E11 Acid deposition (HL)

E.11.1 Describe the mechanism of acid deposition caused by the oxides of nitrogen and oxides of sulfur.

Two primary pollutants that cause acid deposition are sulphur dioxide and nitrogen dioxide. They are converted into acids by a free radical mechanism involving hydroxyl free radicals, formed either by the reaction of water vapour with ozone or by the reaction of water vapour with oxygen free radicals formed when ozone decomposes.

H2O(g) + O3 (g) --> 2HO∙(g) + O2(g)
H2O(g) + O∙(g) --> 2HO∙(g)

Reacts with sulphur dioxide and nitrous oxides in the presence of water to give the dissolved acids.

HO∙(g) + NO2(g) --> HNO3(aq)
HO∙(g) + NO(g) --> HNO2(aq)
HO∙(g) + SO2(g) --> HOSO2∙(g)
then HOSO2∙(g) + O2(g) --> HO2∙(g) + SO3(g)
followed by SO3(g) +H2O(l) --> H2SO4(aq)

E. 11.2 Explain the role of ammonia in acid deposition.
The atmosphere contains trace amounts of ammonia and also in the soil from the action of rhizobia (bacteria) in the root nodules of leguminous plants. The ammonia in the atmosphere neutralises partially the acids to form ammonium sulphate, which is a slightly acidic as it is a product of a weak base and strong acids. As they are washed out by precipitation or sink to the ground the ammonium ion is deposited and enters the soil where acidification and nitrification an occur.

NH4+(aq) + 2O2(g) --> 2H+(aq) + NO3-(aq) + H2O(l)

E10 Smog

E.10.1 State the source of primary pollutants and the conditions necessary for the formation of photochemical smog.

Smog is a poisonous mixture of smoke, fog, air, and other chemicals, is formed in large cities and is favoured by lack of wind and bowl-shape cities (land that is sounded by higher ground in all directions). Smog is likely to occur when there is a thermal inversion, whereby a layer of still warm air to blanket a layer of cooler air, so that convection currents cannot form). As a result, trapped pollutants cannot rise and can rise to dangerous levels if the stillness persists.

‘Pea smog’ (classically English) occurred before the introduction of clean air controls in the mid 1950s, whereby the combustion of coal and oil produced sulphur dioxide mixed with soot, fly ash and partially oxidising organic material (reducing smog). Photochemical smog is the more common smog now (e.g. Los Angeles) that is converted into secondary pollutants in sunlight, consisting of oxides of nitrogen and VOCs (brown/yellow colour).

E.10.2 Outline the formation of secondary pollutants in photochemical smog.

NO2(g) --> NO(g) + O ·(g)

Nitrogen dioxide is broken down to give an oxygen free radical and nitrous oxide. The oxygen radicals can react with oxygen to form ozone and with water to form hydroxyl radicals. These photochemical oxides can react with a variety of molecules including nitrous oxide to form nitric acid and VOCs to form peroxides (ROOR), aldehydes (RCHO), and ketones (RCOR). Peroxides are extremely reactive and aldehydes and ketones reduce visibility by condensing to form aerosols.

O·(g) + H2O (l) --> 2OH·(g) [hydroxyl radical]

OH·(g) + NO2(g) --> HNO3(aq) [nitric acid]

OH·(g) + RH(g) --> R·(g) + H2O [radical]

R·(g) + O2 --> ROO· [peroxide radical]

E9 Ozone depletion (HL)

E.9.1 Explain the dependence of O2 andO3 dissociation on the wavelength of light.

The amount of energy required to break an O=O bond is 496kJ/mol. Using the formulas E=hf and C=λf gives λ=hc/E, where h is Planck's constant and c is the velocity of light. To achieve this amount of energy, the wavelength of light required would be 242nm, as calculated from the formula, and is in the high energy region of the ultraviolet spectrum.

For ozone dissociation, ozone has two resonance hybrids, each with one double bond and one single bond, and as only the single bond is broken less energy is required (light of a lower wavelength) i.e. Ultraviolet light with wavelength 330nm. Working backwards, this means the strength of the O-O bond in ozone is 362kJ/mol.

E.9.2 Describe the mechanism in the catalysis of O3 depletion by CFCs and NOx.

CFCs catalyse the destruction of ozone because the high energy ultraviolet light in the stratosphere causes the homolytic fission of the C-Cl bond that produces chlorine free radicals, that then go one to break down ozone molecules and generate more radicals in a continuous process until the radicals eventually escape/terminate. One molecule of CFC can break down 100000 molecules of ozone.

