chemistry HSC chemical monitoring summary
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Chemical Monitoring and Management. Construct word and balanced formulae equations of all chemical reactions as they are encountered in this module: EQUATIONS to remember:
o Different products under different conditions: o Complete combustion: propane + oxygen carbon dioxide + water C3H8 (g) + 5O2 (g) 3CO2 (g) + 4H2O (g) o Incomplete combustion: propane + oxygen carbon + carbon monoxide + water C3H8 (g) + 3O2 (g) C (s) + 2CO (g) + 4H2O (g) o Haber Process: o nitrogen + hydrogen ammonia + heat o N2 (g) + 3H2 (g) 2NH3 (g) H = -92 kJ/mol o Sulfate Content in Fertilizer: o barium chloride + sulfate barium sulfate + chloride 2o BaCl2 (aq) + SO4 (aq) BaSO4 (s) + 2Cl- (aq) o How Ozone Protects Us From UV Radiation: o Oxygen reacts with UV, forming 2 radicals: O2 + UV radiation 2O o Radical reacts with oxygen, forming ozone: O + O2 O3 o Ozone reacts with UV, forming oxygen and a radical: O3 + UV radiation O + O2 o Radical reacts with ozone, creating oxygen: O + O3 2O2 o All the CFC-Related Equations: o Formation of chlorine radical: CCl2F2 (g) + UV radiation Cl (g) + CClF2 (g) o Reaction of ozone: Cl (g) + O3 (g) ClO (g) + O2 (g) o Regeneration of chlorine: ClO (g) + O (g) Cl (g) + O2 (g) o Removal of chlorine radical: Cl (g) + CH4 (g) HCl (g) + CH3 (g) o Removal of chlorine monoxide radical: ClO (g) + NO2 (g) ClONO2 (g) o Formation of molecular chlorine: HCl (g) + ClONO2 (g) Cl2 (g) + HNO3 (g) o Decomposition of molecular chlorine: Cl2 (g) + UV radiation 2Cl (g) o The Heavy-Metal Sulfide Test: o The sulfide-test is based on the following equilibrium: S2- (aq) + 2H3O+ (aq) H2S (aq) + 2H2O (l) 1
1. Much of the work of chemists involves monitoring the reactants and products of reactions and managing reaction conditions:
1.1: Outline the role of a chemist employed in a named industry or enterprise, identifying the branch of chemistry undertaken by the chemist and explaining the chemical principle that the chemist uses: Dr John Chapman: see assignment 1.2: Identify the need for collaboration between chemists as they collect and analyse data: As chemistry is such a broad field of knowledge, people tend to specialise within a particular branch, such as polymer chemistry or analytical chemistry. However, in real-life situations, many chemical problems require expertise and in depth knowledge from a wide range of chemical branches. Hence, collaboration between chemists is essential for solving chemical issues, or when dealing with large amounts of data being collected, as the chemists provide input and expertise from their own particular field, for a common goal. As chemists often work in teams, collaboration and communication is required to collectively benefit the team, as they collect and analyse information. o EG: An industrial process would require collaboration between physical chemists (for equilibrium considerations), organic chemists (for how the reaction occurs) and analytical chemists (for monitoring products). 1.3: Describe an example of a chemical reaction such as combustion, where reactants form different products under different conditions and thus would need monitoring: Combustion: o Chemical reactions can form different products under different conditions. o Take, for example, the combustion of a simple hydrocarbon, propane. o In an environment with adequate amounts of oxygen, propane combusts completely, forming only carbon dioxide and water: propane + oxygen carbon dioxide + water C3H8 (g) + 5O2 (g) 3CO2 (g) + 4H2O (g) o In an environment with insufficient oxygen, propane combusts incompletely, and can form a range of different products, such as carbon (soot), carbon monoxide, carbon dioxide and water. propane + oxygen carbon + carbon monoxide + water C3H8 (g) + 3O2 (g) C (s) + 2CO (g) + 4H2O (g) Monitoring: o Hence, under different conditions, chemical reactions can proceed in different ways, as seen by the combustion reaction above. o However, in certain situations (such as in car engines), only one reaction is desired. Thus, the reaction conditions must be monitored to ensure that only (or mostly) the wanted reaction occurs. o Carbon monoxide is a poisonous gas and can affect human health negatively. Carbon (soot) is carcinogenic to humans and can be irritating to the lungs. Both of these alternative products can also signal a decrease in fuel efficiency and result in a reduced energy yield from the fuel. 2
Complete combustion Carbon dioxide and water
Relative ratio of oxygen to fuel
Unlimited (excess) oxygen available (high air to fuel ratio) Maximum energy Greenhouse gas, carbon dioxide, produced
Relative amount of energy released Environmental effects
Incomplete combustion Water and any combination of carbon monoxide, carbon (soot) and unburned hydrocarbons Insufficient oxygen for the amount of fuel. (low air to fuel ratio) Much less energy obtained from fuel. Much more polluting as CO is very toxic, hence, harmful to humans and animals. Carbon (soot) particles from the air are carcinogenic and irritating to the lungs. Other greenhouse gases may be produced.
1.4: Gather, process and present information from secondary sources about the work of practising scientists identifying:o the variety of chemical occupations: o a specific chemical occupation for a more detailed study: THE VARIETY OF CHEMICAL OCCUPATIONS: o The large range of jobs available in the chemical industry includes: Analytical chemistry. Bio-molecular chemistry. Colloid and surface science chemistry. Environmental chemistry. Industrial chemistry. Inorganic chemistry. Electrochemistry. Organic chemistry. Physical chemistry. Polymer chemistry . A SPECIFIC CHEMICAL OCCUPATION: o A summary of the job of an environmental chemist: Job includes reviewing operation of effluent water treatment systems and ensuring compliance with government environmental regulations. Reviewing industrys compliance with government environmental noise standards. Assessing levels of potential contamination in wastes (e.g. soil) intended for landfill disposal and classifying them in accordance with government guidelines. Managing disposal of contaminated wastes. Investigating reports of contamination in soil or groundwater to determine source and then arranging to correct it. Determining whether gas stack emissions contain unacceptable levels of regulated materials. Advising 3
engineers/managers of corrective actions needed if any of the above parameters show faults in systems. Answering public or professional enquiries or complaints regarding environmental performance. 2. Chemical processes in industry require monitoring and management to maximise production 2.1: identify and describe the industrial uses of ammonia Ammonia (NH3) is a colourless, toxic gas with a pungent odour The industrial uses of ammonia: o Solid and liquid fertilisers (through a reaction with sulfuric acid, nitric acid or phosphoric acid to form ammonium sulfate fertiliser, ammonium nitrate fertiliser or ammonium hydrogen phosphate fertiliser). The nitrogen in the ammonia is an essential plant nutrient. o Production of chemicals: Nitric acid (through the Ostwald Process) which in turn is used to manufacture synthetic fibres such as nylon and explosives such as TNT. Cyanide used in plastics manufacture and gold extraction o Cleaning agent: ammonia solution (ammonium hydroxide) destroys bacteria so it is a component of many household cleaners such as floor and bathroom cleaners. o Weak base: ammonia is used to safely neutralise acidic by-products in petroleum refining and in acid spills. Being a weak base, it generates heat slowly during neutralisation (an exothermic process) so does not cause further burning. o Manufacture of some pharmaceuticals (i.e. sulphonamides) and refrigerants. 2.2: identify that ammonia can be synthesised from its component gases, nitrogen and hydrogen The synthesis of ammonia from hydrogen and nitrogen: o N2(g) + 3H2(g) 2NH3(g) For the synthesis of ammonia, nitrogen is obtained by fractional distillation of liquefied air and hydrogen is obtained by the reaction of steam with methane obtained from natural gas. o CH4(g) + 2H2O(g) H2(g) + CO(g) o The carbon monoxide gas can either be liquefied, reacted with a base (i.e. CaOH) or undergo the following reaction with water: o CO + H2O CO2 + H2
2.3: describe that synthesis of ammonia occurs as a reversible reaction that will reach equilibrium TERMINOLOGY: o a reversible reaction is a reaction that can proceed in both directions o equilibrium is a reversible reaction proceeding in a closed system and with the rate of the forward reaction equal to the rate of the reverse reaction THE SYNTHESIS OF AMMONIA: o Hydrogen and nitrogen react very slowly to form ammonia, and at the same time, the ammonia decomposes into nitrogen and hydrogen. Thus, the reaction is reversible: nitrogen + hydrogen ammonia N2 (g) + 3H2 (g) 2NH3 (g) 4
