Environmental Chemistry Option

1. Environmental Chemistry Option

2. E1.1 - Pollution • •Pollution refers to changes in the equilibrium (or balance) of biological and non-biological systems, as a result of human activity • •Although many so-called pollutants are substances that occur naturally, such as ozone or carbon dioxide, human activity has led to an increase in the concentrations of such substances, which upsets the delicate balance of natural cycles • •The atmosphere consists of a relatively thin layer of gas surrounding the Earth. (to 100km) • By comparison, the earth is 6400km in radius

3. E1.1 – Layers of the Atmosphere • •The atmosphere consists of four layers, separated by a change in temperature gradient • Troposphere • -Most of human activity takes place here • -Up to 10-12 km • -90% of matter in the atmosphere • Stratosphere • Mesosphere • Thermosphere

4. E1.1 - Troposphere •In the troposphere temperature falls with increasing height, so that at a height of 12 km the temperature is about -55oC •This temperature gradient allows convection currents (warm gases rise, cool gases sink) causing mixing in the atmosphere pollutants. •Pollutants at ground level quickly spread throughout the troposphere (not true in stratosphere). •Also, horizontal movement of air masses (wind) causes lateral dispersion of pollutants as well.

5. E1.1 – Atmospheric Pollutants •As discussed, many gases we consider pollutants, also occur naturally (CO, CO2, etc) •Anthropogenic, or man-made, output is often not even a large proportion of the worldwide total but can cause issues due to high concentrations in localized areas. •Many atmospheric pollutants arise from the combustion of fossil fuels in motor vehicles or in power stations. This effect has the potential to be minimized

6. E1.1 – Sources of Pollutants

7. E1.2 – Reduction of Pollution •In the internal combustion engine, hydrocarbon fuels are mixed with air, injected into a cylinder and ignited with a spark •The resultant explosion forces the piston to move and is converted to the rotation of the crankshaft, which in turn drives the wheels of the vehicle •We can have complete, partial, and incomplete combustion of the hydrocarbon, ideally it would be complete as follows: 2C8H18+ 25O2→ 16CO2 + 18H2O  Ratio of air/fuel (mass) is approx 15:1

8. E1.2 – Incomplete Combustion • •When the ratio of air/fuel (mass) is less than 15:1, the mixture is said to be ‘rich’ and incomplete combustion ensues • •This results in the formation of carbon monoxide • C8H18+ 10O2→3CO2 + 5CO + 9H2O • These unburnt hydrocarbon molecules (CO) are called volatile organic compounds (VOC’s) from exhaust • •A very poorly designed (or maintained) engine may emit solid particles (soot) from the exhaust (rich) • C3H8+ 2O2→ 3C + 4H2O

9. E1.2 – Reaction with Nitrogen •When the air/fuel ratio is increased the mixture is said to be ‘lean’. A lean mixture will not produce carbon monoxide •When a mixture becomes ‘lean’ (excess O2) the likelihood that oxygen will react with nitrogen in the air (78%) increases. •Under extreme conditions (like in an engine) the two elements can combine to form nitrogen oxides NOx which leads to ‘misfire’ in the engine (known as knocking) because the fuel ignites before the spark

10. E1.2 – Rich vs Lean •An engine that runs ‘rich’ will produce a lot of power, but with poor fuel consumption and high emissions of CO and VOC’s •An engine that runs ‘lean’ will produce less power, less CO and VOC’s, consume less fuel, but may produce more nitrogen oxides. When mixture is too lean, misfiring causes rise in VOC production N2+ O2→ 2NO• (remember the radical?) 2NO• + O2→ 2NO2 Localized in urban areas, causing health effects and contribute to the formation of acid rain

11. E1.2 – Catalytic Converter •Three way catalytic converter is placed in exhaust systems of cars to treat the exhaust gases •Consists of a fine mesh or honeycomb of ceramic material or metal, coated with a thin layer of finely divided platinum, rhodium, or palladium (these T-metals act as heterogeneous catalysts) •Three way refers to: 1.Reduction of nitrogen monoxide to nitrogen 2.Oxidation of unburnt hydrocarbons to carbon dioxide and water vapor 3.Oxidation of carbon monoxide to carbon dioxide

12. E1.2 – Catalytic Converter •First, gases pass over metal catalyst, reacting NO and CO 2NO + 2CO → N2 + 2CO2 •Then, pass over oxidation catalyst (rhodium). If a proper ratio of air/fuel exists, there is proper O2 remaining for this: 2CO + O2→ 2CO2

13. E1.2 – Sulfur as Pollutant •SO2 (sulfur dioxide) is an important primary pollutant. - It’s a pungent smelling toxic gas - damages the respiratory system and may lead to asthma attacks. - Highly soluble in water - Contributes to formation of acid rain

