Acid Rain

ACID RAIN

What is Acid Rain? Most people think rain isn't a damaging kind of weather. However the increase in acidity of rain is 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 in1852 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 below5.6. 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) Ðo SO32-

After this the sulphite ion forms sulphuric acid when it comes into contact with the moisture in the atmosphere.

SO32- +H2O (l)Ðo 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)]Ðo[SO2(aq)] + [HSO32]+[SO32]

There is a dissociation that consists of two parts:

SO2 (aq)Ðo H++HSO3-

HSO3- (aq)ÐoH+ + 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)ÐoHNO3 (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.