o In a closed system, equilibrium will be reached when the rate of the forward reaction is the same as the rate of the reverse reaction. However, for this reaction the equilibrium is slow to be reached and lies to the left. o The synthesis of ammonia has a high activation energy because a lot of energy is needed to break the covalent bonds between the hydrogen molecules and the nitrogen molecules and form atoms that can rearrange to form ammonia. Nitrogen and hydrogen do not exist as single atoms; they both exist as diatomic molecules held together by strong covalent bonds. Nitrogen molecules are held together by very strong triple covalent bonds and the single covalent bond between hydrogen atoms is also quite strong. 2.4: identify the reaction of hydrogen with nitrogen as exothermic The reaction of hydrogen with nitrogen (Haber Process) can be represented as: o N2 (g) + 3H2 (g) 2NH3 (g) H = -92 kJ/mol From the equation we can see that H is negative; hence, the reaction is exothermic. Overall energy is released in the reaction of hydrogen with nitrogen to form ammonia. There is less chemical energy in the product than in the reactants. The energy needed to break the bonds in hydrogen and nitrogen is less than the energy given out when the new bonds in ammonia are formed. The bends in ammoniua are weaker than the bonds in the reactants. 2.5: explain why the rate of reaction is increased by higher temperatures RECALL: o Before any chemical reactions can occur, there must be an energy input. o The activation energy of a reaction describes the minimum amount of energy input necessary for that reaction to occur. The rate of reaction is a term used to describe how FAST a reaction occurs. There are 2 reasons why higher temperatures result in a higher rate of reaction: o As temperature is increased, energy is delivered into the reaction as thermal energy. This thermal energy is converted into kinetic energy, and particles begin to move faster. This causes more collisions between particles, and hence more reactions occur. o Also, if the temperature is higher, there is more chance that colliding particles will have the necessary activation energy for the reaction to take place. For the Haber process (and all reversible reactions in general), at higher temperatures, the rate of reaction of both the forward and reverse reactions are increased, and hence equilibrium is reached faster. 2.6: explain why the yield of product in the Haber process is reduced at higher temperatures using Le Chateliers principle Le Chateliers principle states if a chemical system at equilibrium is subjected to a change in conditions, the system will readjust itself to partially counteract that change In terms of industrial reversible reactions, the term yield of product describes how much product is formed at the point of equilibrium: o It has nothing to do with RATE of reaction. 5
Since the reaction is an exothermic reaction, applying Le Chateliers Principle we can deduce that: o If an increase in temperature is imposed onto the system (the reaction) then the system will shift itself to counteract this imposed change. o Since the system will want to cool itself, to oppose the heating, the reaction that is endothermic will be encouraged. o The forward reaction is exothermic; hence, the reverse reaction is endothermic. o Thus, at higher temperatures, the reverse reaction occurs more o But this produces reactants, not products; thus the yield of products reduces. Thus, high yields of ammonia favour lower temperatures o However, at low temperature, the reaction rate is too slow to produce ammonia at a reasonable rate o ammonia to be obtained at a satisfactory rate o In practice, temperatures of about 400oC are used
2.7: explain why the Haber process is based on a delicate balancing act involving reaction energy, reaction rate and equilibrium In order for the Haber process to be economically viable, we need to consider yield of products, rate of reaction, as well as costs. Hence, a compromise (balancing act) of all the above must be made: o Temperature: Higher temperatures will produce ammonia faster, but lower temperatures will produce more ammonia. Hence a moderate temperature of about 400-500C is used, together with the iron/iron-oxide catalyst. o Pressure: Increased pressure will produce more ammonia, also faster, but it will be expensive to build and maintain high-pressure equipment. The benefits of high pressure outweigh the costs, and so a pressure of 250-350 atmospheres is used. EXTRA: Another step taken to encourage the economic formation of products is the continuous liquefaction and removal of ammonia as it is produced. This reduces the concentration of ammonia, and hence encourages the forward reaction, according to Le Chateliers principle. 2.8: explain that the use of a catalyst will lower the reaction temperature required and identify the catalyst(s) used in the Haber process A catalyst is a substance that increases the rate of reaction of a chemical reaction, but does not itself take part in the reaction. Catalysts work by reducing the activation energy of reactions and hence allow the reaction to proceed more quickly and at lower temperatures. The Haber process uses the catalyst iron oxide (magnetite; Fe3O4). The catalyst is finely ground, to produce a large surface area, and has its surface partly reduced to elemental iron. How the use of a catalyst in the Haber process lowers the reaction temperature required: o The Haber process involves a reversible, equilibrium reaction. N2(g) + 3H2(g) 2NH3(g) H= -92kJ mol-1 o The forward reaction is exothermic, so the high temperature needed to produce a fast reaction rate would push the equilibrium to the left and result in a lower yield. 6
o The reaction rate for both forward and reverse reaction is increased by using a catalyst. A lower temperature can be used with the catalyst to achieve a faster reaction rate. This helps in the delicate balance between the effect of temperature on rate and the effect on equilibrium position. 2.9: analyse the impact of increased pressure on the system involved in the Haber process The Haber process involves the synthesis of nitrogen gas and hydrogen gas to form ammonia: o N2(g) + 3H2(g) 2NH3(g) According to Le Chateliers principle, increasing the pressure will favour the side that will reduce the pressure (i.e. has LESS moles of gas): o In this reaction, there are less moles of gas on the product side (gas ratio is 4:2) and hence increasing the pressure will encourage the forward reaction. o In addition, higher pressures also increase the reaction rate because the gas molecules are closer and at higher concentrations. 2.10: explain why monitoring of the reaction vessel used in the Haber process is crucial and discuss the monitoring required Monitoring is needed to maintain optimum conditions for maximum yield with the least possible waste. Getting maximum yield of ammonia from the Haber process involves balancing reaction energy, reaction rate and equilibrium. A compromise is reached, and the reaction is monitored, maintaining the following conditions in the reaction vessels to ensure optimum yield: o nitrogen and hydrogen are fed into the reaction chamber in a ratio of 1:3 o a pressure of 250-350 atmospheres o a temperature of 400-500oC o iron oxide (Fe3O4) catalyst lowers the activation energy o unused gases are returned to the reaction vessel o ammonia is constantly removed as a liquid Cool the mixture and liquefy as they have different boiling points, they will turn into liquid at different temperatures. If all components are liquefied, they can be separated by fractional distillation. 2.0.1: gather and process information from secondary sources to describe the conditions under which Haber developed the industrial synthesis of ammonia and evaluate its significance at that time in world history The development of the industrial synthesis of ammonia: o Fritz Haber: Haber succeeded in making small amounts of ammonia from hydrogen and atmospheric nitrogen in the laboratory. He discovered a catalyst for the reaction and worked out the conditions of temperature and pressure that allowed a reasonable yield.
o Carl Bosch: Bosch developed the process to an industrial scale, including inventing the necessary high pressure technology that would enable the process to be carried out on a large scale. Germany had succeeded in implementing this process by 1913. The conditions used by Haber and Bosch to allow the production of commercial quantities of ammonia: o Nitrogen and hydrogen used in ratio of 1:3 o 250-350 atmospheres pressure o about 450oC temperature o catalyst of finely divided iron or iron oxide o ammonia liquefied and removed as it is produced The significance of the industrial synthesis of ammonia: o Before the invention of the industrial Haber process in 1913, the global source of nitrates (for fertilisers and explosives) came from saltpetre (bird poo/guano) from Chile. o During WWI, Germanys supplies of nitrogen compounds were cut off by the British; this threatened to cause widespread starvation, as well as cause Germany to rapidly lose the war (they relied on saltpetre to make explosives). o Historians believe that Germany would have run out of nitrates by 1916 if it had not developed the Haber process. On the other hand, the Allies had to rely on Chilean nitrates, which became more expensive as the war progressed. o However, with the invention of the Haber process, Germany (and later the rest of the world) had a cheap source of nitrates from elemental nitrogen and hydrogen. o Thus, the Haber-Bosch process had a significant impact on the course of history during the early 20th C as it allowed Germany to continue the war for much longer than otherwise would have been possible.