14. E1.2 – Primary/Secondary Pollutants •Primary pollutants are emitted directly from the sources and remain unchanged once they enter the environment (particulate matter, inorganic gases, SO2, etc) •Secondary pollutants are formed in the atmosphere by chemical reactions involving primary pollutants and gases normally present in the air. •Most man-made sulfur dioxide emissions arise from the sulfur that exists as an impurity in coal which is burned extensively in many power plants

15. E1.2 – Sulfur reactions •Sulfur is oxidized during the combustion process S + O2→ SO2 •Sulfur dioxide dissolves and reacts with water to produce sulfurous acid, H2SO3 SO2+ H2O → H2SO3 •Sulfur dioxide also undergoes photochemical oxidation in the atmosphere. This occurs in water droplets in which SO2 is dissolved, and is catalyzed by particulates (soot, etc) 2SO2+ O2→ 2SO3 •Sulfur trioxide dissolves and reacts with water to produce sulfuric acid SO3+ H2O → H2SO4

16. E1.2 – Reducing SO2 Emissions •There are three methods by which sulfur dioxide emissions from power stations can be limited The coal or oil can be refined to remove sulfur before combustion Fluidized bed combustion (FBC) reduces the amount of sulfur oxides resulting from combustion Flue gas desulfurization (FGD) removes sulfur dioxide from the exhaust gases before they leave the power station flue (chimney)

17. E1.2 – Fluidized Bed Combustion •This process suspends the solid coal on an upward flowing jet of air during combustion •Coal dust is mixed with limestone powder (CaCO3) and blasted into the furnace with a jet of air. The jet of air suspends the solid particle so they flow like a fluid CaCO3 → CaO + CO2 2CaO + 2SO2 + O2→ 2CaSO4 •The Calcium sulfate can then be removed by electrostatic precipitation

18. E1.2 – Flue Gas Desulfurization •Sulfur dioxide emissions can be removed from the flue gases by passing the gases through a suspension of calcium carbonate and calcium oxide in water. Product is calcium sulfite CaCO3+ SO2→ CaSO3 + CO2 CaO+ SO2→ CaSO3 •Calcium sulfite is then further oxidized, producing calcium sulfate 2CaSO3+ O2→ 2CaSO4

19. E1.2 – Particulate Emissions •Particulate emission refers to the generation of small particles of solid or liquid. Some such pollutants are visible to the naked eye, but most are too small to be seen. Metal particles Metal oxide particles Fly ash Asbestos dust Organic particles Aerosol mist

20. E1.2 – Fly Ash •Fly ash is a combination of very fine carbon, hydrocarbon and metal oxide particles released during the combustion of fossil fuels •Most fly ash is filtered out in flues, but some very fine particles escape into the atmosphere

21. E1.2 – Removal of Particulates •Via sedimentation which relies on letting heavy particles settle out under gravity, or by filtration, in which simple fabric filters capture particles. •Most effective method is electrostatic precipitation – which has two sections Ionization section, consists of a mesh with thin wires, carrying an electrical charge. Flue gas passes through this mesh, causing any solid or liquid particles to acquire charge Collection section, consists of metal plates carrying the opposite charge. Particulates are attracted to the plates, and stick. Plates are shaken at intervals to dislodge the build-up layer of particles, then they are removed.

23. EnviroChemsitry Part 2 – Acid Deposition

24. Acid Deposition refers to the process by which acidic particles leave the atmosphere. The most well known example is acid rain • but acidic substances may also be removed by snow and fog, as well as by dry processes involving gases and solid particles. • Production of SO2 (as discussed in Part 1) aids in this process

25. E2.1 – Carbonic Acid •Natural rain water is acidic, with a pH around 5.6 •The acidity of rain is a result of CO2 naturally present in the atmosphere •When CO2 is dissolved in water it’s referred to as carbonic acid (H2CO3) but only a very small amount actually exists as a solution H2O + CO2→ H2CO3 •Carbonic acid molecules immediately dissociate in water to form hydrogencarbonate ions, HCO3-, and hydronium ions, H3O+ H2CO3+ H2O → HCO3- + H3O+

26. E2.1 – Wet Deposition •The most important sources of acid rain are the sulfur oxides produced in power stations •When sulfur oxides dissolve and react in rain water, solutions of sulfuric acids are formed (as discussed in E.1) SO2+ H2O → H2SO3 SO3+ H2O → H2SO4 Another route to sulfuric acid is a gas-phase reaction of a sulfur dioxide molecule with a hydroxyl radical, OH•, to give sulfuric acid SO2+ OH• → HSO3• + • OH → H2SO4