CCI2F2 (g)-->CCIF•(g) + CI•(g)
CI•(g) + O3(g) --> ClO•(g) + O2(g)
CI O•(g) + O•(g) --> O2(g) + Cl•(g)

Evidence for this mechanism can be seen in how an increase in chlorine monoxide in the antarctic mirrors the decrease in ozone concentrations.

Nitrogen oxides also catalytically decompose ozone by a radical mechanism, whereby oxygen radicals are generated by the breakdown of NO2 in ultraviolet light.

NO2 (g) --> NO(g) + O•(g)
O•(g)+ O3(g) --> 2O2 (g)
NO(g) + O3(g) -->NO2 (g)+ O2(g)

Overall: 2O 3(g) --> 3O 2 (g) [NO2- homogenous catalyst]

E.9.3 Outline the reasons for greater ozone depletion in polar regions.

During winter, low temperatures cause small amounts of water vapour in the air to freeze and form ice crystals, which contain small amounts of chlorine compounds (e.g. HCl, ClONO2) that react in the presence of catalysts to form hypochlorous acid (HClO) and chlorine (Cl2). In spring, when the sun shines, these molecules break down to produce chlorine radicals which catalyse the destruction of ozone, with the largest holes in the ozone occuring during early spring. As the temperature increases and warmer winds blow into the region, the ice crystals disperse and the ozone concentration gradually increases.

OTHERS:

Sun-screening compounds --> Sunlight can be harmful; causes genetic mutations and damage to proteins. Ozone decreases more harmful ultraviolet light in reaching the Earth's surface. Other sun-screens include glass, melanin (pigment in body that is increased by the action of the sunlight on the skin), commercial substances (e.g. PABA [para-aminobenzoic acid) that absorb light of a particular frequency (e.g. for PABA, 265nm) as they contain delocalised π electrons with conjugated double bonds (alternate between C-C and C=C) which are excited to higher energy levels when they absorb UV light. Sunblocks include white pigments, like titanium dioxide or zinc oxide, which reflect and scatter sunlight.

E8 Waste

E. 8.1 Outline and compare the various methods for waste disposal.

Landfills: efficient method to deal with large volumes, filled land can be used for other purposes. However local residents may object to new sites, filled land needs time to settle and requires maintenance as methane is released.

Open dumping: Convenient and inexpensive. However causes air, ground and water pollution; health hazard and attracts pests, and unsightly.

Ocean dumping: Source of nutrients, convenient and inexpensive. However dangerous to marine animals, pollutes the sea.

Incineration:Reduce volume, requires minimal space, produces stable odourless residue, can be source of energy. However expensive to build/operate, cause pollutants if inefficiently burned, requires energy.

Recycling: provides a sustainable environment. However expensive, difficult to separate different materials (sometimes impossible).

E.8.2 Describe the recycling of metal, glass, plastic and paper products, and outline its benefits.

Metal: Mainly aluminium and steel. Sorted and melted, or re-used directly or added to purification stage of metals formed from their ores. Important for metals like aluminium (large amounts of energy needed to produce direct from ore).

Glass: Sorted by colour, washed, crushed and then melted and moulded into new products. Not degraded during the recycling process so can be recycled many times.

Plastic: Sorted (though this may be problematic), degraded to monomers by pyrolysis, hydrogenation, gasification and thermal cracking, then repolymerised. Recycling causes less pollutants and uses less energy than producing new plastics from crude oil.

Paper: Sorted into grades, washed to remove inks, made into a slurry to form new types of paper. High transportation costs- may be more efficient to compost.

E.8.3 Describe the characteristics and sources of different types of radioactive waste.

Two types of radioactive waste: High level waste and low level waste.

Low level waste is where activity is low and the half-lives of the radioactive isotopes are generally short lived. Such items include rubber gloves, paper towels and protective clothing that have been used to handle radioactive materials.

High level waste is where activity is high and the half-lives of the radioactive isotopes are generally long and so the waste remains active for a long period. Most high level waste comes from spent fuel rods or the reprocessing of spent nuclear rods.

E.8.4 Compare the storage and disposal methods for different types of radioactive waste.

Low level waste:

-is sometimes discharged straight into the sea, but this is now banned by many governments and so the following methods are used:

-produces heat during decay and so is stored in 'ponds' of cooled water where it loses much of its activity. Before discharge, it is filtered through an ion exchange resin which removes stronium and caesium (elements responsible for radioactivity).

-Storage of waste in steel containers inside concrete-lined vaults.

High level waste:

-96% uranium is recovered during reprocessing for reuse.

- 1% is plutonium (valuable fuel)

-3% is level liquid waste that is vitrified. The liquid waste is dried in a furnace and then fed into a melting pot together with glass making materials. This molten material is then poured into stainless steel tubes where it solidifies, with air flowing around the containers to keep them cool. High activity and long half-lives means that the waste will remain active for hundreds of years, and hence the solid matter should be buried in a geologically stable place such as disused mines or in granite rock. However there is concern that the radioactive material may eventually leach into the water table and then into drinking water.