3. Manufacture products, including food, drugs and household chemicals, are analysed to determine or ensure their chemical composition:
METHODS OF DETERMINING CONCENTRATION: o Concentration is defined as the amount of solute in a specified amount of solvent. There are many ways of expressing concentration depending on the purpose and circumstances. o Mass per unit volume (grams/litre) used when describing solubility or making solutions involving dissolving a solid in a solvent, usually water i.e. 2g fertiliser is dissolved in enough water to make a 50mL solution m = 2g, V = 0.050L o Per cent by mass, % (w/w) uses a comparison of weight of solute compared to weight of solution, converted to per cent for ease of comparison household cleaners often express concentrations of active ingredients in this way i.e. 35g NaCl is dissolved in 100g water msolute = 35g, msolution = 100 + 35 = 135g o Per cent by volume, % (v/v) used when liquids are dissolved in other liquids alcohol concentration in drinks is expressed in this way i.e. 2.5mL hydrogen peroxide is dissolved in enough water to make 50mL of solution vsolute = 2.5mL, vsolution = 50mL o Parts per million (ppm) means parts of a particular solute per million parts of mixture often used in environmental contexts when the concentrations are very small, especially when considering pollutants i.e. if the maximum allowable concentration of DDT in beef is 1.5ppm, would a 500g sample of beef containing 0.075g of DDT be acceptable? mDDT = 0.075g, mbeef = 500g The beef is not acceptable as this is 100 times the allowable limit
3.1: Deduce the ions present in a sample from the results of tests: Ions that are being tested: o Cations: Ba2+, Ca2+, Pb2+, Cu2+, Fe2+ and Fe3+ o Anions: PO43-, SO42-, CO32- and ClTest Result Add KI Yellow precipitate Add OH- ions, drop wise Deep blue precipitate that dissolves in excess hydroxide Spray into flame Apple green colour Add AgNO3 White precipitate that darkens in exposure to sunlight Add dilute HCl Gas produced that turns limewater milky
Ions present Lead Copper Barium Chloride Carbonate/hydrogen
When given a flowchart, it is very easy to deduce the ion(s) present. However, you need to memorise the tests for the above ions. We use precipitation reactions to identify ions in solution. There are 2 situations possible: when there is only one ion present in a sample, or when there is a mixture of ions in a sample. CATIONS (when only one ion is present): o To determine the ion present, HCl, H2SO4 and NaOH is added in that order, and the formation of a precipitate would indicate which ion it is:
o Firstly, HCl (Cl- ions) is added to the sample. If a white precipitate forms, then the unknown ion is LEAD (Pb2+); the precipitate is lead chloride (PbCl2). To verify, add a few drops of potassium iodide (KI) to a fresh sample. Pb 2+ ions form a vibrant yellow precipitate with iodide (I-) ions. o Next, H2SO4 (SO42- ions) is added to a FRESH SAMPLE. If a precipitate forms, the unknown ion is either calcium (Ca2+) or barium (Ba2+): To distinguish the two ions, add a few drops of sodium fluoride (NaF) to a new sample. Ca2+ forms a precipitate with fluoride (F-), but Ba2+ does not. o Lastly, NaOH (for OH- ions) is added to a fresh sample. If a precipitate forms, the ion is either copper (Cu2+), iron(II) (Fe2+) or iron(III) (Fe3+):
If the precipitate is blue then the ion is Cu2+. To further verify that it is copper, dissolve the precipitate in ammonia (NH3), and if it is Cu(OH)2 it will form a deep-blue solution. If the precipitate is NOT blue, then the ion is either Fe2+ or Fe3+. Using a fresh sample, add: A few drops of potassium thiocyanate (KSCN) solution. Fe3+ reacts with thiocyanate ions (SCN-) to form a blood red precipitate. A few drops of PURPLE potassium permanganate (KMnO4) solution. Fe2+ decolourises the permanganate ion (MnO4-), changing the solution from purple to colourless. CATIONS (when there is a mixture of ions): o When there is a mixture of ions, there is the risk that the test for one ion will interfere with the test for another ion. o Once a precipitate is formed, the reagent is added until no more precipitate will form (this is to remove all traces of a specific cation). o The mixture is then filtered or centrifuged to remove the precipitate, and then further tests are performed on the remaining solution. o The procedure for identifying ions in a mixture: Add HCl solution; if a precipitate forms, Pb2+ is present in the mixture. Keep adding HCl until no more precipitate forms, then filter. Divide the filtrate into 2 portions: Add NaF to one portion; precipitate indicates the presence of Ca2+. Add H2SO4 to the other portion; if a precipitate forms, Ba2+ or Ca2+ ions are present in the mixture. Completely precipitate out all the barium and calcium ions, then filter. Add NaOH solution; if there is a precipitate, mix NH3 into the solution and filter. A presence of any blue precipitate indicates Cu2+, while any green or brown precipitate indicates iron ions (to distinguish iron(II) and iron (III), use thiocyanate and permanganate tests on new samples). ANIONS (when only one ion is present): o Carbonate (CO32-): Add dilute nitric acid (HNO3); if bubbles of a colourless gas form (that when passed through limewater turns it cloudy) carbonate is present. If the solution has a pH of 8-11 then carbonate is likely present. o Sulfate (SO42-): If the addition of Ba(NO3)2 (barium nitrate) in to an acidified sample produces a thick white precipitate, then sulfate is present: Acidify solutions using dilute nitric acid (does not form precipitates). If acidification and addition of Pb(NO3)2 (lead nitrate) produces a white precipitate, then sulfate is present. o Phosphate (PO43-): If the addition of Ba(NO3)2 (barium nitrate) to an ALKALINE sample produces a white precipitate, phosphate is present. 11
Use ammonia (NH3) to make solutions basic. If acidification followed by the addition of (NH4)MoO4 (ammonium molybdate) solution produces a yellow precipitate, phosphate is present. o Chloride (Cl-): If the addition of AgNO3 (silver nitrate) to an acidified sample produces a white precipitate that dissolves in ammonia solution and darkens in sunlight, then chloride is present. ANIONS (when there is a mixture of ions present): o Again, care must be taken not to cause interference between tests: o The procedure: Add nitric acid; any bubbling indicates the presence of carbonate. Keep adding acid until all bubbling stops. Acidify solution with more nitric acid. Add Ba2+ (through barium nitrate). If a precipitate forms, sulfate is present. Filter solution. Make the filtrate alkaline by adding ammonia. Add more Ba2+ ions. If a precipitate forms, phosphate is present. Filter solution. Acidify solution again and add Ag+ ions; precipitate indicates Cl- ions.
3.0.1: PRACTICAL Perform first-hand investigations to carry out a range of tests, including flame tests, to identify the following ions: Cations: Barium, calcium, lead, copper and iron: Anions: Phosphate, sulfate, carbonate and chloride: In this practical, students were given vials of unknown solutions; vials were labelled as mixtures of ions or only one ion. Using both precipitation and flame tests, the contents of the vials were determined. Precipitation Tests: o Please see ABOVE for a very in-depth coverage of precipitation tests used to determine anions and cations present in a sample. Flame Tests: o Flame tests were only used to identify cations. o Anions cannot be identified by flame tests. o Out of the given list of cations, only barium, calcium, and copper produce a distinctive flame colour when sprayed into a flame: Ba2+ gives a pale-green flame. Ca2+ gives a brick-red flame. Cu2+ gives a blue-green flame. 3.2: Describe the use of atomic absorption spectroscopy (AAS) in detecting concentrations of metal ions in solutions and assess its impact on scientific understanding of trace elements:
ATOMIC ABSORPTION SPECTROSCOPY: A method of quantitatively determining the concentrations of metal ions in solutions; it is extremely sensitive and measures in ppm (usually) or ppb (if a graphite furnace is used). Developed by Australian scientist, Alan Walsh, and his CSIRO team in early 1950s. HOW AAS WORKS:
o A hollow-cathode lamp containing the element being analysed generates wavelengths of light specific to the metal ion being tested for. (for example, if cobalt is being tested for, a cobalt lamp is used, where the cathode in the lamp is cobalt metal).
o A solution test sample being analysed is fed into a nebulizer where the liquid solution is made into a spray or mist which is then mixed with a fuel and its oxidant (usually a mixture of oxygen and acetylene). o The flame vaporises the atoms in the mixture. o The light beam is then passed through the sample which absorbs some of the light. (The more concentrated the sample, the greater intensity of light that is absorbed). o This changed light is passed into the monochromator which selects that specific wavelength(s) which have been absorbed, and compares it to a reading without the flame/atoms to determine a reading called absorbance. o By measuring the fraction of the light at a specified wavelength that is absorbed, we can determine the concentration of the element; absorbance is proportional to concentration, and hence the unknown concentration of the element is determined. Absorbance is an arbitrary unit, so the AAS device must be calibrated using standard solutions (solutions of an element of known concentration): prior to the unknown sample being tested, standard solutions of known concentration are aspirated one at a time, and the light absorbed by each solution is measured and recorded. The absorbance of the light is then graphed against the concentration of the samples. This process is calibrating the AAS. USES OF AAS: o It is used to monitor concentrations of heavy-metals in the environment as well as in food, for health and safety reasons. AAS is used as even at low concentrations these metals can be very dangerous. o It is used to measure concentrations of micronutrients in soils. o Used to measure levels of pollutants in water, soil and air. o It can determine the concentration of trace elements in living things and hence determine the cause of deficiency-related diseases. The Impact of AAS On Understanding of Trace Elements: o Trace elements are elements that are needed in very small amounts by living things (in the range of 1-100 ppm). o Trace elements are needed in minute quantities for the proper growth, development, and physiology of organisms. o Common trace elements in humans are: zinc cobalt copper nickel molybdenum iodine, and selenium. o Before the invention of AAS, analytical methods were not sensitive enough to measure the concentration of trace elements in organic samples. o The presence of these elements were often unnoticed, and the causes of diseases relating to trace-element deficiency (such as goitre; iodine deficiency) were unknown. 14
o AAS showed that not only that there were trace amounts of elements in all organisms, but that these were also essential for their well-being. o Hence, AAS had a great impact on the understanding of trace elements: o Examples of AAS use related to trace elements: In coastal SW Australia, animal health was very poor even though they were grazed on seemingly good pasture. AAS showed that there were cobalt deficiencies in the soil and the pasture, and this was corrected. Arid areas of Victoria could not support legume crops until molybdenum deficiencies were detected and rectified. Before the 1930s large areas of Australia couldnt be farmed very well, even when heavily fertilized with superphosphate. If the grass grew, sheep that fed on it grew very patchy wool. Research using wet methods of chemistry showed that the soils were deficient in trace elements such as Cu, Zn, Mn and Mo. As soon as these elements were provided, farming could be established. AAS has allowed the levels of trace elements in our soils to be more readily monitored. 3.0.5: Gather, process and present information to interpret secondary data from AAS measurements and evaluate the effectiveness of this in pollution control: AAS is an extremely effective tool in pollution monitoring and management. It can be used to measure levels of pollutants in the soils, food, water and has many other applications in relation to pollution control. EG: o AAS was used in Bangladesh to monitor levels of arsenic in drinking water. o Arsenic is a serious pollutant as it is extremely toxic. o The following AAS absorbance data was obtained by using standard solutions of arsenic of known concentration: Total Arsenic Concentration Absorbance 50 g/L 0.12 100 g/L 0.23 150 g/L 0.35 This data was then graphed on a calibration curve:
Samples of drinking water were taken and their absorbances were measured: Sample Absorbance 1 0.28 2 0.13 3 0.31 These absorbances were then plotted on the graph, and from there the arsenic concentrations were estimated: o Sample 1 (124 ppb) o Sample 2 (61 ppb) o Sample 3 (163 ppb) Arsenic levels higher than 100 ppb were considered polluted. Hence, AAS is a very effective tool in pollution monitoring and control.
