The Chemistry and Effects of Acid Rain

Topics of discussion Overview Sources of the acidity in Acid rain Natural Protection Against Acid Rain Enviromental Problems with Acid Rain Glimmer of Hope

Overview As the name suggests, acid rain is just rain which is acidic. The rain becomes acidic because of gases which dissolve in the rain water. Rain is naturally slightly acidic because of the carbon dioxide dissolved in it (which comes from animals breathing), and to a lesser extent from chlorine (which is derived from the salt in the sea). This gives rain a pH of around 5.6.

Some of the inventions we depend on for convenience and entertainment cause pollution.  Fossil fuels (gas, coal and oil), are necessary to produce electricity in most power plants.  The smoke and fumes from burning these fuels are released into the air and combine with moisture to produce acid rain.

Any precipitation with pH < 5. 6 is called Acid rain Any precipitation with pH < 5.6 is called Acid rain. The following chart will give you an indication of what different pH’s are:

About 70 percent of acid rain comes from sulphur dioxide (SO2 ), which dissolves into the water to form sulphuric acid. The rest comes from various oxides of nitrogen (mainly NO2 and NO3, collectively called NOx). These gases are produced almost entirely from burning fossil fuels, mainly in power stations and road transport:

Sources of Acidity in the Acid Rain Over 90% of the SO2 and NOx present in the atmosphere over eastern North America is a result of human activities. The major sources of sulfur emissions are coal (and to a lesser extent, oil) burning electric power plants, and industries such as ore smelting. Together, these two industries account for over 70% of the sulfur emissions in North America.

About 40% of NOx emissions come from the transportation sector, (cars, trucks, planes etc.). The rest are emitted from fossil fuel fired power plants and other combustion processes Most fuels including coal and oil, are mixtures of many different hydrocarbons, some of which contain sulphur.

When fuels containing suphur are burned, the sulphur present forms sulpur dioxide. S + O2 SO2 Subsequent reaction of the SO2 with air gives 2 SO2 + O2 2SO3 When the gases SO2 and SO3 join with water acids are formed. SO2 + H2O H2SO3 (sulphurous acid) SO2 + H2O H2SO3 (sulphuric acid)

Combustion reactions (such as automobiles) cause small amounts of N2 to react with oxygen in the air. N2 + O2 2 NO N2 + 2 O2 2 NO2 Subsequent reaction of the NO with atmospheric O2 2 NO + O2 2 NO2 In addition some of the NO2 also reacts with water . 2 NO2 + H2O HNO2 + HNO3

The following also contribute to acid rain problems The combined “soup” of H2SO3, H2SO4, HNO2 and HNO3 constitutes “acid rain” The following also contribute to acid rain problems Volcanic eruptions (produces sulphur dioxide) Gas from rotting vegetation. Lighting (produces nitric oxide)

Natural Protection against Acid Rain Most lakes have a moderate CO2/HCO3 buffer Once this buffer is exceeded, damage occurs Once acid rain stops, the absorption of CO2 from the atmosphere eventually reverses, most of the effects of acid rain Many lakes are limestone-rich which neutralize the acid rain. Dumping limestone from airplanes reverses the effect of acid rain as well

Environmental Problems with Acid Rain Fish and plant grow is seriously affected by acidified water and soil. Many lakes are now fishless Forests begin to die due to acidified soil The following slides demonstrate some of the effects of acid rain on the environment.

Acid rain has a particularly devastating effect on forests

Acid rain leaches minerals out of rocks and soil Aluminum ions leached out of rocks are poisonous Beneficial nutrients can be leached out of top soils and down to subsoils where it’s nutrients are unavailable for plant growth.

A water steam with a strong acid rain content.

Metal and stone structures, especially buildings made from limestone, ( a material favoured in the past) are damaged by acid rain Many ancient buildings and statues are now completely destroyed or unrecognizable.

Long term exposure to acid rain can have dramatic effects, the first picture is the original sculpture, the second one is how it looks right now

Acid rain frequently falls to earth far from where it is created Acid rain frequently falls to earth far from where it is created. Nations that create the problem do not usually have to deal with the consequences People’s health suffers directly and indirectly from water contaminated by acid rain and chemicals from leached soil and rock.

Hope An upsurge in public awareness of environmental issues such as acid rain is helping. New technologies offer a way to halt acid rain Alternative nonpolluting energy sources Industrial processes are being modernized to cut pollution and harmful waste International cooperation on pollution problems is accelerating; people are concerned and governments are listening

Bibliography Chemistry 12: A workbook for students : James Hebden p. 186 to 188