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The majority of people consider rain to be an undamaging weather
occurrence. However the increase in acidity of rain is both unsafe and
damaging.
In order to fully understand the term acidity, it is essential to
know something about the pH scale. This scale has a range of 0 to 14,
with 7 being neutral. Anything below 7 (0-6) is known to be acidic and
anything above 7 (8-14) is alkaline. A change in only one unit is equal
to a tenfold increase in the strength of the acid or base. Therefore a
unit change from pH 6 to pH4 is equal to a 10 x 10 increase in it
acidity.
Taking the above into consideration, it is easy to see how the
normal phenomenon “rain” is becoming more and more acidic as its pH has
dropped from around 6and 7 to about 4.3and 5.3.This occurrence is known
as Acid
Rain and was first noted in 1852 by the English chemist called Robert Angus Smith.
Acid rain in other words is the term used to describe rainfall that
has a pH level below 5.6. It is a form of air pollution that is
currently a theme of huge debate due to its wide spread damages. It is
responsible for the destruction of thousands of lakes and streams in
the United States, Canada and parts of Europe.
How Acid Rain is Formed
The two most important primary sources of acid rain are sulphur
dioxide (SO2) and oxides of nitrogen (NOx). Sulphur is a colourless,
pungent gas produce during the combustion of fossil fuels containing
sculpture. A variety of industrial processes such as the production of
steel and iron and crude oil processing produce this gas. This gas is
also emitted into the atmosphere by natural means. Ten percent of the
sculpture in the atmosphere comes from volcanoes, sea spray, plankton
and decomposing vegetation.
The other gas primarily accountable for the formation of acid rain
is nitrogen oxide. The term ‘oxides of nitrogen’ describes any compound
of nitrogen with any amount of oxygen atoms. The only oxides of
nitrogen are nitrogen monoxide and nitrogen dioxide. These gases are
produced by firing processes at very high temperatures (vehicle) and
chemical industries. There are natural processes such as forest fires,
volcanoes and bacterial action in soil that also emit nitrogen oxides.
Transportation and industrial combustion also contribute to the
emissions of nitrogen oxides.
The acidity of acid rain does not only depend on the release levels
of these gases but also on the chemical combination sulphur dioxide and
nitrogen oxides interrelate in the atmosphere. Both gases undergo a few
chemical reactions before they become the acids present in acid rain.
The two main phases are the gas phase and the aqueous phase.
As far as sulpur dioxide is concerned there is a range of possible
reactions that may lead to the oxidation of the gas in the atmosphere
each having a different degree of success. One option is the
photooxidation of the gas by ultraviolet light. This reaction has been
found to be an unimportant supplier to the creation of sulphuric acid.
Another route that is more common is the one where sulphur dioxide is oxidized to form a sulphite ion.
SO2 (g) + o2 (g) à SO32-
After this the sulphite ion forms sulphuric acid when it comes into contact with the moisture in the atmosphere.
SO32- +H2O (l)à H2SO4 (aq)
This reaction takes place quickly therefore the construction sulphur
dioxide in the atmosphere is thought to lead to this kind of oxidation
and form sulphuric acid. The photooxidation method is much slower due
to the absence of a catalyst.
A third reaction for sulphur dioxide to become sulphuric acid is by
the oxidation by ozone. All the reactions mentioned to this point are
gas state reactions. In the aqueous phase sulphur dioxide exists as
three kinds:
[S(IV)]à[SO2(aq)] + [HSO32]+[SO32]
There is a dissociation that consists of two parts:
SO2 (aq)à H++HSO3-
HSO3- (aq)àH+ + SO32-
The oxidation of aqueous sulphur dioxide depends on catalysts like iron and manganese.
The oxidation of sulphur dioxide is most widespread in clouds and
particularly in greatly polluted air where there are abundant amounts
of ammonia and ozone, which act as catalysts in the formation of
sulphuric acid from sulphur dioxide. However, not all the sulphur
dioxide becomes sulphuric acid, as a lot of it floats up into the
atmosphere and is transported to other areas and returns to earth
unchanged.
