STAGE 2 CHEMISTRY
Elemental and Environmental Chemistry
Unit 7
Acid rain:
READING
The Essentials book pg 44-48 ,
Q 1.14, 1.15, 1.16, 1.17
calculate pH in acidic and basic solutions,
describe and write equations showing the formation of
acid rain, describe the effects of acid rain.
The Syllabus statement says:
Key Ideas
Intended Student Learning
pH is a measure of the concentration of hydrogen
ions where this is taken as the concentration of
hydrogen ions relative to the standard 1 mol L-1 , that
is
 concentration of H  ions 

pH = -log10 
mol L–1


+
commonly used as pH = -log [H ].
Calculate the concentration of H+ of solutions,
given their pH, and vice versa.
The low pH of acid rain is due to the presence of
sulfuric and nitric acids.
Calculate the pH of solutions of strong bases
and strong monoprotic acids.
Rain containing dissolved carbon dioxide is acidic.
Write equations to show how carbon dioxide
produces acidic rainwater.
Rainfall with a pH of less than 5.6, known as ‘acid
rain’, is formed when oxides of nitrogen and sulfur
dissolve in water in the atmosphere.
Describe and write equations for the formation
of acid rain.
Acid rain has harmful environmental effects.
Describe the environmental effects of acid
rain, including its action on metals and
carbonates (with equations), and on the
mobilisation of toxic cations such as
aluminium.
The KEY IDEAS here are:

pH is a measure of hydrogen ion concentration and pH = -log[H+] ,

pH measures the acidity of a solution.

Acids donate protons, (hydrogen ions, H+) and so the concentration of hydrogen ions is a measure of
acidity. Acid solutions have a pH < 7, alkaline solutions have a pH > 7 and neutral solutions have a pH
= 7.

Rainwater is naturally acidic because of dissolved carbon dioxide.

Rainwater of pH less than 5.6, is defined as acid rain.

Acid rain forms when oxides of nitrogen and sulfur dissolve in water in the atmosphere.
The low pH of acid rain is due to the formation of sulfuric and nitric acids in rain water.

