environmental chemistry study of the effect of human activity on the chemical processes in the...
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Environmental chemistryEnvironmental chemistry study of the effect of human
activity on the chemical processes in
the environment concerns political and natural borders global issues applied chemistry
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Main topics: coreMain topics: core air pollution acid deposition greenhouse
effect ozone depletion
dissolved oxygen in water
water treatment soil waste
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Main topics: AHLMain topics: AHL
ozone depletion smog acid deposition water and soil
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Environmental chemistryEnvironmental chemistryE1: air pollutionE1: air pollution Describe the main sources of carbon
monoxide (CO), oxides of nitrogen (NOx), oxides of sulfur (SOx), particulates and volatile organic compounds (VOCs) in the atmosphere.
Evaluate current methods for the reduction of air pollution.
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the the atmosphereatmosphere
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composition of dry clean airhttp://www.dep.state.pa.us/earthdaycentral/97/air_teachers/TAB1.htm
component content component content
% by Vol. ppm % by Vol. ppm
Nitrogen 78.09 780,900 Hydrogen .00005 0.5
Oxygen 20.94 209,400 Methane .00015 1.5
Argon .93 9,300 Nitrogen dioxide
.0000001 0.001
Carbon dioxide .0318 318 Ozone .000002 0.02
Neon .0018 18 Sulfur dioxide .00000002 0.0002
Helium .00052 5.2 Carbon
monoxide .00001 0.1
Krypton .0001 1 Ammonia .000001 0.01
Xeon .000008 0.08
Nitrous oxide .000025
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Units used to express amount of a gas % by volume ppm = parts per million/particles per million
particles Remember in gases, assuming conditions are
the same:
same volume = same number of particles So 78.09 % by volume is 78.09 % particles
per million particles or 780,900 ppm
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primary air pollutantsprimary air pollutants waste products from human activity primary = added directly to the air pollutant = chemical in the wrong
concentration in the wrong place primary air pollutants:
CO NOx particulates SOx
volatile organic compounds (VOCs)
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Air pollutantsAir pollutantsFor each air pollutant you need to know:
sources: natural and man-made methods of reducing its emissions any relevant balanced symbol
equations
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carbon monoxidecarbon monoxide: sources Natural:
atmospheric oxidation of methane
CH4 + ½ O2 CO + 2H2
natural forest fires
Man-made: incomplete combustion of carbon-containing
fuels; mainly in cities; localized problem as well as fluctuations during the day – heavy traffic.
forest fires
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carbon monoxidecarbon monoxide: health effect
CO combines with Fe in haemoglobin in blood – bonds 320 times stronger than oxygen – oxygen cannot bond onto heamoglobin.
Less oxygen supplied to body cells. Effects:
headaches, shortness of breath, in case of high concentration (e.g. rush hour):
unconsciousness, death.
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carbon monoxide: reduction (1)
Lean burn engines
Decrease fuel /air ratio (in mass) from 1:14.7 (stoichiometric ratio/green band in diagram) to 1:18. Why?
(or increase air /fuel ratio to 18:1)
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carbon monoxidecarbon monoxide: reduction (2)
catalytic converter in car exhaust system reduction of nitrogen oxide oxidation of CO and unburnt
hydrocarbons/VOCs
2CO (g) + O2 (g) 2CO2 (g)
2NO (g) + 2CO (g) 2CO2 (g) + N2 (g)
2C8H18 (g) + 25O2 (g) 16CO2 (g) + 18H2O (g)
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Catalytic converter
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catalytic converter
To achieve high rate of reaction for this redox process: exhaust gases are passed over platinum
which acts as a heterogeneous catalyst; converter has a honey comb structure to
increase surface area; very high temperature of the exhaust gases
also increase the rate of reaction dramatically.
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Thermal exhaust reactorHeat from engine makes CO react with more air to oxidise it further to carbon dioxide.
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sulphur oxidessulphur oxides: sources Natural:
volcanoes sea spray biological decay of organic matter that
contains sulphur reduction of sulphates
Man-made: coal-burning power stations (equation
starting from S in coal) roasting of metal sulphides e.g. ZnS and
Cu2S (equation)
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sulphur oxidessulphur oxides: health effects acidic oxides lung irritants, affect in particular those
suffering from respiratory problems e.g. asthma sufferers
formation of sulphuric acid aerosols (droplets of sulphuric acid) (equation) (often catalysed by metal particulates); effects of aerosols: irritant to the eyes irritate vessels in lungs causing impaired breathing
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sulphur oxidessulphur oxides: methods of reduction
use of “low-sulphur content” fuels or removal of sulphur before burning coal/oil
removal of SO2 from fumes before they are
released: limestone based fluidised bed (equations showing
decomposition of calcium CaCO3 reaction of CaO with SO2) alkaline scrubbing (wet scrubber) (equations showing
reaction of CaO and Mg(OH)2)
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Sulphur oxidesSulphur oxides: reduction (1)
Wet scrubber
Limestone based fluidised bed
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nitrogen oxidesnitrogen oxides: sources Natural:
Electrical storms release enough energy to cause oxidation of atmospheric nitrogen: (equations showing oxidation of nitrogen and further oxidation of NO)
Decomposition of organic matter containing nitrogen
Man-made: Combustion of fossil fuels in car engines and furnaces of power stations produces high
temperatures to cause oxidation : (equation showing oxidation of nitrogen)
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nitrogen oxidesnitrogen oxides: health effects
choking irritating gas, affects eyes and people with respiratory problems
forms nitric acid aerosols/acid rain (equation showing dissolution of nitric acid in water)
nitric acid also increases the rate of oxidation of SO2 (see later)
plays an important role in the formation of secondary pollutants e.g. ozone and smog
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nitrogen oxidesnitrogen oxides: reduction
catalytic converter lean burn engines: high air/fuel ratio or low
fuel/air recirculation/reburn of exhaust gases: nitrogen
oxide emissions are reduced by reintroducing exhaust gases into the fuel mixture, lowering peak combustion temperatures as it is the high temperature in the combustion engine which causes nitrogen oxide production.
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ParticulatesParticulates: sourcesparticulates = airborne/suspended liquid and solid
particles. Natural:
volcanic eruptions large forest fires
Man-made: burning fossil fuels e.g. diesel forest fires industrial emissions; chemical processes incinerators
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ParticulatesParticulates: health effects particulates penetrate lungs and may block air
passages some are poisonous e.g. Pb and asbestos adsorb chemicals and can act as catalysts in
reactions producing secondary pollutants by adsorbing also increase concentration and rate of
reaction reduce visibility
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ParticulatesParticulates: reduction
Electrostatic precipitator
Particulates are given a negative charge and then attracted onto positively collection plates
.
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ParticulatesParticulates: reduction
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Volatile organic compoundsVolatile organic compounds: sources: sources Natural sources:
methane: bacterial anaerobic decomposition of organic matter (e.g. in rice paddies)
from plants e.g. terpenes leakage from natural fossil reserves
Man-made: evaporation of fuels partial combustion of fuels leakage from storage reservoirs
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VOCsVOCs: health effects photochemical smog can lead to carcinogenic compounds fatigue, weakness respiratory problems
VOCsVOCs: reduction catalytic converter