Title: Lesson 13: Acid Deposition
Learning Objectives:
– Balancing the equations that describe the combustion of sulphur and
nitrogen to their oxides and formation of their acids
– Distinction between pre-combustion and post combustion methods of
reducing sulphur oxide emissions.
– Deduction of acid deposition equations for reactions with metals and
carbonates
Causes of acid deposition
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All rain water is naturally acidic (carbon dioxide dissolves in water forming weak
carbonic acid)
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Carbonic acid ionizes to form the equilibrium:
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This gives a minimum pH of 5.6.
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Acid rain < pH 5.6
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Oxides of Sulphur and Nitrogen (primary pollutants) are the cause of acid rain
(secondary pollutant)
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Acid deposition is a broader term than acid rain and includes all
process by which acidic components as precipitates or gases leave the
atmosphere.
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Types of acid deposition
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Wet acid deposition: rain, sleet, snow, fog, mist, dew fall to the ground as aqueous
precipitates
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Dry acid deposition: acidifying particles, gases fall to the ground as dust and smoke
 later dissolves in water
Richard Thornley - Acid Deposition Video
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Sulphur dioxides
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Produced from burning fossil fuels (coal/heavy oil to generate electricity)
Released in industrial processes such as metal extraction from ores (smelting)
50% global emissions from coal
SO2 is a colourless gas with sharp smell and it dissolves in water to form sulfurous
acid
SO2 can be oxidized to Sulphur trioxide, which then dissolves in water to form
Sulphuric acid
Simple oxidation by atmospheric Oxygen is not involved. Complex
interactions with hydroxyl radicals, ozone or hydrogen peroxide are
involved. (Don’t worry too much about the detail of this)
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Nitrogen oxides
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NO is produced mainly from internal combustion engines, where burning
fuel releases heat energy causing nitrogen and oxygen to combine:
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A similar reaction gives rise directly to a brown gas, nitrogen dioxide:
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Nitrogen dioxide also forms from the oxidation of nitrogen monoxide:
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Nitrogen dioxide dissolves in water to form a mixture of nitrous acid
(HNO2) and nitric acid (HNO3):
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Alternatively, nitrogen dioxide can be oxidized to form nitric acid:
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The formation of acid deposition
Simple oxidation by atmospheric Oxygen is not involved. Complex
interactions with hydroxyl radicals, ozone or hydrogen peroxide are
involved. Below if an example of how hydroxyl free radicals (.HO)
contribute to the production of nitrous and nitric acid.
Main active components of acid rain are :
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Effects of acid deposition
Acid deposition has a wide range of impacts:
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Materials
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Plant life
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Water
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Human health
TASK: Using the information sheet and your student textbooks, write a summary of the
effects of Acid Deposition.
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Methods of dealing with acid deposition
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Improving design of vehicle engines
Using catalytic converters
Removing Sulphur before burning fossil fuels
Using renewable power supplies
Making greater and more efficient use of public transport
Designing more efficient power stations
‘Liming’ of lakes – calcium oxide or hydroxide neutralizes acidity:
CaO(s) + H2SO4(aq)  CaSO4(aq) + H2O(l)
Ca(OH)2(s) + H2SO4  CaSO4(aq) + 2H2O(l)
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Reductions of S02 Emissions
Pre-combustion methods
 Remove Sulphur in coal or oil before combustion. This is done
by crushing coal and washing with water. High density metal
sulfide sinks to bottom and separates from clean coal.
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Hydrodesulfurization (HDS) is a catalytic process that
removes sulfur by reacting it with hydrogen to form
hydrogen sulfide, H2S. This is toxic so is captured and
converted into elemental sulfur for use in manufacturing
sulfuric acid.
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Reduction of SO2 emissions
Post-combustion methods
 Flue-gas desulfurization removes up to 90% of SO2 from flue gas in the smoke
stacks of coal-fired power stations before it is released into the atmosphere.
 A wet slurry of CaO and CaCO3 reacts with SO2 to form the neutral product
calcium sulfate, CaSO4:
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Calcium sulfate (gypsum) is used in industry in the manufacture of
plasterboard
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Reductions of NOx Emissions
Catalytic converters in vehicles
 Hot gases are mixed with air and passed over a platinum or palladium based
catalyst. This converts toxic emissions into harmless products:
Lower temperature combustion
 The formation of NO is reduced at lower temperatures. Recirculating exhaust
gases back into the engine lowers the temperature to reduced NO emissions.
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Solutions
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