Acid rain has harmful environmental effects on metal structures, carbonates and can affect the
concentration of aluminium ions in soil water.
page 1
H+ is equivalent to H3O+
Acids – some background
An acid is a substance that donates a proton (H+) to a base,
eg HCl, HNO3, H2SO4, CH3COOH, NH4+ are typical acids
A base is a substance that accepts a proton (H+) from an acid,
eg OH–, NH3,
Protons exist in water (aqueous solutions) as hydronium ions, H3O+.
Acid/base reactions are sometimes called proton transfer reactions.
HCl(aq) + H2O(l)
H3O+(aq) + Cl–(aq)
Acids undergo ionisation reactions. These are the reaction in which a molecule reacts with water to form one
or more ions.
HCl(aq) + H2O(l)
H3O+(aq) + Cl–(aq)
A strong acid donates a proton (H+) readily. Hence it will ionise to form a high concentration of ions in solution.
A weak acid does not donate a proton (H+) readily. Hence it will ionise to form a low concentration of ions in
solution.
SUPPORTING QUESTIONS 1
1
2
Identify (YES/NO), which of the following reactions can be classified as proton-transfer reactions?
i
HNO3(aq) + H2O(l)
ii
NH3(aq) + HNO3(aq)
iii
Ag+(aq) + Cl– (aq)
iv
H3PO4(aq) + H2O(l)
H3O+(aq) + H2PO4–(aq)
v
O2–(aq) + H2O(aq)
2OH–(aq)
vi
Mg(s) + 2HCl(aq)
vii
CO32- (aq) + H2O(aq)
i
Write an equation to illustrate the ionization of HNO3.
ii
HNO3 is defined as a strong acid. Explain what this means.
H3O+(aq) + NO3–(aq) ____________________________________________
NH4+(aq) + NO3-(aq)
AgCl(s)
____________________________
____________________________
____________________________
____________________________
MgCl2 (aq) + H2 (g)
____________________________
HCO3– (aq) + OH–(aq) ____________________________
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page 2
MY NOTES
pH
What does pH measure?
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The formula to calculate pH is:
The formula to calculate [H+] is :
EXAMPLES
Calculating pH of a strong monoprotic acid, given the concentration of the acid.
Calculate the pH of a 0.20 mol L-1 of hydrochloric acid solution.
Calculating pH, given the [H+] concentration .
Calculate the pH of a solution with a [H+] of 3.1 x 10-4 mol L-1.
Calculating the [H+] concentration, given the pH.
Calculate the [H+] of a hydrochloric acid solution with a pH of 3.4.
Calculating the pH of strong bases
In these examples you first calculate the pOH value of the base which is equal to –log[OH-] with the [OH-]
being equal to the concentration of the strong base.
You then use the expression: pOH + pH = 14 to calculate pH.
Calculate the pH of a sodium hydroxide solution of concentration of 0.500 mol L -1.
page 3
SUPPORTING QUESTIONS 2
1.
2.
pH calculation exercises
Calculate the pH of the following solutions :
i
0.05 molL-1 hydrochloric acid
ii
0.20 molL-1 HNO3(aq)
iii
0.04 molL-1 sodium hydroxide solution
iv
a solution with an H+ concentration of 5.4 x 10-5 molL-1
v
a solution with an OH- concentration of 3.7 x 10-7 molL-1
Calculate the hydrogen ion concentrations in the following substances, given their pH :
i
tomatoes, pH = 4.2
ii
lemon juice, pH = 2.3
iii
milk, pH = 6.4
page 4
SOME NOTES ON ACID RAIN
Rainwater is naturally acidic. This is due to carbon dioxide dissolving in water to form a solution of carbonic
acid.
Equation is :
Acid rain is rainwater that has a pH of < 5.6.
Formation of acidic non-metal oxides. Sulfur dioxide is formed during the burning of fossil fuels (eg coal) that
contain sulfur impurities. Sulfur dioxide can react with oxygen to form sulfur trioxide
Nitrogen oxide is formed from nitrogen and oxygen at high temperatures, such as those found in industrial
furnaces and combustion engines. Nitrogen oxide can then further react under these conditions to form
nitrogen dioxide
Acid rain is caused by increasing levels of sulfur and nitrogen oxides being released into the atmosphere from
the burning of fossil fuels and the roasting of sulfide ores to produce some metals like zinc. These oxides are
acidic and dissolve in water forming acids which will ionise in water.
Examples are :
SO2
SO3
NO2
Acid rain has had the most damaging effects in North America and Europe.
Acid rain pollutes waterways, has leaching affects on soils and has corrosive effects on building materials such
as steel and marble, (calcium carbonate).
Example :
Fe
+
CaCO3
H+
+
H+
In soils, the increased acidity causes metal ions, like aluminium, which is normally bound up in the soil in
insoluble form, to be dissolved in the water and then to get washed into streams and lakes. Aluminium ions are
toxic to plants in certain concentrations.
Example :
Soils rich in limestone are not affected as much by acid rain. The limestone is able to neutralise the acid.
Example :
page 5
Effects of acid rain
Acid rain increases the rate of corrosion of steel and aluminium structures and can degrade protective coatings
such as zinc oxide.
Acid rain increases soil acidity and stunts plant growth. Low pH causes some essential minerals such as Mg2+
to leach from the soil. Without Mg2+, plants cannot photosynthesise.
In waterways, aquatic life is particularly sensitive to increased levels of acidity. Even a slight decrease in pH
prevents some fish from breeding.
Acid rain has a pH of 5.6 or less and it has been blamed for the destruction of vast forest areas and the