E7 Soil

Soil is composed of inorganic and organic material including living organisms, and its composition varies considerably. Soil degradation is when the quality of soil has been affected in some way so that crop production is lowered, and can be caused by changes in weather pattern or manmade factors (i.e. acidification, desertification, contamination, erosion and salinization, timber cutting, overgrazing, industrailization).

http://geodata.grid.unep.ch/mod_map/map.php


E.7.1 Discuss salinization, nutrient depletion and soil pollution as causes of soil degradation.

Salinization: Constant or excess irrigation using water that contains salts, which upon evaporation, remain in the soil and accumulate in the fertile topsoil. These salts can either build up to toxic levels in the plant or dehydrate plants by preventing the uptake of water, both ways resulting in the death of crops.

Nutrient depletion: Intensive farming, without proper management practises results in nutrients being depleted without replacement. Plants require minerals and nutrients for healthy growth and by harvesting these plants, the normal cycling of nutrients through the soil food web is unable to occur. As a result, there is a amelioration of nutrient depletion. Soil should be left to fallow for some periods and organic material like manure and compost should be added to maintain the levels of nutrients.

Soil pollution: Caused by a variety of factors- industrial discharge and improper dumping of toxic waste material ( long term soil pollution), organic soil pollution from the transport and illegal dumping of spent engine oil (short term effect), use of pesticides and fertilizers disrupt the food chain and reduces soil's biodiversity, pollutants can run into surface waters and move through the soil through throughflow and percolation, polluting groundwater.

E. 7.2 Describe the relevance of the soil organic matter (SOM) in preventing soil degradation, and outline its physical and biological functions.

Soil requires not only minerals to allow healthy plant growth, but also organic matter (e.g. in compost). SOM represents the organic constituents of the soil including undecayed plant and animal tissues, their partial decomposition products (i.e. polysaccharides, proteins, sugers, amino acids) and the soil biomass. Humus is a complex mixture of simple and more complex organic chemicals of plant, animal or microbial origin.

SOM can be used in three main ways:

-biological

-chemical

-physical

BIOLOGICAL: Humus provides a source of energy and essential nutrient elements (N, P, S); contributes to resilience of soil/plant system.

PHYSICAL: Humus is involved in structural stability and water-retention and thermal properties. It helps soil to retain moisture and therefore increases capacity to withstand drought, and encourages formation of good soil structure. Its dark colour causes it to absorb more heat and this hence helps warm the soil in colder seasons.

CHEMICAL: Cation exchange capacity helps it act like clay. Contains active sites that bind to nutrient cations to prevent them from being washed away and making them more available to plants. Toxic cations also bind to humus and this stops them from entering the wider ecosystem. Humus acts as an acid-base buffer and enhances the ability of the soil to maintain a constant pH.

E.7.3 List common organic soil pollutants and their sources.

1) Hydrocarbons, VOCs, SVOC (semi-volatile organic compounds): from transport, solvents and industrial processes.

2) Agrichemicals: pesticides, herbicides, fungicides

3) Polyaromatic hydrocarbons: incomplete combustion of coal, oil, gas, wood and garbage.

4) Polychlorinated biphenyls (PCBs): coolant and insulater in electrical equipment (e.g. transformers, generators)

5) Organotin compounds: Bactericides and fungicides used in paper, wood, textile and anti-fouling paints.

E6 Water treatment

75% of the Earth's surface is covered by water. 97.3% of this is ocean, 2.1% glaciers and ice caps, 0.6% ground water, 0.015% lakes and rivers, and 0.001% in the atmosphere.

E.6.1 List the primary pollutants found in waste water and identify their sources.

Water is capable of hydrogen bonding and is highly polar- this allows is to dissolve many chemicals and as a result some toxic substances, bacteria and viruses can be carried by water. Pollutants in water arise from human usage of water for personal use and industry, and the subsequent release of water into the environment. Water unsuitable for drinking, irrigation, industrial use or washing is considered polluted water.

Pollutants include:

- Heavy metals. i.e. Cadmium from rechargeable batteries, metal plating and pigments. Copper from household plumbing, copper mining and smelting. Mercury from batteries, mercury salts (fungicides), mercury cells in chlor-alkali industry, discharge from pulp and paper mills. Lead, Zinc.

- Pesticides from agricultural practices, like DDT, fungicides and herbicides.