3.0.2: Gather, process and present information to describe and explain evidence for the need to monitor levels of specific ions in substances used in society: Lead: o Lead needs to be monitored because it is a severe neurotoxin. o It retards intellectual development in young children, causes brain damage and can lead to neurological disorders. o Until recently, lead was widely used as an additive in petrol; it was released in great quantities to the atmosphere in vehicle exhausts and deposited in the environment near highways. o Lead also used to be found in many paints, as these were often lead-based pigments; this lead was released into waterways, the soil and air when old houses were demolished. o Monitoring of lead concentrations of soil near highways, in water and in the atmosphere is essential to ensure that people are not exposed to harmful levels of toxic lead ions. Phosphate: o Phosphate occurs in natural waterways at low concentrations and is essential for normal aquatic plant growth. o However, if phosphate levels become too high, it could set off and algal bloom; this will choke the entire waterway, as the waterway will be completely surrounded by algae and drained of oxygen. o This has many detrimental effects on the ecosystem living in the water. o Hence, by monitoring the levels of phosphates in waterways and in consumer goods that reach waterways (e.g. detergents), scientists can guard against the development of algal blooms. Summary of information gathered Identify the ion chosen Lead Why the ion needs to be monitored Lead is a toxic, heavy metal and a neurotoxin Can cause damage to all organisms in the body, especially the brain, kidneys and reproductive system. It disrupts enzyme systems and causes anaemia as it inhibits the formation of red blood
How the ion is monitored Substances monitored for this ion
cells. Causes brain damage and retards intellectual development in young children. Lead bioaccumulates and is difficult to eliminate Bioaccumulation occurs along the food chain so levels can build up in animals at the end of chains, i.e. humans Lead is readily detected and its concentration is measured by AAS atmosphere waters food human tissues soil
Sources of the ion in these substances
Incidents involving this metal
Mining and refining of ores containing lead (i.e. copper smelters) Paints containing lead released as it deteriorates or is sanded or burnt Lead glazing of pottery Corrosion of plumbing materials containing lead Car batteries Burning of rubbish Higher concentrations of lead in the atmosphere along major roads, when lead was allowed in petrol, and around mining towns and copper smelting, caused lead poisoning. Children with higher than normal lead concentrations in blood after eating flaking paint in their homes.
3.0.3: PRACTICAL Identify data, plan and select equipment and perform first-hand investigations to measure the sulfate content of lawn fertiliser and explain the chemistry involved: In this practical, the sulfate content of a lawn fertiliser was calculated. This is an example of gravimetric analysis; it is a quantitative analysis. METHOD: o In a dry beaker, 5 grams (0.01%) of fertilizer was electronically weighed. o All the fertilizer was carefully transferred to a mortar and pestle, where it was ground into a fine powder. o The powder was transferred into a beaker; using a wash bottle with distilled water, all the residue was washed into the beaker up to the 200 mL mark. o 50 mL of 0.1 M hydrochloric acid was added, and the mixture stirred until all of the fertilizer dissolved. 17
o The mixture was then heated to its boiling point; the flame was turned off, and barium chloride was added in drops, which formed a precipitate: BaCl2 (aq) + SO42- (aq) BaSO4 (s) + 2Cl- (aq) o Barium chloride was added until no more precipitate formed. o The mixture was digested (heated just below boiling point) for 30 minutes, stirring every now and then, and then the precipitate was allowed to settle. o The beaker and its contents were then cooled in an ice-bath. o Using a sintered-glass crucible (which had its mass recorded), connected to a vacuum pump, the mixture was filtered. o The crucible was washed with water, and then ethanol, and then dried in an oven to a constant mass. o The difference between the crucibles original weight and weight after drying (which was the mass of BaSO4) was found to be 5.92 g. CALCULATING SULFATE CONTENT: mass of BaSO4 formed = 5.92 g percentage of sulfate in BaSO4 = molar mass sulfate / molar mass BaSO4 = (32.1 + 416.0) / (137.3 + 32.1 + 416.0) = 41.2 % mass of sulfate in BaSO4 = 5.92 41.2 % = 2.44 g percentage of sulfate in fertilizer = mass of SO4 / mass of fertilizer 100 = 2.44 / 5.0 100 = 48.9 %
3.0.4: Analyse information to evaluate the reliability of the results of the above investigation and to propose solutions to problems encountered in the procedure: RELIABILITY: o The use of a very precise electronic scale ensured accurate measurement. o The fertilizer was ground into a powder to ensure that all of it dissolved; if some sulfate stayed in solid form, the measurements would be inaccurate. o The washing of the mortar and pestle into the beaker was to ensure all of the fertiliser was transferred into the beaker. o Hydrochloric acid was added to aid in the dissolving of the fertiliser. o Slowly forming precipitates (using a dropper) at a high temperature ensures the formation of large particles. o Also, the mixture was digested for half an hour to allow the particles to grow. o Large particle size meant fewer loses of BaSO4 through the filter. o The mixture was cooled in ice-water to greatly reduce the solubility of BaSO4 to ensure that virtually the entire sulfate precipitated out. o A sintered-glass crucible was used instead a filter-paper as filter paper pores are too large; they allow an unacceptable level of BaSO4 through. o The washing with water and ethanol removed any traces of contaminants.
SOLUTIONS to Problems Encountered: o Human errors in measurement were the main source of inaccuracies; more careful measurement of fertilizer mass, would ensure better results. o There were small losses of barium sulfate due to its very small solubility. This was why the beaker was cooled in ice-water before filtering (solubility decreases at low temperatures). However, due to time constraints, cooling did not occur to a great extent. o Longer digestion times would allow larger particles to form, and less loss of barium sulfate through the filter would occur. o There was possibly some barium sulfate left on the beaker walls before filtering; this would be solved by washing out the beaker with distilled water. o An incomplete drying of the sintered glass crucible (due to moist environment, as well as time constraints) would be solved by a longer period of heating, as well as by using a desiccator.
4. Human activity has caused changes in the composition and the structure of the atmosphere. Chemists monitor these changes so that further damage can be limited 4.1: describe the composition and layered structure of the atmosphere
Name of layer
Ionosphere (made up of the two layers called the mesosphere and thermosphere)
Minimum and Composition maximum distance from the surface of the Earth 0 approximately 15km Contains 90% of atmospheric gases. 78% N2, 21% O2, 0.9% Ar, small amounts of other gases such as CO2, Ne, He and pollutants such as methane and oxides of sulfur and nitrogen. 15 50km Contains 9% of atmospheric gases. Nitrogen (N2), Oxygen (O2) and Ozone (O3) and other gases in the troposphere 50 150km (atmosphere Contains 1% of gradually fades away atmospheric gases. The there is no sharp upper spacing between boundary) molecules is even greater at these altitudes; the gas particles include ions (i.e. O2+, NO+) together with atoms (such as O) and free electrons that would not be stable at higher altitudes. 20
Variable. This is where weather is determined. For every rise in altitude of 1km, the temperature drops by around 5oC
-50oC to 30oC at the top. Temperature increases largely because of ozone, which absorbs some thermal energy from the suns radiation. Mesosphere: temperature decreases with increasing altitude to about -120oC. Thermosphere: temperature increases with increasing altitude.
4.2: identify the main pollutants found in the lower atmosphere and their sources Pollution refers to an undesirable change in the physical, chemical, or biological characteristics of the natural environment, brought about by man's activities. The main pollutants found in the lower atmosphere and their sources:
PollutionNitrogen oxides (NOx, i.e. NO, NO2) Volatile organic compounds Carbon monoxide (CO)
Artificial SourceMotor vehicles, energy production, jet engines Unburnt fuel, solvents and paints Motor vehicles, incomplete combustion of fuels Combustion of fossil fuels in motor vehicles and electricity production Electricity production, smelting metal sulfide ores, combustion of fossil fuels, sulfuric acid plants, industries, i.e. paper and food processing Combustion of fossil fuels, mining, construction, incinerators Motor vehicles (with leaded petrol), smelting metal ores, incinerators, paint, manufacturing, i.e. batteries Formerly in spray cans, refrigerant/air conditioner coolant, foaming agent High voltage equipment
Natural SourceBiomass burning, lightning, soil bacteria Emitted by vegetation, i.e. eucalypts Biomass burning, volcanoes, decomposition of plants and animals Biomass burning, volcanoes, decomposition of plants and animals Bacteria, volcanoes, hot springs
Carbon dioxide (CO2)
Sulfur dioxide (SO2)
Particles (soot, asbestos etc) Lead
Biomass burning, soil (wind erosion), pollen, spores Erosion of lead ores
Lightning, the reaction between hydrocarbons and the oxides of nitrogen under the influence of UV light.
GAS MIXING: o Hot air rises and cold air falls. o For the troposphere, its bottom is warmer than its top (because temperature decreases as altitude increases); hence, the air from the bottom is always rising to the top. That is, there is constant mixing of gases (convection). This means that pollutants released at the Earths surface by human activity are rapidly dispersed throughout the stratosphere. This is what causes the lower atmosphere to be so rapidly polluted.
o For the stratosphere, its bottom is colder than its top (because temperature increases as altitude increases); hence, there is little movement of gases. The cold air at the bottom and hot air at the top simply stays there. This protects the stratosphere from pollutants released at the surface, as gas mixing stops at the tropopause. The only way pollutants enter the stratosphere is by slow diffusion of gases. Pollutants are very minor constituents in the troposphere and their concentrations are usually measured in ppm. Pollution interferes with ecosystems and our health and quality of life: o Sulphur dioxide: irritates the respiratory system, causes lung damage and may cause asthma. It also contributes to acid rain. o Nitrogen oxides: contribute to acid rain and photochemical smog; they decrease lung function, increase susceptibility to respiratory infections and sensitivity to asthma. o Particles: settle in the lungs and cause irritation, reduce visibility and some are carcinogenic. o VOCs: contribute to photochemical smog and many carcinogenic. o Carbon dioxide: greenhouse gas, contributes to global warming o Carbon monoxide: toxic, combines with haemoglobin better than oxygen, causing asphyxiation o CFCs: ozone depletion in the atmosphere
4.3: describe ozone as a molecule able to act both as an upper atmosphere UV radiation shield and a lower atmosphere pollutant The action of ozone (as well as its value) depends greatly on where it is located. In the upper atmosphere (stratosphere) ozone is found as the ozone layer. It is extremely crucial part to life on Earth. However, in the lower atmosphere (troposphere), ozone is a serious air pollutant. Ozone in the Stratosphere: o Ozone in the stratosphere, in the form of an ozone layer, protects us from harmful ultraviolet radiation (UV light): There are 3 forms of UV light; UV-A, UV-B and UV-C. The ozone-layer blocks the harmful UV-B and UV-C rays from passing through the atmosphere; these can cause many cancers and severe sunburn. The useful UV-A (needed for photosynthesis) can still pass through. Ozone layer in the stratosphere absorbs 97-99% of the Suns UV light o The following equations show how ozone is formed and destroyed as well as how it protects us from harmful UV radiation: O2 + UV radiation 2O O + O2 O3 O3 + UV radiation O + O2 O + O3 2O2 o Every time an oxygen/ozone reacts with UV light, it absorbs it. o Hence ozone can be said to be an upper atmosphere UV radiation shield.
