Option E-Environmental Chemistry

Describe the main sources of carbon monoxide (CO)
Natural Source(s): Incomplete oxidation of methane
CH4 + 1? O2 –> CO + 2H2O

Anthropogenic Source(s): Incomplete combustion of fossil fuels, e.g. C8H18 + 8?O2 –> 8CO+ 9H2O

Describe the main sources of oxides of nitrogen (NOx)
Natural Source(s): Electrical storms and biological processes

Anthropogenic Source(s): At high temperatures inside internal combustion engines N2 + O2 –> 2NO

Describe the main sources of oxides of sulfur(SOx)
Natural Source(s): Oxidation of H2S produced by volcanoes, and decay of organic matter.

Anthropogenic Source(s): Combustion of sulphur-containing coal and smelting of sulphide ores S +O2 –> SO2

Describe the main sources of particulates
Natural Source(s): Dust from mechanical break-up of solid matter. Sulphur from volcanic eruptions. Pollen, bacterial and fungal spores.

Anthropogenic Source(s): • Soot from the incomplete combustion from hydrocarbon, (eg. Diesel) and coal in power stations.
• Arsenic from insecticides .
• Asbestos from the construction industry(released when demolishing).
• Fly ash from the combustion of fossil fuels in furnaces.
• Mercury from the manufacture of fungicides, pulp and paper.

Describe the main sources of volatile organic compounds (VOCs)
Natural Source(s): It is produced by the bacterial anaerobic decomposition of organic matter in water and soil. Unsaturated hydrocarbons called terpenes are given out by plants.
Anthropogenic Source(s): Unburned or partially burned gasoline and other fuels; Released into the atmosphere from solvents and paints
Effects on health and methods of reduction of carbon monoxide (CO)
Effects on health: Prevents haemoglobin from carrying oxygen by forming carboxyhaemoglobin

Methods of reduction: • Use of lean burn engine
• thermal exhaust reactor
• catalytic converter.

Effects on health and methods of reduction of oxides of nitrogen (NOx)
Effects on health: Respiratory irritant leading to respiratory tract infections

Methods of reduction: • Use of lean burn engine
• recirculation of exhaust gases
• catalytic converter.

Effects on health and methods of reduction of oxides of sulfur(SOx)
Effects on health: Respiratory irritant leading to respiratory tract infections

Methods of reduction: • Removal of sulphur from fossil fuels before combustion.
• Alkaline scrubbing.
• Fluidized bed combustion

Effects on health and methods of reduction of particulates
Effects on health: Can affect the respiratory system and cause lung diseases, such as emphysema, bronchitis, and cancer

Methods of reduction: Sedimentation / gravity settling chambers.
Cyclone separators
Electrostatic precipitation
Wet scrubbers

Effects on health and methods of reduction of volatile organic compounds (VOCs)
Effects on health: Some (e.g. benzene) are carcinogenic. Can form toxic secondary pollutants