decimation of fish populations, particularly in Europe and North America.
In humans, acid rain has been linked to irritation of mucous membranes and to respiratory illness, particularly
in the old and very young.
Efforts to reduce the effects of acid rain include:
 using low-sulfur fuels
 removing sulfur from coal before combustion
 removing SO2 from flue gases of power plants
 improving the design of combustion engines to reduce NOx emissions.
Acidic soils and lakes can be partially neutralized by adding powdered limestone
However, acid rain will continue to cause problems until all countries accept their responsibility and limit their
own emissions.
SUPPORTING QUESTION 3
Imagine you were given the following extended response question :
Credit will be given for answers to part (b) which show clear well-expressed ideas and which present accurate
and relevant information in a well-organised, logical manner.
Your answer, which may include equations, should be confined to the space provided and should take
approximately 10 minutes.
Large quantities of sulfur dioxide are produced from the combustion of fossil fuels.
Outline the potential environmental problem associated with the release of this gas
into the atmosphere.
In your answer describe the causes of the problem, two effects of the problem and
two possible solutions to the problem.
page 6
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page 7
A Possible Answer :
Increased concentrations of sulphur dioxide in the atmosphere have contributed to a
form of air pollution called acid rain. Sulphur dioxide is an acidic oxide and will react with
rain-water to form sulphurous acid.
SO2
+
H2O
H2SO3
Along with other non-metal oxides such as NO2, it may result in the rain-water having a
pH of less than 5.6 and hence being classified as acid rain.
Human activity, such as the burning of fossil fuels, and the smelting of sulfide ores (eg
for zinc production), has led to an increase in the concentration of SO2 in the
atmosphere, particularly in densely populated areas or highly industrialised regions.
Acid rain has a detrimental effect on the environment. Buildings made of limestone or
marble statues react with acid rain and are slowly eaten away.
CaCO3
+
H2SO3
CaSO3
+
CO2
+
H2O
Acid rain is also associated with forest dieback and with the killing of aquatic
ecosystems.
Possible solutions to the problem include using low-sulfur fuels such as natural gas, and
making sure the SO2 from power plants does not escape into the atmosphere and is recycled.
page 8
SOME EXTRA INFORMATION ON ACID RAIN
The atmosphere contains low
concentrations of a number of non-metal
oxides, including carbon dioxide, nitrogen
dioxide, sulfur dioxide and sulfur trioxide.
These oxides exist in the atmosphere
from natural sources, but their
concentrations have increased as a result
of human activity, particularly
during the last 100 years. Some of these
acidic oxides are responsible for acid
rain, a serious form of air pollution.
All rain contains dissolved carbon dioxide,
so even unpolluted rain is slightly acidic.
The carbon dioxide reacts with water to
form carbonic acid.
Rain is usually described as acid rain if it has a pH lower than 5.6. The worst examples of acid rain occur
around densely populated and highly industrialized regions. Acid rain is usually the result of rain dissolving
sulfur oxides (SO2 and SO3) and nitrogen dioxide (NO2) to produce a dilute solution of acids such as
sulfurous (H2SO3), sulfuric (H2SO4) and nitric (HNO3).
In addition to the natural sources
of sulfur oxides such as volcanic
eruptions, emission of sulfur
oxides into the atmosphere is
often the result of burning fossil
fuels in electricity-generating
power stations, and the smelting
of sulfideores in metal-smelting
plants. The sulfur dioxide
produced is oxidized in the
air to form sulfur trioxide.
Nitrogen oxide emissions are
usually the result of
high-temperature combustion
processes such as those in
motor vehicles and airplane
emissions. Nitrogen oxides can
also be formed naturally in the
vicinity of lightning, which
provides enough energy to allow
nitrogen to be oxidized to NO
and further to NO2. When
dissolved in rain, these sulfur
and nitrogen oxides produce
acids and, subsequently, acid rain.
SO2(g) + H2O(l)
SO3(g) + H2O(l)
4NO2(g) + 2H2O(l)
H2SO3(aq)
H2SO4(aq)
+
O2(g)
4HNO3(aq)
Although a solution with a pH of 4 or 5 would be described as only weakly acidic, these solutions can cause a
lot of damage over long periods of time. Acid rain can cause several problems. Surface waters and lakes can
become acidic, decreasing the populations of aquatic species that cannot tolerate these acidic conditions.
Many lakes in Europe, Scandinavia and North America are now too acidic to support fish life. Acid rain can
also cause damage to plants, including crops and forests. This occurs through the direct effect of the acid
rain, but also indirectly because of its effects on soil. Another problem is the damage caused to metal and
stone buildings and structures. In particular, many historical statues are slowly dissolving due to the effect of
acid rain on the limestone of which they are composed. The reaction involved is:
CaCO3(s) + 2H+(aq)
Ca2+(aq) + H2O(l)
page 9