Option E
Environmental Chemistry
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E2 and E11: Acid Deposition
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E2 and E11 Assessment Statements
E.2.1
State what is meant by the term acid deposition and outline its origins.
Acid deposition refers to the process by which acidic particles, gases and precipitation leave the atmosphere. Both wet deposition (acid rain, fog and
snow) and dry deposition (acidic gases and particles) will be assessed. Rain is naturally acidic because of dissolved CO2 but acid rain has a pH of less
than 5.6. It is caused by oxides of sulfur and oxides of nitrogen.The equations for the burning of sulfur and nitrogen, and for the formation of H2SO3,
H2SO4, HNO2 and HNO3, will be assessed.
E.2.2
Discuss the environmental effects of acid deposition and possible methods to
counteract them.
E.11.1
Describe the mechanism of acid deposition caused by the oxides of nitrogen and
the oxides of sulfur.
E.11.2
Describe the role of ammonia in acid deposition.
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Formation of Acid
Deposition Review
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Formation of Acid Deposition
Sulfur Oxides - SOx
SO2 is a primary pollutant formed mainly by coal burning.
Sulfur impurities in coal react with oxygen gas to form sulfur dioxide (SO2).
S(s) + O2(g) g SO2(g)
SO3 is a secondary pollutant formed when SO2 is oxidized in the air:
2 SO2(g) + O2(g) g 2 SO3(g)
SO2 reacts with water in the air to form sulfurous acid, a weak acid:
SO2(g) + H2O(l) g H2SO3(aq)
SO3 will also react with water in the air to produce an acid, sulfuric acid.
This acid is a strong acid:
SO3(g) + H2O(l) g H2SO4(aq)
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Formation of Acid Deposition
Nitrogen Oxides - NOx
NO (nitrogen monoxide) is a primary pollutant formed mainly in the
internal combustion engines of vehicles, where high temperatures
promote the reaction between nitrogen and oxygen gases in air.
N2(g) + O2(g) g 2 NO(g)
NO2 is a secondary pollutant formed when NO is oxidized in the air:
2 NO(g) + O2(g) g 2 NO2(g)
NO2 reacts with water in the air to produce a mixture of nitrous and nitric acids:
2 NO2(g) + H2O(l) g HNO2(aq) + HNO3(aq)
NO2 also reacts with oxygen and water to form only nitric acid:
4 NO2(g) + O2(g) + 2H2O(l) g 4 HNO3(aq)
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A Few Other Points about E2
CO2
CaCO3(s) vs CaSO4(aq)
Al3+
+
K,
2+
Mg
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Free Radical
Mechanisms for the
Formation of Acid Rain
HL!
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Free Radical Mechanisms in Acid Rain Formation
1. Formation of Hydroxyl Free Radicals (•OH)
Hydroxyl free radicals are important in the mechanisms
that produce sulfuric acid and nitric acid.
They are produced by two reactions:
Reaction 1: Reaction between water and atomic oxygen free radicals:
H2O + O• g 2 •OH
Reaction 2: Reaction between water and ozone:
H2O + O3 g 2 •OH + O2
BreathingBreathing
air containing
air containing
fine droplets
fine of
droplets
acid and
of acidic
acid and
gases
acidic gases
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Free Radical Mechanisms in Acid Rain Formation
2. Formation of Nitrous and Nitric Acids
Acids form when the hydroxyl free radicals
react with nitrogen oxides:
Reaction 1:
formation of nitrous acid (HNO2):
•OH + NO g HNO2
Reaction 2:
formation of nitric acid (HNO3):
•OH + NO2 gHNO3
BreathingBreathing
air containing
air containing
fine droplets
fine of
droplets
acid and
of acidic
acid and
gases
acidic gases
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Free Radical Mechanisms in Acid Rain Formation
3. Formation of Sulfuric Acid
A three step mechanism is needed to produce sulfuric acid, H2SO4:
Step 1:
•OH + SO2 g HOSO2•
Hydroxyl free radicals react with SO2 to
form a new free radical.
Step 2:
HOSO2• + O2 g HO2• + SO3
The new free radical combines with O2 to
form SO3 and another free radical.
Step 3:
SO3 + H2O g H2SO4
SO3 reacts with water to produce sulfuric
acid.
BreathingBreathing
air containing
air containing
fine droplets
fine of
droplets
acid and
of acidic
acid and
gases
acidic gases
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The Role of Ammonia
in Acid Deposition
(Aren’t you happy you take HL?)
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The Role of Ammonia
Assorted processes in ecosystems add ammonia to the atmosphere.
Ammonia (NH3) is a weak base.
As a base, it can neutralize acids to form these ammonium salts:
NH3 + HNO3 g NH4NO3
2 NH3 + H2SO4 g (NH4)2SO4
When the ammonium salts sink to the ground or fall as acid precipitation, they enter the soil
and dissolve to give free ammonium (NH4+) ions.
The ammonium ions then react in a process called nitrification:
NH4+ + 2 O2 g 2 H+ + NO3- + H2O
Why is this important?
Well, it produces nitrates (NO3-) that plants need as nutrients.
But it also adds H+ to the soil, making it more acidic.
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