Ozone in the Troposphere: o However, when ozone is found in the lower atmosphere (troposphere), it is considered a serious air pollutant. o This is because ozone can cause serious health and environmental problems. o HEALTH ISSUES: Ozone is poisonous to humans; as a strong oxidant, it can react with body tissue, especially with sensitive mucous membranes when breathed in. Ozone causes breathing difficulties, aggravates respiratory problems and produces headaches and premature fatigue. o PHOTOCHEMICAL SMOG: When ozone is found as a component of smog it is a serious pollutant, and can be very hazardous for health. Sunlight splits nitrogen dioxide and the free radical produced joins with oxygen to form ozone: NO2 NO + O. Also, hydrocarbons and PAN (peroxyacyl nitrates) can be present in the smog. Thus ozone is both as a major pollutant, and a UV radiation shield. o Fortunately, most ozone is located in the stratosphere.
4.4: describe the formation of a coordinate covalent bond Non-metallic compounds contain covalent bonds. A covalent bond is a shared pair of electrons that keeps two atoms together. Normally one atom contributes one electron and the other joined atom contributes the other shared electron. A coordinate covalent bond is formed when one atom donates both the electrons in the shared pair. A lone pair from one atom forms the bond. o Once the coordinate bond has formed, cannot be distinguished from other covalent bonds. 4.5: demonstrate the formation of coordinate covalent bonds using Lewis electron dot structures RECALL: o In LEWIS electron-dot structures, atoms are represented by their chemical symbol, surrounded by only valence electrons; e.g. oxygen will be represented as an O surrounded by 6 electrons (see ABOVE). o Use different symbols for the electrons of different atoms (e.g. crosses/dots). A coordinate-covalent bond involves one atom donating a pair of electrons to the other to form a bond, so both can have an octet; all the electrons in the bond come from one atom: o EXAMPLES: o In the OZONE (O3) molecule, one of the oxygen atoms forms a coordinate covalent bond with an oxygen atom:
o Ions, such as the hydronium H3O+ and the ammonium NH4+, contain a coordinate covalent bond. In the formation of the hydronium ion, one of the non-bonding electron pairs on the oxygen atom is used to form a covalent bond between the hydrogen ion H+ (which has no electrons) and the oxygen atom. 23
o Formation of a coordinate covalent bond in the hydronium ion:
o Formation of a coordinate covalent bond in the ammonium ion:
o TIP When asked to draw a Lewis diagram of ozone, check that all oxygen atoms have only 6 electrons of their own, but 8 electrons bonded altogether: Left: 4 unbonded electrons + 4 electrons in covalent bond = 8 Middle: 2 unbonded + 4 in covalent bond + 2 in coordinate covalent = 8 Right: 6 unbonded + 2 in coordinate covalent = 84.6: compare the properties of the oxygen allotropes O2 and O3 and account for them on the basis of molecular structure and bonding
Allotropes are different forms of an element made of the same type of atom, but with atoms arranged differently. Allotropes have the same chemical properties but different physical properties. o For example, allotropes of carbon are graphite, diamond and fullerenes. Both oxygen gas and ozone are composed of oxygen atoms, but they are arranged differently. Oxygen is a diatomic molecule with two atoms joined by double covalent bonds. Ozone molecules have three oxygen atoms, two of them joined by double covalent bonds and one joined to the central atom by a single coordinate bond. Property O2 (oxygen gas) O3 (ozone) ExplanationState and colour Odour Reactivity Colourless gas Odourless Moderately reactive. Decomposed by high energy UV light. Pale blue gas Sharp, pungent odour Highly reactive. Decomposed by medium energy UV light. No explanation No explanation To decompose oxygen, its double bond has to be broken; this requires considerable amounts of energy. However, the single bond (coordinate covalent bond) in ozone requires much less energy to be broken, and hence ozone is much less stable (readily decomposes to O2). The oxidising strength of ozone comes from the weakness of the single bond; it
Moderately strong oxidising agent
Very strong oxidising agent.
Solubility in water
15 times more soluble than oxygen
easily releases an oxygen atom which can then oxidise a compound. e.g. reaction with metals: oxygen forms the oxide as the only product whereas ozone reacts more readily producing the metallic oxide and an oxygen molecule. Non-polar O2 does not form strong intermolecular forces in the polar water. Ozone has a bent structure, which provides for some polarity of the molecule in its interaction with water.
Effect on humans
Essential for life
Melting point and boiling point
MP: -218.75oC BP: -182.96oC
Lethal to microorganisms. Causes coughing, chest pain and rapid heartbeat. Irritates eyes and airways. At concentration > 1 ppm it is toxic. MP: -192.5 oC BP: -110.5 oC
The boiling point of diatomic oxygen is lower than that of the ozone as diatomic oxygen has a lower molecular mass requiring less energy in the boiling process.
Structure and bonding
Diatomic molecule two oxygen atoms held together with a covalent double bond.
Molecule is linear shape. Thus, non-polar.
Three oxygen atoms held together with 1 double covalent bond and 1 single coordinate covalent bond Molecular shape is bent polar molecule
4.7: compare the properties of the gaseous forms of oxygen and the oxygen free radical A free radical is a reactive particle that contains one or more unpaired electrons in its outer shell. EG: The oxygen free radical: o The oxygen free radical has two pairs of electrons, as well as two unpaired electrons (see diagrams); these unpaired electrons are highly reactive. o It is basically an oxygen atom; they have the same electron configuration. The oxygen free radical can be made either by passing electrical current through oxygen gas to decompose it, or by exposure to U.V. radiation: o O2 2O
o the energy absorbed in the splitting of oxygen, and the unpaired electrons, make the free radical very reactive. COMPARISON: o O2, O3 and the oxygen free radical (O) are all forms of the element oxygen, but the free radical is much more reactive and the most toxic. O2 and O3 are relatively stable in the atmosphere unless acted upon by UV, but the free radical only lives briefly, once formed it immediately reacts.
4.8: identify the origins of chlorofluorocarbons (CFCs) and halons in the atmosphere Chloroflurocarbons (CFCs) are halogenated alkanes with all hydrogens substituted with chlorine and fluorine atoms, i.e. dichlorofluromethane and 1,1,2,2-tetrachloro-1,2-difluroethane. o they are man-made (synthetic) chemical compounds. o DO NOT CONTAIN HYDROGEN AT ALL o CFCs were introduced in the 1930s (known as freons) as replacements for ammonia in refrigeration: This was because had the required pressure-dependent properties that refrigerants needed, as well as that they were odourless, non-flammable, non-toxic and inert, much unlike the toxic, foul-smelling ammonia. o CFCs were very widely used as: Refrigerants in fridges and air-conditioners Propellants in aerosol spray cans Foaming agents in the manufacture of foam plastics like polystyrene Cleaning agents in electronic circuitry Solvents in dry cleaning o These many uses released CFCs directly into the lower atmosphere. o As they were very inert and insoluble in water (rain), they remained in the troposphere, where air convection spread them throughout the atmosphere. o Very slowly, the CFCs began to diffuse through the tropopause and into the stratosphere, where problems began to occur. Halons are halogenated alkanes with hydrogens substituted with bromine in addition to chlorine and fluorine atoms, i.e. bromotrifluromethane, bromochlorodifluromethane. o They are dense, non-flammable liquids that were widely used as effective fire-extinguishers (called BCF fire-extinguishers) because of their excellent fire retardant properties. o As they were used onto fires, the halons were released directly into the atmosphere, where they too slowly diffused into the stratosphere. o Halon use has been dramatically reduced because the bromine molecules are even more effective than chlorine atoms in the chain reactions that lead to the depletion of the ozone layer. Because both halons and CFCs are so inert, they remain in the atmosphere unchanged for many years.