-Nitrates: from acid rain and artificial fertilizers. Causes infantile methaemoglobinaemia (oxygen starvation) because of less acid in babies' stomachs that are converted to nitrite by bacteria- this oxidises the iron in haemoglobin irreversibly preventing it from binding with oxygen. In adults, nitrites are converted into nitrosamines (carcinogenic).

-Dioxin: Waste materials containing organochlorine compounds form dioxins when they are not incinerated at high enough temperatures. It accumulates in fat and liver cells and causes malformation of fetuses.

-Polychlorinated biphenyls (PBCs): One to ten chlorine molecules attached to a biphenyl molecule, chemically stable and high electrical resistance. Accumulate in fatty tissues and remain in the environment; affect reproductive efficiency, impair learning in children, carcinogenic.

E.6.2 Outline the primary, secondary and tertiary stages of waste water treatment, and state the substance that is removed during each stage.

Raw sewage can either be discharged untreated into water bodies, or, in rural areas, deposited into cesspits/septic tanks where they are broken down before they leach into the ground. However, the treatment of waste enables fresh water to be recycled from it.

Primary treatment: Removes 60% of solid material and 1/3 of oxygen demanding wastes. Coarse mechanical filters are used to remove large objects and then a sedimentation tank allows for suspended solids to settle out as sludge. Calcium hydroxide and aluminium sulphate are added to form aluminium hydroxide, which precipitates together with suspended dirt particles in flocculation. Grease is removed by skimming, and the effluent is discharged into a waterway or for secondary treatment.

Secondary treatment: removes 90% of oxygen-demanding wastes, where waste are aerobically degraded using oxygen and bacteria. Two methods- a) waste water is left to trickle through a bed of stones with bacteria; b) sewage is aerated with pure oxygen in a sedimentation tank. Sludge contains active microorganisms that digest organic waste (activated sludge) and some of it is recycled. Water is discharged into a water way where chlorine is added for disinfection or ozone.

Tertiary treatment: expensive; removes heavy metal ions, nitrates, phosphates and residual organic compounds.

- Precipitation- Aluminium sulphate and calcium oxide can be used to precipitate phosphates. Heavy metal ions can be precipitated as insoluble hydroxides/basic salts by the addition of calcium hydroxide or sodium carbonate, or insoluble sulphides by the bubbling of hydrogen sulphide. However some metal hydroxides redissolve as the complexes are soluble (Zn, Hg, Cd).

- Activated carbon bed- carbon is activated by heating periodically to high temperatures and this removes dissolved organic material by oxidising the adsorbed organic compounds to carbon dioxide and water and regenerates the carbon surface.

- Removal of nitrates- ion exchange zeolites can be used to exchange hydroxide ions for nitrates but this is very expensive. The other method would be to use denitrifying bacteria to reduce them to nitrogen or to pass the water through algae pools as algae utilize nitrates as nutrients.

E.6.3 Evaluate the process to obtain fresh water from sea water using multi-stage distillation and reverse osmosis.

Desalination removes salts from sea water (3.5%) that make the water unfit for consumption and for industrial or agricultural purposes. Seawater is heated in a series of coiled pipes before being introduced to a partially evacuated chamber where the water boils instantaneously under the reduced pressure and is condensed and piped off as freshwater. A multi-stage distillation system is often used so that heat released from the condensation of steam can be reused to heat more seawater. This distillation system separates more volatile water from the less volatile salts, and is quite a costly system to maintain.

Reverse osmosis is when pressure is applied to a solution across a semi-permeable membrane that is greater than the osmotic pressure (of water diffusing across the membrane down the concentration gradient) which in turn causes osmosis to occur in the reverse direction, leaving the salts behind as water is forced out of the salt solution. In this case, the membrane used is cellulose ethanoate. It does not require a phase change and hence needs less energy, but still requires energy to produce the necessary pressure (70atm) for reverse osmosis.

E5 Dissolved oxygen in water

For animals to survive in aquatic systems, water must contained a minimum concentration of dissolved oxgyen as the organisms require oxygen for aerobic respiration. Oxygen is a diatomic non-polar molecule that is only slightly soluble in water, as water is polar. At 20°C, the solubity of oxygen is ≈0.00028cm3, which is 9mg of water per million mg of water (9ppm). Solubility decreases as temperature rises.

Quality of water is dependent on the dissolved-oxygen (DO) content of a body of water. Water of good quality has a DO content of 8-9ppm, while moderately polluted water is about 4.5ppm, and anything below that concentration is highly polluted. Quality of water depends on factors like oxygen demanding wastes, like organic substances (plants, animals, human waste) and waste from industrial processes (meat packing and food processing plants and paper mills), and diseases causing microorganisms (pathogens).

E.5.1 Outline biochemical oxygen demand (BOD) as a measure of oxygen demanding wastes.

