8.5 Flashcards
What is acid deposition?
Acid Decomposition is all processes by which acidic components leave atmosphere (gas or precipitate)
is all rainwater naturally acidic or basic?
acidic
what does acid rain refer to?
solutions with a pH below 5.6 and which therefore contain additional acids
what are main contributors of acid rain?
oxides of sulfur and nitrogen - primary pollutants
what is a primary pollutant?
an air pollutant emitted directly from a source
What is a secondary pollutant?
not directly emitted as such but forms when other pollutants (primary pollutants) react in the atmosphere
what is acid rain?
Acid rain is a secondary pollutant produced when these acidic gases (sulfur/nitrogen oxides) dissolve in water.
types of acid deposition:
- Wet acid deposition: rain, snow, hail, fog, mist, dew.
fall to ground as precipitates
2.Dry acid deposition: acidifying particles
gases fall to ground as dust and smoke and later dissolve in water to form acids
what is SO2 produced from?
burning of fossil fuels - particularly coal and heavy oil in power plants used to generate electricity
also released in industrial processes of smelting where metals are extracted from their ores.
it is estimated that about 50% of annual global emissions of SO2 come from coal
what kind of a compound is SO2?
colourless gas with a sharp smell
What can be produced from SO2?
SO3; H2SO3; H2SO4
O2 + 2SO2 = 2SO3
H2O + SO2 = H2SO3
H2O + SO3 = H2SO4
what are some mechanisms that might occur in the reactions of producing SO3, H2SO3 and H2SO4
- During sunlight - photo oxidation
- Oxidation may also be catalysed by tiny particles of metal present in the clouds (Fe, Mn)
- Ozone (O3) or hydrogen peroxide ( H2O2) (pollutants) can be invloved
a detailed study of ozone involvement
- hydroxyl free radicals which form by reactions between water and atomic oxygen/ozone
- A free radical possesses an unpaired electron and so is a short-lived and reactive species
- shown with the symbol of a dot
What is NO produced from?
mainly internal combustion engines where the burning of the fuel releases heat energy that causes nitrogen and oxygen from the air to combine
Reactions of nitrogen
N2 + O2 = 2NO
N2+ 2O2 = 2NO2
2NO + O2 = 2NO2
2NO2 + H2O = HNO2 + HNO3
2H2O + 4NO2 + O2 = 4HNO3
what contributes to the production of nitrous and nitric acid?
photooxidation, the presence of ozone and hydroxyl free radicals
what is photooxidation?
oxidation under the influence of radiant energy such as light
* High intensity of light can oxidize the chlorophyll due to which chlorophyll loses the ability to photosynthesize
What are the main active components of acid rain?
H2SO3; H2SO4; HNO3; HNO2
What are effects of acid deposition?
- erosion of structures
- corrosion of metals
- leaching
- lakes become dead
- eutrophication
- irritate the respiratory tract and increase the risk of illnesses such as asthma, bronchitis, and emphysema
- irritation to the eyes
what reactions lead to the erosion of structures?
2CaCO3 + H2SO4 = CaSO4 + 2CO2
CaCO3 + H2So4 = CaSO4 + H2O + CO2
calcium sulfate is somewhat more soluble than calcium carbonate it washes out the limestone or flakes off
calcium su;fate has a greater molar volume that calcium carbonate so its formation causes expansion and stress in the stonework
a similar reaction occurs with nitric acid
CaCO3 + 2HNO3 = Ca(NO3)2 + H2 + CO2
what is corrosion?
natural process that converts a refined metal into a more chemically stable oxide/hydroxide/sulphide state
How does acid deposition cause corrosion?
Components of both dry deposition and acid rain react with metals such as iron, forming the salt
This enables ionic conductivity to occur which leads to increase in the rate of electrochemical corrosion reactions such as rusting
Fe + SO2 + O2 = FeSO4
Fe + H2SO4 = FeSO4 + H2
in addition acid rain is able to reacr with and so remove the protective oxide layer on the surface of metals such as aluminium
Al2O3 + 6HNO3 = 2Al(NO3)3 + 3H2O
As a result acid rain causes significant damage to metallic structures such as bridges, rail and tracks and vehicles
why does this reaction release more pollutant into the air to form more acid rain?