4.9: identify and name examples of isomers (excluding geometrical and optical) of haloalkanes up to eight carbon atoms A haloalkane is an alkane with one or more halogen (elements in group 7) atoms replacing hydrogen atoms. Naming Haloalkanes: o Use prefixes for the halogen group i.e. fluro-, chloro-, bromo-, iodoo If more than one type of halogen atom is present, list them in alphabetical order o Use di, tri, tetra when there is more than one atom of any halogen o Number the carbon chain, giving the lowest set of numbers to the halogens present o If the numbers are the same from both ends of the chain, then give the first named halogen the lowest number Isomers: o Isomers are compounds that have the same molecular formula, but a different structural formula (different arrangement of atoms). o Carbon compounds, such as haloalkanes are able to form many different isomers: Molecules of alkanes and alkenes containing less than 4 carbon atoms can only assume one possible structure. Molecules larger than and including butane and butane have various isomers the number of which increases with increasing molecular size. o Due to the differences in the arrangement of the atoms, isomers have different chemical and physical properties, and hence it is vital that these differences in structure are reflected in systematic names. o EG: Draw 4 isomers of C4H7ClF2 and name them using IUPAC nomenclature:
4.10: discuss the problems associated with the use of CFCs and assess the effectiveness of steps taken to alleviate these problems CFCs were widely used for 50 years The biggest problem associated with the use of CFCs is the destruction of stratospheric ozone (i.e. depletion of the ozone layer) and the greenhouse effect: o CFCs are very inert, and are not washed out by rain. o CFCs remain in the troposphere for many years, because they are very stable, and eventually diffuse into the stratosphere. CFCs are broken down by the presence of UV radiation in the atmosphere (photo dissociation), releasing halogen free radicals. These free radicals can then react with ozone, destroying it. o This increases the UV radiation penetrating the Earth, leading to: increases in sunburn and skin cancer damage to plants possible harmful effects on sensitive ecosystems, such as those found in the Antarctic the breaking down of chemical bonds, in particular in biological polymers like DNA and proteins, causing considerable damage to living systems increases in eye cataracts increased rate of deterioration of synthetic polymers left in the sin damage to aquatic plants and animals decreases in phytoplankton decreases in crop productivity o The production and destruction of ozone is an equilibrium process until the release of CFCs o CFCs catalyse the destruction of ozone depleting the layer faster than it can be made naturally, and hence they decrease the amount of shielding against UV radiation. How CFCs Destroy The Ozone Layer: o Firstly, short wavelength UV radiation (that has not been removed by the ozone layer) attacks the CFC molecule and breaks off a chlorine atom: CCl2F2 (g) + UV radiation Cl (g) + CClF2 (g) o The chlorine radical reacts with ozone, forming oxygen gas, and a chlorine monoxide radical: Cl (g) + O3 (g) ClO (g) + O2 (g) o The chlorine monoxide radical then reacts with an oxygen radical, and the chlorine radical is regenerated: ClO (g) + O (g) Cl (g) + O2 (g) o The net result is that an ozone molecule and an oxygen radical have been converted into 2 oxygen molecules AND the chlorine has not been used up. o The chlorine radical can then attack another ozone molecule and repeat the whole process thousands of times; this is a chain reaction. o Thus, a very small amount of CFC in the stratosphere can do significant damage. How Chlorine Radicals Are Removed: o According to the above, theoretically, a single chlorine atom could destroy the entire ozonelayer; however certain natural reactions remove this radical. 28
o The chlorine atom reacts with stratospheric methane, ending the chain reaction: Cl (g) + CH4 (g) HCl (g) + CH3 (g) o Neither HCl nor the methyl-radical has any effect on ozone. o Another important reaction for stopping ozone depletion involves the ClO species reacting with nitrogen dioxide, forming chlorine nitrate: ClO (g) + NO2 (g) ClONO2 (g) The Greenhouse effect: o The greenhouse effect is the heating of the Earth as a consequence of some gases in the atmosphere which absorb and emit infrared radiation. Without these gases, heat would escape back into space and Earths average temperature would be much colder. Because of how they warm our world, these gases are referred to as greenhouse gases. o CFCs are greenhouse gases o Many people are worried that all of the extra greenhouse gases being generated because of human activities might increase the greenhouse effect causing more heat to be retained by the Earth, leading to global warming The Antarctic Spring Ozone Hole: o There is a serious periodic depletion of the ozone-layer that occurs every spring over Antarctica; it is called the Ozone Hole. o This is due to the conditions of Antarctica in winter, as well as spring. o Antarctic winters are perpetually dark; the cold conditions, as well as solid particulate catalysts in the air, encourage the following reaction to occur: HCl (g) + ClONO2 (g) Cl2 (g) + HNO3 (g) o This has zero effect on ozone levels during the winter. o However during early spring, the Sun begins to rise, and the situation changes dramatically; sunlight is able to split chlorine molecules: Cl2 (g) + UV radiation 2Cl (g) o Hence in spring there is another source of chlorine radicals to destroy more ozone; the concentration of ozone is reduced dramatically, causing a hole. o Eventually, the fixed amount of Cl2 created over the winter is blown away by Antarctic winds and the ozone layer slowly regenerates. Dealing With The CFC Problem: o The only way to stop ozone depletion is to STOP releasing CFCs of any form. o INTERNATIONAL AGREEMENTS: The main way the CFC problem is being dealt with is by international agreements based on the common goal of phasing out CFCs. The Montreal Protocol on Substances That Deplete the Ozone Layer (1987) is an international treaty designed to protect the ozone layer by phasing out the production of a number of substances believed to be responsible for ozone depletion. Its goals included ceasing the manufacture, and banning the use, of CFCs and certain haloalkanes by 1996, the end of halon use by 1994, the phasing out of HCFCs, as well as the provision of financial assistance to developing nations in order to help them reach the goals of Montreal. 29
o CFC REPLACEMENTS: Finding alternative compounds to fulfil the roles of CFCs is a major step forward in preventing ozone depletion. Alternatives include: Replacing CFC aerosols propellants with hydrocarbons and the use of normal pressure packs Alternative chemicals: o HCFCs are less damaging to the ozone layer than CFCs because they have more reactive carbon-hydrogen bonds and so react before they can diffuse into the stratosphere. HCFCs that do diffuse into the stratosphere will be a source of chlorine radicals o HFCs do not contain chlorine and so do not deplete the ozone layer even if they are released into the stratosphere. HFCs are the best chemicals to replace CFCs. o they also contribute less to global warming o Dealing With Increased UV Radiation: Increasing UV levels have meant that more UV inhibitors need to be included in polymers (such as PVC) and Cancer Councils have advised the use of only sunscreens with a rating of SPF 30+ or greater. Effectiveness of These Solutions: o The Montreal Protocol is only effective if member nations ratify the protocol and adhere to its regulations; so far, the Montreal Protocol has been a huge success in international agreement and environmental health. o Certain CFC replacements are not as effective as the CFCs themselves; future technological advancement hopes to find better replacements. o There are still, however, significant levels of CFCs in the atmosphere, and current technology has no way of removing them.
4.0.1: present information from secondary sources to write the equations to show the reactions involving CFCs and ozone to demonstrate the removal of ozone from the atmosphere All the CFC-related equations from above: o Formation of chlorine radical: CCl2F2 (g) + UV radiation Cl (g) + CClF2 (g) o Reaction of ozone: Cl (g) + O3 (g) ClO (g) + O2 (g) o Regeneration of chlorine: ClO (g) + O (g) Cl (g) + O2 (g) o Removal of chlorine radical: Cl (g) + CH4 (g) HCl (g) + CH3 (g) o Removal of chlorine monoxide radical: ClO (g) + NO2 (g) ClONO2 (g) o Formation of molecular chlorine: HCl (g) + ClONO2 (g) Cl2 (g) + HNO3 (g) o Decomposition of molecular chlorine: 30
Cl2 (g) + UV radiation 2Cl (g) Since the chlorine atom is not being used up in the above reactions, it is able to attack more ozone molecules, and continue breaking ozone up indefinitely. This continues like a chain reaction until the chlorine reacts with nitrogen or hydrogen forming compounds such as ClONO2 and HCl
4.11: analyse the information available that indicates changes in atmospheric ozone concentrations, describe the changes observed and explain how this information was obtained How Are Ozone Levels Measured? o Stratospheric ozone levels are measured from ground-based instruments, from instruments in satellites and from instruments in weather-balloons. Ozone levels are measured in Dobson Units (DU) the lower the DU, the less ozone is present/the lower the concentration of ozone One Dobson Unit is equivalent to about 2.69 1016 ozone molecules in a column with a cross section of 1 square centimetre. The instrument that measures ozone concentrations in Dobson units is called the Dobson spectrophotometer. It works by measuring the intensity of sunlight at two wavelengths that are absorbed by ozone, and two that aren't. o The measurements made indicate that changes in ozone levels have occurred. o GROUND-BASED: The ground-based instruments used are UV spectrophotometers that are able to measure the intensity of light at specific wavelengths. These instruments are pointed directly upwards towards the sky and are set to measure light intensity at the wavelengths of light at which ozone absorbs (such as UV-B and UV-C) and at wavelengths on either side (for light which ozone does not absorb). A comparison of these 2 measurements gives a measure of the total ozone in the atmosphere per unit of area of Earth surface at that location. o BALLOON-BASED: These spectrophotometers can also be placed in high-altitude weather balloons that can rise above the stratosphere. The instruments are pointed downwards, and measure ozone from above. o SATELLITE-BASED: TOMS (total ozone mapping spectrophotometers): used on board probe satellites They measure the total ozone in a column of atmosphere below it by comparing the UV radiation coming from the column of atmosphere below with the UV radiation directly coming from the sun. The data is usually presented as a computer generated colour map. TOMS are able to scan the entire globe and measure ozone concentrations as a function of altitude and geographical location. TOMS are no longer used since a failure in 2006, OMI is now used OMI (Ozone Mapping Instrument) modified spectrophotometer 31
The Changes Observed: o Measurements of the total amount of ozone in a column of atmosphere have been recorded since 1957. o The main depletion of ozone has occurred over the Antarctic. o Scientists identified that a dramatic decline in springtime ozone occurred from the late 1970s over the entire Antarctic. The decline reached approximately 30% by 1985. In some places, the ozone layer had been completely destroyed. o The ozone decline over Antarctica during springtime is now not so dramatic, but often exceeds 50%.