BOD is the measure of the amount of oxygen consumed by biodegradable organic wastes and ammonia in a given amount of water over a time period, normally 5 days, at 20°C. A sample is diluted in oxygen saturated water and enclosed without any air in a BOD sample bottle for a 5 day incubation period, after which the decrease in dissolved oxygen is measured using an oxygen electrode. Almost pure water has a ppm BOD of less than 1, while water of doubtful purity has a ppm BOD of 5, and water of unacceptable purity a ppm BOD of 20 (e.g. untreated sewage has a BOD of 100-400ppm, while food processing plants can be up to 10000ppm).

E.5.2 Distinguish between aerobic and anaerobic decomposition of organic material in water.


Aerobic decomposition
-Oxidation process where complex organic matter is broken down into simple organic material, carbon dioxides and water. Simple organic material can be further converted to nitrates, sulphates, and phosphates.

-DO decreased as oxygen is used up, and if too much organic matter is present, DO may decrease to zero and kill aquatic life that depends on oxygen.

Anaerobic decomposition
-Typical products include ammonia and amines from nitrogen and hydrogen (strong fishy smell), methane/biogas/marsh gas from carbon and hydrogen, hydrogen sulphide (rotten egg smell) from organic sulphur, and phosphine (PH3) phosphorus.
-Oxygen is not used up.

E.5.3 Describe the process of eutrophication and its effect.

Plant nutrients are often soluble and leach into water bodies. This leads to excessive growth of aquatic plant life, often in the form of algal blooms.

Effects:

-water smells and tastes bad, becomes lifeless.
-red tides: marine plankton produce chemical toxins.

-Dead plants are decomposed anaerobically, depleting oxygen

-Fish die from asphyxiation (lack of oxygen)

-Anaerobic processes produce toxic substances like phosphine and hydrogen sulfide.

E.5.4 Discuss the source and effects of thermal pollution of water.

Thermal pollution occurs when water that is heated in power plants/industrial processes is dumped into streams, rivers or lakes. Two major effects would be on dissolved oxygen and the metabolic rates of aquatic life.

OXYGEN:

Concentration of oxygen decreases as temperature increases. Furthermore, warm water is less dense than cool water and stays near the top. This water is unable to absorb as much oxygen from the atmosphere; at 0°C, DO is 15ppm, at 20°C, DO is 9ppm, and at 40°C, DO is 6.5. This is for pure water (polluted water will have even lower DO contents).

AQUATIC LIFE:

Increased temperatures increase the rate of biochemical processes, and hence the metabolic rates of aquatic animals increase and, in so doing, require more oxygen. The rate of consumption of oxygen increases.

E4 Ozone Depletion

E.4.1 Describe the formation and depletion of ozone in the stratosphere by natural processes.

Ozone is a naturally occuring component of the stratosphere 15 to 45 km above the Earth's surface. It is a powerful oxidising agent and pale blueish gas with an acrid odour. Ozone can have harmful effects on living matter but is also essential to life and health (ozone layer).

Ozone is formed from the photo-dissociation of molecular oxygen by uv light. Free radicals are formed from the splitting of oxgyen molecules into oxygen atoms by high energy, short wavelength UV light from the sun. These oxygen atoms then react with other oxygen molecules to form ozone.

The resonance structure of ozone is as follows:

It suggests that the two bonding electrons from the pi bonds are spread over the entire structure of the molecule with a bond order of 1.5 (weaker bond than oxygen). As a result, less energy (i.e. longer wavelength light) is required to break the bonds in ozone than in oxygen.

The depletion of ozone hence occurs when the reverse reaction occurs and ozone absorbs longer wavelength UV light to form an oxygen molecule and an oxygen free radical. An oxygen free radical reacts with ozone to form two molecules of oxygen gas.

Ozone is hence constantly being formed and broken down, and acts as a shield by absorbing 93-99% of the sun's harmfulUV light of longer wavelength than that absorbed by oxygen and nitrogen.

E.4.2 List the ozone-depleting pollutants, and their sources.

Depletion of the ozone layer has been observed from satellite data. There are holes in the ozone layer over the north and south poles, although this is partially seasonal (e.g. with lowest levels in the antarctic spring).

Ozone is depleted by chlorofluorocarbons. CFCs are chemically inert when released but become chemically reactive chlorine atoms when they reach the unfiltered UV rays of the sun in the stratosphere.

The bond enthalphy for C-F is higher than that of the C-Cl bond, and as a result a chlorine free radical is formed, that reacts readily with ozone to produce oxygen.

CCl2F2 + uv light --> ∙Cl +CClF2
∙Cl + O3 --> ClO∙ + O2

Nitric oxide (from aircraft exhausts) react with ozone to form nitrogen dioxide and oxygen.