Nitrogen oxides like sulphur dioxide rise into the atmosphere and
are oxidized in clouds to form nitric acid. Very polluted clouds where
traces of manganese, hydrogen peroxide and iron are present provide
catalysts for theses reactions. Nitrogen oxides are mainly emitted into
the atmosphere by vehicle exhausts. They react with water to form
nitric acid in the atmosphere.
NO2 (g)+H2O (l)àHNO3 (aq)+HNO2(aq) [gas phase]
In the aqueous phase there are three equilibria that have to be considered for the oxidation of nitrogen oxide.
* 2NO2(g)+H2O(l) 2H++NO3- +NO2-
* NO (g)+NO2(g)+H2O (l) 2H+ +2NO2-
* 3NO2(g)+H2O(l) 2H+ +2NO3-+2NO(g)
The limited pressures of nitrogen oxides present in the low
atmosphere, and the low solubility of nitrogen oxides limit the above
reactions. There is an increase in the rate of the reactions only with
the use of metal catalysts like in the case of the aqueous oxidation of
sulphur dioxide.
The effects of Acid Rain
Scientists have noticed that over the years some forests seem to be
growing more and more slowly for no obvious reason. After the
collection of information on the chemistry and the biology of the
forests it has been proved that this is due to acid rain.
When acid rain falls, the acidic rainwater dissolves the nutrients
and helpful materials from the soil. These minerals are then washed
away before trees other plants can use them to grow. The acid rain also
helps release toxic substances such as aluminum into the soil. This is
o due to the fact that metals like this are bound to the soil under
normal conditions, but the added dissolving act of hydrogen ions causes
rocks and small bound soil particles to break down. When acid
precipitation is regular leaves are likely to lose their waxy
protective coat. This loss makes the plant open to any disease. When
the leaves are damaged the plant cannot produce adequate food energy
for it to survive. When the plant is weak it is defenseless to disease,
insects that may kill it.
Acid rain also affects aquatic organisms. It enters lakes in several
ways but the most common route is when it is drained through sewage
systems that lead to lakes. The euphoric acid present in acid rain
interferes with the fish’s’ ability to take in oxygen, salts and
nutrients. Acid molecules cause the formation of mucus to form in their
gills not allowing them to absorb oxygen well. Additionally the low pH
level does not allow the calcium levels of the fish to be maintained
causing the eggs during reproduction to be weak and brittle. Sometimes
when acid rain runs off the land it carries some fertilizers along into
the lakes .The fertilizers induce the growth of sea plants causing
eutrophication.
Apart from natural ecosystems, acid rain also affects man-made
materials and buildings. Limestone, marble and sandstone are all very
easily dissolved by acid rain. Textiles, paints, metals and ceramics
can readily be decayed. Man-made materials are slowly depreciated with
normal rainfall but acid rain speeds up this process.
However, most importantly acid rain can affect the health of human
beings. It harms us via the atmosphere or through the soil from where
we get our food. Acid rain causes the release of toxic metals that may
be absorbed by drinking water, crops or animals that may later consumed
by humans. These could cause nerve damage to children or acute brain
damage or even death.
Acid precipitation is very harmful to the respiratory system.
Sulphur dioxide and nitrogen oxide secretion may lead to dry coughs,
asthma, headaches, eye, and nose and throat irritation. It has the
ability of making breathing very difficult and may give rise to
diseases, which could possibly lead to death.
How acid rain can be prevented
Sulphur dioxide in the atmosphere can be reduced either by fitting
desulphurization plants to power stations or by changing the fuel or
the way it is burned. The main suppliers of nitrogen oxides are motor
vehicles and industries. Flue gases can be dealt with so that most of
the nitrogen oxides are removed. Catalytic converters can be installed
in the exhaust systems of vehicles. These convert nitrogen oxides,
carbon dioxides and unburned hydrocarbons into a cleaner state.
Conclusion
Overall, acid rain is a problem that should not be ignored. This
occurrence ruins anything it comes into contact with. By affecting the
environment acid rain affects humans on the long run as well. What we
can do, to reduce the emission of gases that cause acid rain is to
reduce the use of fossil fuels. Walking and public transportation can
reduce tons of nitrogen oxide emissions. The use of less energy is
beneficial to the atmosphere as energy comes from fossil fuels, which
may lead to acid rain.
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