4.0.2: gather, process and present information from secondary sources including simulations, molecular model kits or pictorial representations to model isomers of haloalkanes In this experiment, molecular modelling kits were used to show different isomers of a haloalkane. The class was divided into groups, and each group was provided with a kit. Each group was provided with a haloalkane, which they were require to draw structural formula for, and then using the kit, 3 different isomers were formed. JUSTIFY the method: o The models created a good visual 3D representation of a chemical property; that is, isomerism. RESULTS: o Four isomers of C4H7ClF2 with their IUPAC names:
o 4-chloro-1,1-difluorobutane. 32
4.0.3: present information from secondary sources to identify alternative chemicals used to replace CFCs and evaluate the effectiveness of their use as a replacement for CFCs Ammonia: o Large scale (industrial) refrigeration has reverted back to using ammonia as a refrigerant, as was done prior to the discovery of CFCs. o However, great care is exercised, as ammonia is dangerous and toxic. HCFCs: o Hydro chlorofluorocarbons are CFCs that contain hydrogen. o These were the first replacements for CFCs. o HCFCs contain CH bonds that are susceptible to attack by reactive radicals in the troposphere and so are decomposed rapidly to a significant extent. o This means that only a very small proportion ever reaches the stratosphere. o HCFCs replaced CFCs in domestic refrigeration, as propellants in spray cans, as an industrial solvent and as a foaming agent. o Effectiveness: Small amounts of HCFCs do reach the stratosphere, and hence they are also ozonedepleting (10% the ozone-depleting potential of CFCs). They are seen as only a temporary solution. HCFCs also contribute massively to the greenhouse effect, and so their use is being phased out (complete ban by 2030). HFCs: o Hydro fluorocarbons are compounds that contain only carbon, hydrogen and fluorine (NO chlorine or bromine). o They are widely seen as a viable CFC and HCFC alternative, as they contain reactive CH bonds (so they degrade in troposphere) as well as the fact that they do not contain any chlorine (and hence cannot form Cl radicals). o Their ozone depleting potential is zero. o HFCs are very widely used in refrigeration and air-conditioning applications. o Effectiveness: As they have zero ozone-depleting potential, HFCs are a good alternative to using CFCs in terms of atmospheric health. However, they are not as effective refrigerants as CFCs, and are slightly more expensive. They are also strong greenhouse gases, and so further research is required. 33
5. Human activity also impacts on waterways. Chemical monitoring and management assists in providing safe water for human use and to protect the habitats of other organisms 5.1: identify that water quality can be determined by considering: concentrations of common ions, total dissolved solids, hardness, turbidity, acidity, dissolved oxygen and biochemical oxygen demand Water quality can be determined by considering the following factors: o Concentrations of common ions: CATIONS: Most common cations in water: Na+, Mg2+, Ca2+, K+, Sr2+, Fe3+ Other metal ions are analysed routinely, like Al3+, which is mobilised by increased acidity, toxic heavy metals, like Hg2+, Pb2+, and Cd2+, and As (arsenic) in its various forms. AAS is the preferred method for determining the concentrations of most of these ions. ANIONS: Most common anions in water: Cl-, SO42-, HCO3 Chloride: gravimetric analysis is used to determine chloride levels by precipitation as AgCl. Alternatively, volumetric analysis can be used by titration with AgNO3 solution. Sulfate: concentration usually determined gravimetrically as BaSO4 Hydrogen carbonate: concentration determined volumetrically by titration with standard HCl Relative concentrations of ions in water depend on three factors: their prevalence in the Earths crust their solubilities, and the degree to which they are extracted from the water by living things Factors affecting solubility of substances in water: the nature of the solute ionic, covalent network, covalent molecular either polar or non-polar (water is a good solute for ionic and polar substances) temperature the solubilities of most solids increase with increasing temperature; the solubilities of gases decrease with increasing temperature gas pressure the solubilities of gases are directly proportional to the pressure of the gas above the water The concentrations of sodium and chloride are important indicators of the salinity of the water system; any significant change in salinity (increase or decrease) can greatly affect aquatic life. Magnesium and calcium ions are measured to indicate water hardness, which is covered in more detail below. Phosphate and nitrate ions are essential to aquatic life; however, excess levels these ions leads to eutrophication and algal blooms, destroying waterways. o Total dissolved solids TDS are usually ionic salts TDS is commonly referred to as measuring the salinity of the water
high concentrations are usually uninhabitable for many species contrastingly, some species can only live in a marine or brackish environment MEASURMENT: TDS determined by evaporation of a filtered sample and weighing the remaining solids concentrations measured in ppm or mg L-1 alternative method: measure the electrical conductivity of the water using a conductivity meter and estimate the TDS based on a series of standards o Hardness: hard water is water that contains significant concentrations of calcium and magnesium ions together with hydrocarbonate, sulfate and chloride ions temporary hardness: hardness that can be removed by boiling the water. It is caused by the presence of calcium and magnesium hydrogen carbonate. These form the insoluble carbonate when heated, thus removing calcium ions from the water. permanent hardness: caused by the presence of heat stable salts such as sulphates. These cannot be removed by boiling. Permanently hard water can be softened by adding sodium phosphate (calogen) or sodium carbonate (washing soda).
Advantages of hard water: the scum that builds up inside pipes helps prevent metals leaching from old pipes or joints into the water supply some industries, such as the brewing industry, prefer hard water Ca for growth of strong bones and teeth Disadvantages of hard water: difficult to lather, leaving scum on the washing calcium carbonate deposits inside hot water systems and pipes, eventually blocking the flow of water How to turn hard water into soft water: ion exchange process in an ion exchange process hard water is passed through a bed of sodium zeolite and calcium and magnesium ions in the water are removed and replaced by sodium ions in the zeolites, leaving soft water. A deioniser is an ion exchanger containing zeolites that softens water by replacing either calcium and magnesium ions with hydrogen ions or negative ions with hydroxide ions. NOTE desalination is a technique that may also be used but is not currently viable due to the excessive amount of energy required MEASUREMENT: concentrations of calcium and magnesium ions in a water sample can be determined by volumetric analysis by titration with ethylenediaminetraacetic 35
acid (EDTA) the EDTA forms stables complexes with calcium and magnesium ions o hardness of water is usually reported as mg L-1 or ppm CaCO3 assuming all the calcium ions are present as calcium carbonate and no magnesium ions are present can also be assessed qualitively using height of the lather head technique
o Turbidity turbidity is a measure of suspended solids in water sources of suspended solids include clay, silt, plankton, industrial wastes, sewage high turbidity is a problem: reduces the penetration of sunlight, reducing the effectiveness of aquatic plants photosynthesis, affecting oxygen concentration and pH suspended sediment can carry nutrients and pesticides into waterways small particles in the upper layers of a water body can absorb infra-red radiation from sunlight, raising water temperature suspended solids also affect animals i.e. they smother oysters, clog fish gills and make it difficult for predators to find food MEASUREMENT can be measured visibly (qualitive test) can be measured using a turbidity or Secci disk the disk is lowered into the body of water until the cross or mark can no longer be seen; the greater the concentration of the suspended solids in the water, the more quickly the mark will disappear as the disk is lowered can be measured by pouring water into a turbidity tube which allows turbidity to be measured in NTU. Potable (drinking water) needs to have a reading lower than 3 NTU o Acidity the pH of an aquatic system depends on the source of the water, the geology of the catchment and levels of biological activity within the system good indicator of the water bodies health changes in acidity/alkalinity affect the usability of the water MEASUREMENT pH probe or universal indicator may be used potable water needs to have a pH range between 6.5 and 8.5 o Dissolved oxygen the concentration of dissolved oxygen in an aquatic ecosystem is an excellent indicator of its health dissolved oxygen is sensitive to temperature, the presence of organic wastes and the overall health of the system imperative because aquatic organisms require oxygen to survive two main sources of dissolved oxygen: dissolved from the atmosphere which is mixed through the water body by turbulence created by wind, water flow and thermal currents
oxygen produced by photosynthesis by aquatic plants, algae and phytoplankton low levels of dissolved oxygen are often an indicator of pollution MEASUREMENT: Winkler method titration (quantitive) meter with calibrated O2 sensitive electrode (quantitive) special tablets can be used that dissolve in water and change colour depending on the level of dissolved oxygen (qualitive) o Biochemical oxygen demand (BOD) BOD is a method of assessing the capacity of the organic matter in a water sample to consume oxygen also in indirect measure of organic waste present in the water MEASUREMENT assessing the quantity of oxygen used up in decomposing the organic matter the BOD test uses anaerobic organisms to decompose (oxidise) the organic matter over a five day period at 25oC o the difference between the dissolved oxygen levels at commencement and at the end of the five day period represents the BOD of the sample o water samples in excess of 4ppm are regarded as being likely polluted o Nitrogen to phosphorus ratio: Because of the excessive nutrient inputs into our aquatic systems, it is imperative for chemists to regularly monitor their levels and identify their major sources. Both nitrogen and phosphorous are essential for the growth of plants and plants differ in their requirements for these elements. If the N:P ratio is >10:1, or if N and P respectively > 1 and 0.1 ppm, then eutrophication tends to occur. Thus, the NSW EPA recommends a N:P ratio of >10:1, total nitrogen in the range of 0.1-1 ppm and total phosphorous in the range 0.01-0.01 ppm MEASUREMENT: Nitrate and other ionic forms of nitrogen (inc. NO2- and NH4+) are generally analysed by visible spectrometry following the addition of particular reagents Alternatively, (school lab) drop a few drops of sulfuric acid and a small piece of copper foil to 1mm of the sample. This is then heated gently in a fume hood and brown fumes of noxious nitrogen dioxide should result. Phosphate analysed by visible spectrometry; depends on the addition of a particular reagent that forms a coloured complex in the presence of a phosphate ion.