The depletion of ozone means that more harmful UV rays reach the Earth, increasing skin cancer, eye cataracts, genetic mutations, sunburn, damage to animals and plant (suppression of plant growth) and causing changes in the world's climate.

E.4.3 Discuss the alternatives to CFCs in terms of their properties.

Properties of CFCs that make it popularly used in aerosols and as solvents would be their lack of reactivity, low toxicity and low flammability. Alternatives to CFCs should have no C-Cl bonds and show little absorption of infrared radiation so as to to behave like a greenhouse gas.

1. Hydrocarbons are used as refrigerator coolants and do not cause ozone depletion, but are flammable and are greenhouse gases.

2. Hydrochloroflurocarbons decompose less easily because of the presence of hydrogen as the bond enthalpy for the C-H bond is higher than that of the C-Cl bond. However, ozone is still depleted because of the presence of a C-Cl bond.

3. Fluorocarbons have low reactivity, are neither toxic nor flammable and are stable to UV radiation because of the strong C-F bond. However, these are greenhouse gases.

4. Hydrofluorocarbons are also ideal as they do not contain chlorine atoms and have low reactivity, low toxicity and low flammability but contribute to global warming.

For more on ozone depletion, click here.

E3 Greenhouse Effect

E.3.1 Describe the greenhouse effect.

The Greenhouse Effect is the trapping of heat in the atmosphere. Short wavelength sunlight (visible light and UV) penetrates the atmosphere to warm the Earth's surface which partially absorbs the energy before radiating it back out as longer wavelength radiation (infrared). This long-wavelength radiation is partially retained in the atmosphere and only some is re-emitted back out.

http://arcticclimatemodeling.org/Movies/greenhouse_effect_dvd_sample.html

Greenhouse gases, like carbon dioxide, methane, CFCs, water vapour and nitrous oxides, trap as a one-way filter, trapping heat like in a glass/plastic in a greenhouse. This heating effect is necessary for the maintanence of life on Earth and is what maintains global temperatures. However, with increased concentrations of greenhouse gases, the greenhouse effect is 'enhanced', with even more radiation being trapped in the atmosphere, causing a significant rise in global temperatures known as global warming. The rate of temperature change is significantly faster than any observed in the last 10000 years.

E.3.2 List the main greenhouse gases and their sources, and discuss their relative effects.

Water vapour: main source is the evaporation of water bodies and the combustion of hydrocarbons. Occupy 0-4% of the atmostphere, and

E.3.3 Discuss the influence of increasing amounts of greenhouse gases on the atmosphere.

Over the last century worldwide:

1. Increase in temperatures by 0.5ºC

2. 1% increase in precipitation

3. 15-20cm rise in sea levels from glacial meltdown and expansion of ocean water by warmer temperatures.

--> Particularly in the Arctic, global warming from rising levels of carbon dioxide is particularly irreversible because of how the melting of permafrost results in further decomposition of the previously frozen matter, causing a continual increase in carbon dioxide and methane levels.

http://arcticclimatemodeling.org/Movies/permafrost_dvd_sample.html

Scientific models are also being exceeded- the prediction of scientific models are taking place 30 years sooner than expected.

IMPACT OF CLIMATE CHANGE ON HEALTH, AGRICULTURE, FORESTS, WATER RESOURCES, COASTAL AREAS, SPECIES DIVERSITY, SPECIES NUMBERS AND NATURAL AREAS:

-Health
Life cycles of pathogens and insects affected (e.g. Mosquitoes). Water-borne diseases may become more prevalent.
-Agriculture
Crop yield and crop distribution will be affected. Flooding of land from sea water may result in salination of the water table and affect crops requiring fresh water.

-Forests
Insects and diseases may increase, summer droughts may produce forest fires, higher temperatures and higher increased precipitation may cause increase in vegetation growth, but plants requiring little rainfall may become extinct.

-Water Resources
Decreased water quality due to flooding, more resources needed to turn water into potable sources. Both floods and droughts are more likely, from increased precipitation and increased rates of evaporation.

-Coastal Areas
Eroding of beaches, flooding of low lands and coastal flooding, and resulting loss of such ecosystems.

-Species and Natural Areas
Loss of cold water fish habitat, shift in ecological areas (organisms in temperate regions may migrate upwards to previous uninhabitable regions). Desertification. Loss of habitats and species.

E2 Acid Deposition

E.2.1 State what is meant by the term acid deposition and outline its origin.



Acid deposition refers to acidic particles and gases that deposit or fall to the Earth.

e.g. WET deposition: acidic gases like oxides of sulphur and nitrogen and acidic particles brought down by precipitation by rain, fog and snow. DRY deposition: absense of precipitation.