27. E2.1 – Wet Deposition •Nitrogen Oxides also contribute to acid rain. Formed in vehicle engines: HO• + NO• + M → HNO2 + M M represents the ‘third body’ which is an inert molecule which absorbs some of the excess energy of the reaction (in the atmosphere M is generally N2). •NO2 is formed by the oxidation of NO in the atmosphere and reacts with HO• HO• + • NO2 + M → HNO3 + M

28. E2.1 – Wet Deposition •These acids may be deposited in places other than water, such as snow and fog. •Fog is a particular problem for high-altitude forests •The lower temperature at high altitudes causes water vapor to condense out of the atmosphere, forming a moist ‘blanket’ of acidic fog which surrounds trees.

29. E2.1 – Dry Deposition •Dry deposition refers to acidic substances such as gases and particulates leaving the atmosphere in the absence of precipitation (without rain or fog) •Heavy particulate particles may settle out of the atmosphere under gravity. •Acidic gases such as sulfur dioxide may have directly harmful effects on the environment without first being dissolved in rain water.

30. E2.2 – Environmental Effects •Acid deposition effects the environment in 5 ways 1.It affects the pH of lakes/rivers, which impacts organisms living there 2.It affects the availability of metal ions in soil, which goes on to affect nearby plant life and surface water 3.It directly affects plants 4.It affects buildings and other materials 5.It directly affects human health

31. E2.2 – Impact(1): Lakes/Rivers •Below a pH of 5.5 -Some species of fish (salmon) are killed -Algae, zooplankton, which are food for larger organisms -Prevents hatching of fish eggs •Fish are also killed when aluminum, leached from the soil by acid rain, enters lakes and rivers. •The function of fish gills is affected by Al, leaving the fish unable to extract oxygen from the water

32. E2.2 – Impact(2): Soil •The pH of soil is a key factor which species of plants will grow •Aluminum (naturally present in soil) forms insoluble hydroxide (Al(OH3) at high pH values. •When pH falls due to acid rain, Al becomes soluble and is released into soil. •Other ions (Mg, Ca, etc) which are essential for plant growth are washed away in the same fashion. 2Al(OH)3+ 2H2SO4→ Al2(SO4)3 + 3H2O

33. E2.2 – Impact(3): Plants •Beyond damaging soil, and lowering available nutrients, acid rain can also damage plants directly •Acid deposition can damage leaf chlorophyll, turning leaves brown and reducing the photosynthetic ability of the plant

34. E2.2 – Impact(4): Buildings •Limestone and marble are forms of CaCO3 which can be eroded by acid rain: CaCO3+ H2SO4→ CaSO4 + H2O + CO2 •Metallic structures (mainly steel, Fe, Al) are readily attacked. The sulfur dioxide gas may attack directly as follows: Fe + SO2 + O2→ FeSO4 •Sulfuric acid may attack Fe as well: Fe + H2SO4→ FeSO4 + H2 Fe + 2H+→ Fe2+ + H2

35. E2.2 – Counteract Acid Dep. •Limit (lower) the amount of acidic substances released to the atmosphere NOx are removed from vehicle emissions with a catalytic converter SO2 emissions from coal power plants can be decreased in several ways (scrubbers, etc) •Addition of compounds that will aid in neutralizing acidic effects Addition of limestone (CaCO3) Addition of calcium hydroxide (Ca(OH)2)

36. Topic E – Enviro Chemistry Part 3 – Greenhouse Effect

37. E3.1 – Greenhouse Effect •Greenhouse gases allow the passage of incoming solar short-wavelength radiation but absorb the longer-wavelength radiation from the Earth. Some of the absorbed radiation is re-radiated back to Earth. •TOK: Some people question the reality of climate change and question the motives of scientists who have “exaggerated” the problem. How do we assess the evidence collected and the models used to predict the impact of human activities?

38. •The greenhouse effect is the cause of the phenomenon of global warming in which the average temperature of Earth rises, causing various environmental disasters •The greenhouse effect itself is absolutely necessary for the Earth to regulate its temperature at a habitable level. •Humans are thought to impact this delicate balance by disrupting the natural equilibrium in the atmosphere, causing the planet to become warmer

39. Earths “Average” Temp. • • Average temperature in the troposphere 14-15oC • Maintained bc the energy incident on Earth (from sun), is balanced by the energy leaving Earth (to space) • • Most radiation from the sun is in the visible region, also along with “near UV” and near “IR radiation” • • Only 47% of energy directed at the earth is absorbed, remainder is reflected back to space • • The peak radiation is 500nm and is not absorbed by atmospheric gases so is absorbed and radiated by the earth to the rest of the atmosphere