5.2: identify factors that affect the concentrations of a range of ions in solution in natural bodies of water such as rivers and oceans Ion concentration in solutions of natural water bodies (i.e. rivers, lakes, oceans) are affected by a number of factors, both natural and unnatural o Natural Sources of Ions: rocks and soils of over which the river flows. i.e. water which contains dissolved carbon dioxide will be acidic enough to dissolve carbonates from rocks such as limestone: CaCO3(s) + 2H2CO3 CO2(g) + H2O(l) + Ca(HCO3)2(aq) If rain falls on bushland, and then runs-off into streams and rivers, it will only pick up small amounts of nitrates and phosphates from surface nutrients, as well as some Mg2+ and Ca2+ from minerals; TDS will be low (50). If rain soaks into the ground and flows into aquifers (layers of permeable rock) and then into rivers, the water will have increased levels of Ca2+, Mg2+, Cl-, SO42- and CO32- which are dissolved from the soils and rocks the water flows through. TDS will be moderate (200). If water seeps down into rocks (percolates) in deep underground basins (artesian basins) and only reaches the surface centuries later, then the levels of the above ions will increase massively, as may contain other ions (such as Fe3+, Mn2+, Cu2+ and Zn2+. TDS values are very high (>1000). o Land Clearing: When land is cleared of vegetation, the soil loses the stabilising effects of plant roots and so soil is easily moved and displaced. If rain flows over cleared land, it will disturb dirt and sediment and carry it into the water body it flows into (this will greatly increase TURBIDITY). The level of dissolved solids and ions (TDS) will increase, due to high levels of mineral ions in the soils (such as Na+, K+ etc.) o Agriculture: When rain flows over land used for the growing of crops and pasture-land it leads to increased levels of phosphates and nitrates due to fertilizer run-off. The turbidity will also increase, as well as the BOD, as organic matter (such as animal faeces) enters the water-ways. o Raw Sewage and Other Effluents: Raw sewage, if discharged directly into the water, will greatly increase the levels of many ions (especially nutrient ions such as nitrates and phosphates). Sewage can increase a water-ways TDS by 200 or more; it also greatly increases turbidity, BOD and pathogen levels. Stormwater run-off in urban areas can also carry high levels of ions. Industrial effluent as well as leaching from rubbish dumps can increase the levels of dangerous HEAVY-METAL ions in water (such as Hg2+ and Cd2+). o Other factors: Discharge of wastes from mines, power stations and other industries Increased soil salinity due to irrigation, clearing of land, water logging of soil and rising water tables 38
Solubility of a substance. The solubility of most substances increases with temperature and can vary with pH (i.e. aluminium becomes more soluble in acidic water) Rate of river flow Gases from the atmosphere dissolving in falling rain
5.3: describe and assess the effectiveness of methods used to purify and sanitise mass water supplies TREATMENT OF DRINKING WATER: Monitoring Catchment: o The first step to ensure water used for human use is clean is to ensure that the area the water flows over (the CATCHMENT area) is kept clean. o This involves banning any land-clearing, swimming, boating, industry and agriculture in the entire catchment area, to prevent sediment, animal waste or bacteria to build up in water supplies. o EFFECTIVENESS: This is a very cheap and effective way of ensuring the purity of water for human use; by removing the sources of contamination, purity is ensured. Screening: o Before the water from catchment areas is allowed to enter treatment plants or storage dams, it is passed through metal screens that remove large debris such as sticks, leaves, trash and other large particles which may interfere with subsequent purification steps o EFFECTIVENESS: This step is effective for its purpose, but more treatment is needed. Flocculation: o Some suspended particles (called colloidal particles) cause water to become turbid, but are too small to be removed by conventional filtration. o During flocculation, chemicals, such as iron (III) chloride and aluminium hydroxide and a cationic polymer, are added to coagulate the fine particles together, forming larger particles, which can then be removed by filtration. NOTE: the precipitate is first formed as very tiny particles, but as the water is gently agitated or stirred, the particles flocculate into larger particles. o EFFECTIVENESS: Flocculation removes most of the suspended particles, as well as bacteria, which are caught up in the particle aggregates. It is very cost-effective, and relatively fast. Clarification or Sedimentation: o Water that has been flocculated is then allowed to settle in large tanks; this causes the dirt and other particles to fall to the bottom of the tank as sludge, where it is removed. o The clear water is then pumped to the filtration systems. o EFFECTIVENESS: Sedimentation employs a natural force (i.e. gravity) to separate the sludge from clean water, and so reduces plant running costs. The slow speed of sedimentation may affect its effectiveness.
Filtration: o Water from the settling tank is then passed through a 2m deep filter bed of fine sand and gravel; removing the rest of the particulate matter, as well as the material that did not settle to the bottom of the tank. o Over 300 000 small nozzles, under the sand bed, filter 24m3 of water per square meter of sand per hour. They are cleaned by backwashing with water and air to remove solid particles. These settle out in tanks, are centrifuged and dried, then used for compost. o Sometimes anthracite (metamorphic coal) is added to the filter beds, as it adsorbs organic matter and removes odours. o EFFECTIVENESS: Sand filtration removes a high proportion of the particulate matter that aggregated during flocculation, however extremely small particles are not removed (such as some bacteria and viruses). It is suitable for providing water to urbanised areas. Chlorination and Fluoridation: o Following filtration, chlorine gas is bubbled through the water to destroy any residual bacteria and prevent the growth of algae in pipes. Chlorine reacts with water as shown: The hypochlorite ion (OCl-) can kill disease causing agents, such as bacteria and some viruses. o The quantity of chlorine added is dependent on the quality of the water. o Monitoring of the microorganisms in water before it is chlorinated and the use of an automatic metering device constantly alter the amount of chlorine added. As a result, the water from household taps can at times have a very strong chlorine smell though not at other times. o Fluoride is added as Na2SiF6 or NaF to reduce dental decay in children. o EFFECTIVENESS: Chlorination is an effective way of removing most pathogenic organisms, but, it is not so effective at killing viruses Also, chlorine may impart an unpleasant odour on the water. Fluoridation has proved an effective method to increase tooth hardness. Alternative - Membrane Filters: o All the above are the steps taken to sanitize water in most Australian plants. o Alternatively, membrane filters can be used, which would replace many of the steps used above. o Membrane filters would remove the need for flocculation, sedimentation, sand filtration and also chlorination, because: Their pore size is sufficiently small to remove suspended particles. Membranes are very thin and allow the use of pressurization; this greatly speeds up the process, and hence removes the need for sedimentation. Sand filtration is a slow process that membrane filtration supersedes. o Membrane filters can remove all bacteria AND viruses, as these organisms are much too large to fit through the pores of a membrane filter. 40
However, membrane filters are considerably more expensive than current methods used, and so their use is limited by costs. CHLORINATION OF SWIMMING POOLS o Because of their oxidising ability, chlorine and hypochlorites are also used extensively in sterilising swimming pool water, the treatment of waste water, and many household disinfectants and bleaches. o Municipal water supplies and public swimming pools are usually treated with metered doses of free chlorine gas. In domestic swimming pools, calcium hypochlorite granules or sodium hypochlorite solution are often used Both chlorine and hypochlorite react with water to form hypochlorous acid which kills microorganisms. o Alternative: Ozone treatment More expensive treatment, but more effective. Ozone must be produced on the site it is to be used as it is very reactive.
5.4: describe the design and composition of microscopic membrane filters and explain how they purify contaminated water Australia is a very dry continent, suffering regular droughts and hence, an unreliable water supply. To conserve water in the future, as population increases, it will be necessary to recycle water, desalinate sea water or even to treat sewage and bring it back to potable water. One possible technique is membrane filtration. A membrane filter is basically a thin film of synthetic polymer throughout which there are small pores of uniform size. They are made from polymers such as cellulose acetate, PVC and polypropylene. Filters are classified according to the size of their pores, as this determines what type of particles can pass through. MEMBRANE FILTERS: o There are many designs in which the filter can be formed; this depends on the purpose for which the filter was made. o Filters can be classified according to the size of their pores and hence the particles they remove. o Depth Filters: Made of fibrous materials compressed together to form a maze of flow channels in a thick mat that traps particles Used as pre-filters as they can remove a large load of suspended solids economically o Surface Filters: Thin sheet of filter-membrane folded around a hollow core (such as a thin tube) and surrounded by a casing. When water is pumped in at one end and flows over the polymer sheets at right angles, exiting via the hollow core. o Screen Filters: Design involves forming the membranes into capillaries (tiny tubes) with a diameter of about 500 m. 41
Millions of capillary pores are in each square centimetre of filter, allowing for a very fast flow rate. s are bounded together to form a filtering unit that has a very large surface area. Water is passed over the surface of the capillary and clean water passes through to the middle of the capillary and flows out. Types of screen filters: Microfiltration: can remove inorganic and biological particles including suspended solids, algae, protozoans such as giardia and cryptosporidium and fecal bacteria such as Ecoli. Ultrafiltration (10-8m): can remove viruses and some metal ions. Nanofiltration (10-9m): can remove metal ions, thus can reduce hardness as well as some toxic heavy metals. ADVANTAGES OF MEMBRANE FILTERS: o Operation is not expensive o Heavy metals could be removed and recycled o Ensure tiny micro-organisms such as the escape of gardia and cryptosporidium are kept out of our water supply o Could be used to purify catchment water and also water from households and industry to be recycled ensuring water supply during droughts DISADVANTAGES OF MEMBRANE FILTERS: o High cost to purchase and install o N