Pure rain water has a pH of ≈5.6, because carbon dioxide dissolves in rain to form carbonic acid. As a result, environmental pollution called acid rain is any rain with pH less than 5.6, and occurs commonly in industrailised areas (pH≈4-5 [4 to 40 times greater than pure rain water]). Extreme case of acid rain resulted in pH of 1.7 (LA, Dec 1982), and also a pH of 4.0 can kill fish life to form dead lakes (North America, China, Russia).

Acid deposition is associated with parts of a country with heavy industries and down-wind from industrial sites. Oxides of sulphuur and nitrogen are mostly responsible for this acidity and precipated become contaminated with acids when these oxides are present as pollutants.

Formation from sulphur oxides:

- sulphur dioxide + water --> sulphurous acid (stronger than carbonic acid)

-sulphur dioxide+ oxgyen--> sulphur trioxide (formed in the presence of oxygen, ozone, hydroxyl free radical and sunlight)

sulphur trioxide + water --> sulphuric acid

Formation from nitrogen oxides:

-nitrogen dioxide + water --> nitric acid + nitrous acid (absence of oxygen)

-nitrogen dioxide + water + oxygen --> nitric acid (consists of more complicated steps, summarised in this equation)

- other pollutants like the hydroxyl free radical (formed during photochemical smog) can react with nitrogen dioxide (also a free radical with a lone electron) to form nitric acid [accelerates conversion of nitrogen dioxide to nitric acid].

E.2.2 Discuss the environmental effects of acid deposition and possible methods to counteract them.

Acid deposition affects humans, aquatic life, materials, soil, vegetation and visibility.

Humans:
· Irritates whole respiratory tract and the eyes.
· Sulphate particles penetrate the lungs where they become embedded, adverse effects on asthmatics, the elderly and the young. (Sulphate aerosols are powerful irritants)
· Increased concentrations of aluminum ions in water due to acidic conditions may lead to Alzheimer’s disease.

Aquatic life:
· Eutrophication from nitrates in acid deposition.
· Aluminum ions from leaching of soil by acid rain affect the function of the gills.
· Acidification of water bodies may result in the death of some species of fish.

Materials:
· Corrosion of basic materials (e.g. marble, limestone, dolomite).
· Insoluble carbonates are converted to more soluble sulphates that then dissolve- destroys structural and artists’ stone (stone leprosy).
· Corrosion of iron and steel promoted by acid rain, worsened by high humidity, high temperatures, presence of particulates.
· Leaching of toxic heavy metals (e.g. Lead, Cadmium, Mercury) into the water system.
· Bleaching and weakening of fabrics and leather.
· Dicolouration and embrittlement of paper.
· Deterioration of electrical appliances.

Soil and vegetation:
· Acute injury from short term exposure to high acid concentrations consists of dead areas of leaves which dry out and usually become bleached.
· Chronic injury from long term exposure to low acid concentrations consists of bleached spots, yellowing of leaves, suppression of plant growth and reduction in yield (acid rain disrupts chlorophyll synthesis).
· Leaching and removal of important nutrients such as magnesium ions from the soil.

Visibility:
· Mist can causes great loss of visibility and affect air flights.

METHODS OF COUNTERACTING ACID DEPOSITION:
-pre-combustion methods or post-combustion methods can be used to prevent the formation of acidic particles/gases.
-Promotion of alternative energy sources and conserving energy would decrease combustion of fossil fuels.
-Adding calcium carbonate or calcium hydroxide to soil and lakes neutralizes acidity.

For more on acid deposition, click here.

E1 Air pollution

E.1.1 Describe the main sources of carbon monoxide (CO), oxides of nitrogen (NOx) oxides of sulphur (SOx), particulates and volatile organic compounds (VOCs) in the atmosphere.

Unpolluted air contains 78% nitrogen, 21% oxygen, 1% argon, 0.04% carbon dioxide and trace amounts of other gases with up to 4% water vapour. An air pollutant is a substance normally absent in air, or a substance that is normally present but in excess amounts. Primary pollutants are carbon monoxide, oxides of nitrogen, sulphur dioxide, particulates and hydrocarbons. Secondary pollutants are compounds formed when primary pollutants react in the air.

CARBON MONOXIDE:
-natural source: incomplete oxidation of methane
-man-made source: incomplete combustion of fossil fuels
-effect on health: prevents haemoglobin from carrying oxygen by binding irreversibly to form carboxyhaemoglobin.

-methods of reduction: use lean burn engine, thermal exhaust reactor, or catalytic converter.

