The Chemistry and Effects of Acid Rain

Topics of discussion

OverviewSources of the acidity in Acid rainNatural Protection Against Acid RainEnviromental Problems with Acid RainGlimmer of Hope

OverviewAs 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. 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 RainOver 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 processesMost 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 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 RainMost lakes have a moderate CO2/HCO3 bufferOnce this buffer is exceeded, damage occursOnce acid rain stops, the absorption of CO2 from the atmosphere eventually reverses, most of the effects of acid rainMany 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 fishlessForests 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 poisonousBeneficial 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 rainMany 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. Nations that create the problem do not usually have to deal with the consequencesPeople’s health suffers directly and indirectly from water contaminated by acid rain and chemicals from leached soil and rock.

HopeAn 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 sourcesIndustrial processes are being modernized to cut pollution and harmful wasteInternational 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