Al2O3(s) + 6HNO3(aq) → 2Al(NO3)3(aq) + 3H2O(l)
Though that nitric acid, HNO3, does not react as a typical acid releasing hydrogen from metals, as it is an oxidizing agent and releases NO instead
How does acid deposition impact water?
causes dead lakes - unable to support life
Many fish can’t survive when pH<5
below ph 4 rivers are effectively dead as toxic Al3+ ions normally trapped in the rank as insoluble aluminium hydroxide leach out under acid conditions
Al(OH)3 + 3H+ = Al3+ + 3H2O
Aluminium ions interfere with the operating of the fish’s gills and reduce their ability to take in oxygen
what is eutrophication?
over-fertilization of bodies of water and can be caused by nitrates present in acid rain
it results in algal booms leading to oxygen depletion and sometimes the death of the lake/ stream
how does acid deposition affect human health?
does not directly affect it
but its components can react to form fine sulfate and nitrate particles that can travel long distances and be present in inhaled air.
these particles irritate the respiratory tract and increase the risk of diseases such as asthma/bronchitis/emphysema
can also cause irritation to the eyes
The release of toxic metal ions such as Al3+, Pb2+, and Cu2+ by the reaction of acid rain on metal structures such as pipes is also a potential health risk.
How does acid deposition affect plant life?
direct cause of slower growth, injury, or death of plants
cause important minerals such as Mg2+, Ca2+, and K+ held in the soil to become soluble and so wash away in a process called leaching, before they can be absorbed by plants
Without sufficient Mg2+ ions, for example, a plant cannot synthesize chlorophyll and so cannot make its food through photosynthesis
causes the release of substances that are toxic to plants, such as Al3+, which damage plant roots
can directly affect plants by blocking the pores for gas exchange, known as stomata
Forests in hilly regions seem to be particularly vulnerable because they tend to be surrounded by acidic clouds and mists
worst effects on forests have been in Europe, though the impact is seen worldwide
What are some responses to acid deposition?
- Reduction of SO2 emissions
- Reduction of CO2 emissions
- lowering the demand for fossil fuels
- more efficient energy transfer systems
- greater use of public transport
6.switching to renewable energy sources - neutralization of acids
methods of reducing SO2 emissions
- pre-combustion methods
- post-combustion methods
pre combustion methods of reducing SO2 emissions
processes that reduce/remove the sulfur present in coal/oil before combustion
sulfur ( presented as metal sulfade) can be removed by crushing the coal and washing with water
the high density metal sulfide sinks to the bottom and so seperates from the clean coal
Hydrodesulfurization (HDS) is a catalytic process that removes sulfur from refined petroleum products by reacting it with hydrogen to form hydrogen sulfide, H2S.
This is a highly toxic gas, so it is captured and later converted into elemental sulfur for use in the manufacture of sulfuric acid, H2SO4
post combustion methods of reducing SO2 emissions
Flue-gas desulfurization can remove up to 90% of SO2 from flue gas in the smoke stacks of coal-fired power stations before it is released into the atmosphere
The process uses a wet slurry of CaO and CaCO3 which reacts with SO2 to form the neutral product calcium sulfate, CaSO4.
CaO(s) + SO2(g) → CaSO3(s) CaCO3(s) + SO2(g) → CaSO3(s) + CO2(g) 2CaSO3(s) + O2(g) → 2CaSO4(s)
The calcium sulfate has industrial uses such as making plasterboard.
Reduction of NOx emissions ( there is an x because it can be either 1 2 or 3)
1.Catalytic converters in vehicles
2. Lower temperature combustion
Catalytic converters in vehicles to reduce NOx emissions
Exhaust gases can be controlled by the use of catalytic converters in which the hot gases are mixed with air and passed over a platinum- or palladium-based catalyst. The reaction converts toxic emissions into relatively harmless products. For example:
2CO(g) + 2NO(g) → 2CO2(g) + N2(g)
Lower temperature combustion to reduce NOx emissions
the formation of nitrogen monoxide is reduced at lower temperature. Recirculating the exhaust gases back into the engine lowers the temperature to reduce the nitrogen oxide in the emissions
neutralization of an acid to reduce the affect of acid deposition
The restoration of ecosystems damaged by acid rain is a long-term process. One method is to use calcium oxide (CaO), known as lime, or calcium hydroxide (Ca(OH)2) to neutralize the acid.
CaO(s) + H2SO4(aq) → CaSO4(s) + H2O(l) Ca(OH)2(s) + H2SO4(aq) → CaSO4(s) + 2H2O(l)
