of sulfur dioxide, 70% of that in the world.

Automobiles produce about half of the world's nitrogen oxide. As the number of automobiles in use increases, so does the amount of acid rain. Power plants that burn fossil fuels also contribute significantly to nitrogen oxide emission.

Though human causes are primarily responsible for acid rain, natural causes exist as well. Fires, volcanic eruptions, bacterial decomposition, and lightening also greatly increase the amount of nitrogen oxide on the planet.

Once the tiny pollutant molecules have entered the atmosphere, they can travel for thousands of miles. Eventually, the particles will combine with other compounds to produce new, often harmful, chemicals.

Acid rain comes down to the earth in the form of rain, snow, hail, fog, frost, or dew. Once it reaches the ground, the acidity in the substance can harm and even destroy both natural ecosystems and man-made products, such as car finishes.

The Effects of Acid Rain
Acid rain can be carried great distances in the atmosphere, not just between countries but also from continent to continent. The acid can also take the form of snow, mists and dry dusts. The rain sometimes falls many miles from the source of pollution but wherever it falls it can have a serious effect on soil, trees, buildings and water.

Forests all over the world are dying, fish are dying. In Scandinavia there are dead lakes, which are crystal clear and contain no living creatures or plant life. Many of Britain's freshwater fish are threatened, there have been reports of deformed fish being hatched. This leads to fish-eating birds and animals being affected also. Is acid rain responsible for all this? Scientists have been doing a lot of research into how acid rain affects the environment.

What are prevention methods?
Modern science has proven that acid rain is a dangerous and highly destructive problem. As a result, various ways to limit acid rain have been invented, and some are now being used.

Reduce emissions: The installation of sulfur cleaning scrubbers in factories, washing sulfur out of coal, and finding new methods of burning coal.

Conserving Resources: Individuals can help by conserving energy or driving their cars less. Governments can pass laws restricting pollution levels, or can use a variety of methods such as tradable emission permits to reduce acid rain. Whatever way it is done, acid rain will certainly have to be limited in the future.

Find alternative sources of energy: use hydroelectric, nuclear power, solar energy or windmills.

Restoring the Damage done by Acid Rain: Lakes and rivers can have powdered limestone added to them to neutralise the water - this is called "liming".


Phạm Bá Việt Anh

Department of Analytical Chemistry
Faculty of Chemistry
Hanoi National University of Education
Mobile - Tel: (84) 943 919 789
 
Diễn đàn » Hóa học đại học và sau đại học » Tiếng Anh chuyên ngành hóa học (English for Special Purposes) » Lesson twenty (Acid Rain)
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