OXIDES OF NITROGEN:
-natural source: electrical storms and biological processes
-man-made source: at high temp.s inside internal combustion engines.
-effect on health: respiratory irritant that results in infections
-methods of reduction: use of lean burn engine, recirculation of exhaust gases or catalytic converter.

SULPHUR DIOXIDE [can be oxidised to form sulphur trioxide]:
-natural source: oxidation of H2S produced by volcanoes and decay of organic matter
-man-made source: combustion of sulphur-containing coal and smelting of sulphide ores
-effect on health: respiratory irritant that results in infections
-methods of reduction: removal of sulphur from fossil fuels before combustion, alkaline scrubbing, fluidised bed converter.

PARTICULATES:
-natural source: soot, ash, dust, asbestos, sand, smoke, pollen, bacterial and fungal spores
-man-made source: burning of fossil fuels (esp. coal and diesel)
-affects respiratory system, causing lung diseases (e.g. emphysema, bronchitis, cancer)
-methods of reduction: sedimentation chambers, electrostatic precipitation.

VOCs (CxHy or R-H):
-natural source: plants (like rice; emit unsaturated hydrocarbons called terpenes)
-man-made source: unburned or partially burned gasoline and other fuels, solvents.
-effect on health: carcinogens (e.g. benzene), can form toxic secondary pollutants (e.g. PANs)
-methods of reduction: catalytic converter

E.1.2 Evaluate current methods for the reduction of air pollution.

1) Thermal exhaust reactor: Exhaust from car engine is combined with more air and reacts (at high temperature from the heat of the exhaust gas). CO becomes CO2 and unburned hydrocarbons are combusted.

2) Lean burn engines: Carburettor is adjusted altering ratio of air:fuel. Higher ratios results in less CO emitted (more complete combustion). However, this results in higher temperatures that increase the formation of oxides of nitrogen. Conversely, lower ratios cause less oxides of nitrogen to form but more carbon monoxide.

3) Catalytic converter:

Hot exhaust gases are passed over catalysts (i.e. platinum/palladium/phodium) that complete combust CO and unburned VOCs, as well as catalysing the reaction between nitrous oxide and carbon monoxide to give carbon dioxide and nitrogen gas.

2CO + 2NO --> 2CO2 + N2

4) Removal of sulphur from fossil fuels (e.g. metal sulphides in coal) [pre-combustion]

i) Coal gasification removes most of the sulphur in high sulphur coal when converting it to synthetic natural gas (SNG). Any sulphur is converted to H2S (acidic and easily removed). However, this process requires a lot of energy.

ii) use low sulphur, cleaner burning coal (e.g. anthracite) with high heat content. However, such coal is scarcer and more expensive.

iii) remove sulphur in petroleum refining and natural gas processing with a catalyst to hydrogen sulphide.

iv) coal washing: sulphur removed from coal by crushing the coal and mixing it with water, so that the denser sulphides sink and the cleaned coal can be skimmed from the surface. However, a lot of sulphur is trapped below the surface of the particles and organic sulphur (chemically bonded) cannot be physically removed.

5) Alkaline scrubbing [post-combustion]: alkaline slurry of limestone and lime (large amounts required, and heating to convert limestone to lime requires large amounts of energy which is expensive) [or magnesium hydroxide] are used to remove sulphur dioxide from the exhausts of coal-burning plants, with the resulting sludge used to for landfills, or gypsum [CaSO4.2H2O] for plasterboards. A wet alkaline scrubber uses liquid (usually water-based) to remove contaminants by sprayin ghte alkaline liquid in downwards as the gas streams upwards).

CaCO3(s) + SO2(g) --> CaSO3(s) + CO2(g) (note CO2 is a greenhouse gas)

CaO(s) + SO2(g) --> CaSO3(s)

2CaSO3(s) + O2(g) + 4H2O(g) --> 2CaSO4.2H2O(s)

6)Fluidized bed combustion [post-combustion]: burning of coal on a bed of limestone removes sulphur as calcium sulphite or sulphate as the coal burns.

7) Electrostatic precipitation: Soild or liquid particles are suspended in the air, with larger particles allowed to settle in sedimentation chambers (gravity) while an electrostatic precipitation chamber can be used for smaller particles. Charged particles are attracted to oppositely charged electrodes which are shaken periodically to remove aggregated particulates from the bottom of the precipitator.

Environmental Chemistry (Option E)

This blog aims at covering the Environmental Chemistry Option in the IB Diploma using computer simulations and data bases. Be awed.

* Information taken from Chemistry 3rd Edition, by John Green and Sadru Damji.

Databases:
http://www.agnet.org/library/
http://geodata.grid.unep.ch/
http://www.brighthub.com/environment/science-environmental/articles/16695.aspx