Download Acid Precipitation

Acid Precipitation
The problem of acid rain (also known as acid deposition or acid precipitation) has been building
since the 1970’s. At that time, environmentalists sent out warnings that smokestacks and tailpipes
were emitting chemicals that were increasing the acidity of rain and snow. Acid precipitation has
many effects on plants, animals, buildings and ecosystems.
Normal Rain
A common misconception is that normal rainwater is neutral and has a pH of 7.0.
Rain fall through the atmosphere and therefore passes through atmospheric gases which can
dissolve into raindrops.
Identify some of the gases in our atmosphere.
Write a combination reaction between carbon dioxide and water.
Use the product of this reaction as an acid in a Bronsted-Lowry reaction with water.
When carbon dioxide dissolves in water, it actually produces a dilute carbonic acid (a weak acid)
solution with a pH of about 5.6. Since this is ‘normal’ rainwater, anything less than this is
considered acid precipitation. Some acid precipitation has a pH as low as 4.2.
Determine the [H+] in normal rain water and in acid precipitation.
Acid precipitation is caused by emissions of
sulfur dioxide and nitrogen oxides. Although
these chemicals exist naturally, over 90% of
sulfur dioxide emissions and 95% of nitrogen
oxide emissions are the result of human
activities, such as fuel combustion in
vehicles, coal-burning power plants, and
metal smelting (Figure 2).
Laws have been passed to control the amounts of chemicals (such as sulfur dioxide and the
nitrogen oxides) that can be released by electricity generating plants, vehicles, and manufacturing
facilities. C4
Sulfuric Acid Production
Fossil fuels, such as coal and petroleum, were formed over millions of years from once-living plants
and animals (that’s why they are called fossil fuels). When the ancient organisms died, the
carbohydrates that they contained were transformed into organic molecules called hydrocarbons.
The ancient organisms also contained proteins, however, and these proteins contained sulfur and
nitrogen. The sulfur, therefore, remains as a contaminant in fossil fuels.
When the fossil fuels are burned, the sulfur is heated to very high temperatures and reacts with
oxygen in the air.
Write a combustion reaction for sulphur in oxygen.
Sometimes the sulfur is present in the fuel as hydrogen sulfide, which can also react with oxygen.
Write a combustion reaction for hydrogen sulphide gas and oxygen.
Both reactions produce sulphur dioxide, SO2(g). This compound reacts with oxygen in the
atmosphere to form sulfur trioxide in a synthesis reaction.
2 SO2(g) + O2(g) → 2 SO3(g)
The sulfur trioxide then combines with atmospheric water to produce sulfuric acid.
Write a reaction to show the formation (through synthesis) of sulphuric acid.
Nitric Acid Production
Fossil fuel combustion, especially from cars and trucks, also results in the formation of nitric acid.
The high temperatures inside automobile engines allow atmospheric oxygen and nitrogen to react,
producing nitrogen oxides.
Write a similar series of reactions (3) to show the formation of nitric acid from atmospheric
nitrogen.
What activities in your daily life contribute to the acid precipitation problem?
The Impact of Acid Precipitation
Acid precipitation is a problem across large sections of Canada. More than 80% of all Canadians
live in areas that are polluted by acid precipitation. About 96% of the land in eastern Canada that is
considered to have a high capability for forestry is subjected to excess levels of acid precipitation.
Acid rain causes an estimated $1 billion worth of damage in Canada every year.
As acidic precipitation falls to the ground, it can cause various problems to plant life, animals and
even buildings.
Plant Life: acid rain causes leaf damage,
growth rates and even poor sap production.
Aquatic Organisms: organisms that live in
water are very susceptible to pH changes.
Water from their environment can enter their
cells and acidic water will result in reproductive
problems, increased death rate and can effect
entire populations (Figure 4)
What effect could acid precipitation have on
terrestrial plants and animals.
Acid precipitation can also have an effect on non-living things. Lakes and rivers have obvious
changes to pH which can result in increased erosion to rocks and shores. As well, the absorption
rate of different ions and minerals is affected by pH.
What man-made structures would be affected by acid precipitation? How?
One of the more direct concerns for humans is the impact of acid precipitation on toxic metals. The
concentration of metals such as copper and lead in water and soil increases. These toxic metals
can cause long-term chronic health effects.
Why can acid precipitation result in an increase in the concentration of metals in water and
soil?
Not all bodies of water are equally affected by acid precipitation.
How can the effect of an acid be prevented?
The impact varies, depending on the type of rock underneath the lake or stream. Some rocks
contain ions that are able to counteract the effect of the acid precipitation in a neutralization
reaction. These compounds slow down the erosion of the rock and prevent the pH from changing
drastically.
Limestone is also known as calcium carbonate. Carbonates react with acids to form carbon dioxide
as follows. The calcium ion is a spectator ion.
CaCO3 (aq) + 2 H+(aq) from the acid precipitation → CO2 (g) + H2O (l) + Ca2+(aq)
Another rock, dolomite, as well as clay, contains ions that can also help reduce the effect of the
acid precipitation.
Write a reaction between dolomite (calcium magnesium carbonate) and an acid (H+).
There have been many proposed solutions to the acid precipitation problem, each addressing a
different part of the problem.
Suggest ways to fix the pH of affected bodies of water.
Suggest ways to prevent the formation of acid precipitation.