40. •When Earth absorbs energy, surface temperature rises, and energy flows from hot (earth) to atmosphere (cold), etc • •Once energy is re-radiated from the earth, it’s no longer in the visible region, it’s wavelength is much longer and in the infrared region • If all this energy released into space instead, our average atmospheric temp would be -20oC • Gases such as H2O and CO2 help to re-radiate energy in the atmosphere and toward the earth as well. • It can be seen that an increase in [IR absorbing gases] such as CO2 and H2O results in a decreased amount of energy escaping from the Earth by moving toward the surface

41. E3.2 – Factors for GH Gases •A contribution of a greenhouse gas to the warming of the atmosphere depends on three factors: 1.The abundance of the gas in the atmosphere 2.The ability of the gas to absorb infrared radiation 3.The lifetime of the gas molecules in the atmosphere, before being removed by chemical processes The 2nd and 3rd factors are often combined to give a figure called the Global Warming Potential (GWP)

42. E3.2 – Greenhouse Gases •Major Contributors as Greenhouse Gases Water vapor, H2O Carbon dioxide, CO2 Methane, CH4 Nitrous Oxide, N2O Chlorofluorocarbons, CFC’s Ozone, O3

43. GH Gases – H2O •Most important GH Gas, has a GWP of 0.1 •Percentage of H2O(g) in atmosphere 1-4%, ranges •Absorbs IR over a broad range of frequencies •Increased atmospheric temperatures lead to more rapid evaporation of the oceans, and larger capacity of the air to carry water vapor (humidity) •Estimates of H2O’s contribution to Global Warming is 36%-75%

44. GH Gases – CO2 • •Percentage of CO2 in atmosphere is 0.035% • •CO2 has a GWP of 1. • More efficient than water in absorbing IR radiation • •Absorbs IR in a “window” that H2O does not • •[CO2] rise due to the following human activities: • Combustion of fossil fuels • Manufacture of cement (CaCO3→CaO + CO2) • Deforestation in tropics, lower rate of photosynthesis, meaning CO2 is entering atmosphere more quickly than removed

45. GH Gases – CH4 •Percentage in atmosphere CH4 = 1.7x10-4 •It’s GWP is 25 (compare to H2O=0.1 and CO2=1) •Estimates say 4%-9% contribution to Global Warmíng •It is removed from the atmosphere relatively quickly •Formed when cellulose (plant fiber) decomposes anaerobically via bacteria (CH2O)n → CH4 + CO2 •Occurs on large scale as a result of human actions: - Rice cultivation (paddy fields) - Fermentation of grass in cows, and rotting manure - Leaking gas pipelines - Fermentation of organic materials in covered landfills

46. GH Gases – N2O •GWP of 296. It’s less efficient at absorbing IR than CO2 but it’s high number comes from a long residence in the atmosphere •Percentage in atmosphere 0.031% •Accounts for 5% of Global Warming effects •Human activity only accounts for 10-12% of it’s production, but anthropogenic NO2 from: - Industrialized agriculture, N fertilizers - Industrialized livestock farming, poor handling of animal waste - Chemical industry, HNO3 and nylon production

47. GH Gases – CFC’s • •Chlorofluorocarbons (CFC’s) have largely been replaced in aerosols, propellants, and refrigerants by hydrochlorofluorocarbons (HCFC’s) and hydrofluorocarbons (HFC’s) • These gases are less damaging to the ozone layer but still have GWP values much higher that CO2 and are important contributors to global warming

48. E3.3 – Influence of Greenhouse Gases • Rising Sea Levels • •As atmospheric temperature increases, sea levels will rise for two reasons: • The increased atmospheric temperature causes accelerated melting • This does not include floating ice in the arctic as it already displaces water while it floats • As oceans warm up, the water in them will expand, occupying more volume (even minor amounts could be significant due to the quantity of water in the ocean!)

49. Glacier Retreat •Glaciers undergo a seasonal melting and freezing as temperatures vary throughout the year. •In the Himalayas glacial melt water is an important source of fresh water, feeing the rivers of South Asia •Increased melting increases erosion and risk of flooding downriver, a particular problem in low-lying countries

50. Changing Patterns of Agriculture • •In temperate regions (such as Europe) yields of grain will most likely increase due to higher temperature, longer growing season and increased [CO2] available for photosynthesis • •But, increased humidity and rainfall could lead to increased incidence of fungal crop diseases, and migration of tropical insects to higher altitudes. • •At higher latitudes, more workable land may become available due to thawing and temperature changes. • Worldwide, the possibility of extreme weather increases the likelihood of ruined harvest