Methods of reduction: Catalytic converter

Evaluate current methods for the reduction of air pollution: Thermal exhaust reactor
• Exhaust from the car engine is combined with more air and reacts due to the heat of the exhaust gases. Carbon monoxide is converted into carbon dioxide and unburned hydrocarbons are also combusted.
• 2CO(g) + O2(g) –> 2CO2(g)
Evaluate current methods for the reduction of air pollution: Lean Burn engines
• By adjusting the carburettor the ratio air:fuel can be altered. The higher the ratio the less carbon monoxide emitted as more complete combustion occurs. Unfortunately this produces higher temperatures so more NOx is produced. At lower ratios less NOx but more CO will be emitted.
Evaluate current methods for the reduction of air pollution: Catalytic converter
• The hot exhaust gases are passed over a catalyst of platinum, rhodium, or palladium. These fully oxidize carbon monoxide and unburned volatile organic compounds, VOCs and also catalyse the reaction between carbon monoxide and nitrogen oxide.
• 2CO(g) + 2NO(g) –> 2CO2(g) + N2(g)
Evaluate current methods for the reduction of air pollution: Sulfur dioxide: • Removal of sulphur from fossil fuels before combustion
o Some sulphur is present in coal as metal sulphides (e.g. FeS) and can be removed physically by crushing the coal and mixing with water. The more dense sulphides sink to the bottom and the cleaner coal can be skimmed off.
Evaluate current methods for the reduction of air pollution: Sulfur Dioxide: • Alkaline scrubbing
o Sulfur dioxide can be removed from the exhaust of coal burning plants by scrubbing with an alkaline slurry of limestone (CaCO3) and lime (CaO). The resulting sludge is used for a landfill
o CaCO3(s) + SO2(g) –> CaSO3(s) + CO2(g)
o CaO(s) +SO2(g) –> CaSO3(s)
o 2CaSO3(s) + O2(g) + 4H2O(g) –> 2CaSO4*2H2O(s)
Evaluate current methods for the reduction of air pollution: Sulfur dioxide: • Fluidized bed combustion
o It involved the burning of coal on a bed of limestone which removes the sulfur as CaSO3 or CaSO4 as the coal burns
Evaluate current methods for the reduction of air pollution: Particulates: • Sedimentation Chambers
o Large particles can be allowed to settle under the influence of the gravity in sedimentation chambers.
Evaluate current methods for the reduction of air pollution: Particulates: • Electrostatic precipitation
o For smaller particles, a precipitation chamber can be used. Charged particulates are attracted to the opposite electrodes, which are shaken periodically so that the aggregated particulates fall to the bottom of the precipitator where they can be removed.
State what is meant by acid deposition
Acid deposition: the process by which acidic particles, gases and precipitation leave the atmosphere
Wet deposition: acidic oxides react with water in the air. Known as acid rain, it includes fog, snow and dew as well as rain. Rain is naturally acidic with a pH of 5.65 due to the presence of CO2, acid rain has a pH lower than 5.6: CO2 + H2O ? H+ + HCO3
Dry deposition: includes acidic gases and particles.
Outline the origins of acid deposition: Oxides of sulfur
Oxides of sulphur SOx
o Occurs naturally from volcanoes, and industrially it is produced from the combustion of sulphur containing fossil fuels and the smelting of sulphide ores.
o S(s) + O2(g) –> SO2(g)
o In the presence of sunlight SO2 is oxidized to SO3:
SO2(g) + 1?O2(g) –> SO3(g)
o Oxides can react with water to form sulphurous acid and sulphuric aid:
o SO2(g) + H2O(l) –> H2SO3(aq)
o SO3(g) +H2O(l) –> H2SO4(aq)
Outline the origins of acid deposition: oxides of nitrogen
Oxides of nitrogen NOx
o Occurs naturally from electric storms and bacterial action. Nitroge monoxide is produced in the internal combustion engine and in jet engines
o N2(g) + O2(g) –> 2NO(g)
o Oxidation to nitrogen dioxide occurs in the air: 2NO(g)+ O2(g) –> 2NO2(g)
o Nitrogen dioxide then reacts with water to form nitric acid and nitrous acid:
2NO2(g) + H2O(l) –> HNO3(aq) + HNO2(aq)
o It can also be oxidized directly to nitric acid by oxygen in the presence of water:
4NO2(g) + O2(g) + 2H2O(l) –> 4HNO3(aq)
Discuss the environmental effects of acid deposition: Vegetation
• Vegetation
o Increased acidity in the soil leaches important nutrients, such as Ca2+, Mg2+, and K+.
Reduction in Mg2+ can cause reduction in chlorophyll and consequently lowers the ability to photosynthesize.
o Many trees are affected by acid rain. Symptoms include stunted growth, thinning of tree tops, and yellowing and loss of leaves. The main cause is the aluminium leached from rocks into the soil water. The Al3+ion damages the roots and prevents the tree from taking up enough water and nutrients to survive.
Discuss the environmental effects of acid deposition: Lakes and rivers
o Increased levels of aluminium in water can kill fish. Aquatic life is also highly sensitive to pH. Below pH 6 the number of sensitive fish decline as so do insect larvae and algae. Snails cannot survive below a pH of 5.2 and below 5.0 many microscopic animal species disappear. Below pH 4.0 lakes are effectively dead.
Discuss the environmental effects of acid deposition: Buildings
o Stone, such as marble, that contains calcium carbonate is eroded by acid rain.
o Sulphuric acid react with calcium carbonate to form calcium sulphate, which can be washed away by rain water.
o Salts can also form within the storm that can cause the stone to disintegrate.
o CaCO3(s) + H2SO4(aq) –> CaSO4(aq) + CO2(g) + H2O(l)
Discuss the environmental effects of acid deposition: Human Health
o The acids formed when NOx and SOx dissolve in water irritate the mucus membranes and increase the risk of respiratory illnesses, such as asthma, bronchitis and emphysema.
o In acidic water there is more probability of poisonous ions such as Cu+ and Pb2+, leaching from pipes and high levels of aluminium in water may be linked to Alzheimer’s disease.
Methods to lower or counteract the effects of acid rain
1. Lower the amounts of NOx and SOx formed, e.g. by improved engine design, the use of catalytic converters, and removing sulphur before, during and after combustion of sulphur containing fuels
2. Switch to alternative methods of energy (e.g. wind and solar power) and reducing the amount of fuel burned, e.g. by reducing private transport and increasing public transport and designing more efficient power stations.
3. Liming of lakes – adding calcium oxide or calcium hydroxide (lime) neutralizes the acidity, increases the amount of calcium ions and precipitates aluminium from solution. This has proven to be effective in many, but not all lakes where it has been tried
Describe the greenhouse effect
Greenhouse gases in the atmosphere allow the passage of incoming solar short/wavelength (visible and ultraviolet) radiation but absorb the longer-wavelength infrared radiation from the Earth. Some of the absorbed radiation is re-radiated back to Earth’s surface, and some is reradiated back into space.
List the greenhouse acids
Water H2O
Carbon Dioxide CO2
Methane CH4
Dinitrogen oxide N2O
Ozone O3
CFCs
Sulfur hexafluoride
List the source, relative abundance and contribution o increased global warming of: Water H2O
Source: Evaporation of oceans and lakes

Relative abundance: 0.10

Contribution to global warming: –

List the source, relative abundance and contribution o increased global warming of:
Carbon Dioxide CO2
Source: Combustion of fossil fuels and biomass

Relative abundance: 0.036

Contribution to global warming: 50%

List the source, relative abundance and contribution o increased global warming of: Methane CH4
Source: Anaerobic decay of organic matter caused by intensive farming

Relative abundance: 0.0017

Contribution to global warming: 18%

List the source, relative abundance and contribution o increased global warming of: Dinitrogen oxide N2O
Source: Artificial fertilizers and combustion of biomass

Relative abundance: 0.0003

Contribution to global warming: 6%

List the source, relative abundance and contribution o increased global warming of: Ozone O3
Source: Secondary pollutant in photochemical smogs

Relative abundance: 0.000 004

Contribution to global warming: 0.000004

List the source, relative abundance and contribution o increased global warming of: CFCs
Source: Refrigerants, propellants, foaming agents, solvents

Relative abundance: 0.000 01

Contribution to global warming: 14%

List the source, relative abundance and contribution o increased global warming of: Sulfur hexafluoride
Source: Used as an insulator in the electrical industry

Relative abundance: Very low

Contribution to global warming: 0.05%

3 Discuss the influence of increasing amounts of greenhouse gases on the atmosphere.
The predicted consequences of global warming are complex and there is not always agreement. The two most likely effects are:
1. Changes in agriculture such as crop yield
2. Changes in biodistribution as the climate changes (floods, droughts, changes in temperature)
3. Rising sea-levels due to thermal expansion and the melting of polar ice caps and glaciers.
Describe the ozone layer
The ozone layer occurs in the stratosphere between about 12 km and 50 km above the surface of the Earth. Stratospheric ozone is in dynamic equilibrium with oxygen and is continuously being formed and decompose.
Describe the formation of the ozone layer
The strong double bond in oxygen is broken by high energy ultraviolet light from the Sun to form atoms. These oxygen atoms are called radicals as they possess an unpaired electron and are very reactive. One oxygen radical can react with an oxygen molecule to form ozone
UV (high energy)
O=O(g) ————————> 2O*(g)
O*(g) + O2(g) –>O3(g)
Describe the depletion of the ozone layer
The bonds in ozone are weaker so ultraviolet light of less energy will break them. When they are broken the reverse process happens and ozone breaks down back to an oxygen molecule and an oxygen radical. The oxygen radical can then react with can then react with another ozone molecule to form two oxygen molecules.
UV (lower energy)
O3(g) ———————->O2(g) + O*(g)
O3(g) + O* –> 2O2(g)
List the ozone-depleting pollutants and their sources
Measurements have shown that the amount of ozone in the ozone layer has been decreasing. This decrease is due to particular chemicals produced and released by man.
• Chlorofluorocarbons (CFCs): Most common is dichlorodifluoromethane, CCl2F2. These were developed and used for refrigerants, propellants and aerosols, foaming agents for expanding plastics, and cleaning solvents.
• Oxides of nitrogen (NOx): Formed from internal combustion engines, power stations and jet aeroplanes.
Discuss the alternatives to CFCs in terms of their properties
CFCs destroy the ozone layer because the ultraviolet light breaks the relatively weak C-Cl bonds. Substitutes therefore must have similar properties but not contain a bond that can be broken in ultraviolet light to form radicals

The properties required are:
• Low reactivity
• Low toxicity
• Low inflammability
• No weak C-Cl bonds
• Not absorb infrared radiation (otherwise they will act as greenhouse gases)

Replacements:
• HCFCs (hydrochlorofluorocarbons) such as CHF2Cl. They decompose more readily and do not build up in the stratosphere. These are used at the moment.
• HFCs (hydrofluorocarbons), such as CF3CH2F and hydrocarbons, such as 2-methylpropane C4H10, for refrigerants, but they suffer from being flammable and also contribute to global warming.

Importance of oxygen in water
Importance of oxygen in water: At a pressure of one atm and at a temperature of 20°C the maximum solubility of oxygen in water is only about 9ppm (0.009g dm-3). This value is crucial as aquatic plants and animals require this oxygen for aerobic respiration. The content of oxygen should not be less than 6 ppm
Define BOD in water
The BOD is the quantity of oxygen needed to oxidize organic matter in a sample of water over a five-day period at a specified temperature
How can BOD be measured
The BOD can be measured by saturating the water sample with oxygen so that its concentration is known at 0.009g dm-3. The sample is then left at 25°C for five days. This allows the bacteria time to use some of the dissolved oxygen to decompose any organic material in water. The oxygen remaining is measured using a redox titration called the Winkler method.
Distinguish between aerobic decomposition anaerobic decomposition of organic material in water
• If sufficient oxygen is present, organic material will decay aerobically and oxides or oxyanions are produced.
• Anaerobic decay involves organisms which do not require oxygen. The products are in the reduced form and are often foul selling and toxic
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