Topic 3 – Chemical Monitoring and Management
By: Raymond Chen
Topic 3 – Chemical Monitoring and Management
11 – THE INDUSTRIAL CHEMIST
11.1 – THE WORK OF AN INDUSTRIAL CHEMIST
THE ROLE OF A WORKING CHEMIST Chemists employed in the industrial sector have many roles including:
o Development Chemist - Designing chemical processes for the manufacture of a chemical product to ensure the rate of the reaction and yield product are optimised.
o Production Chemist - Working with chemical engineers to designing the equipment to carry out the industrial process
o Research Chemist - Undertaking ongoing research to improve the product or process or to develop new products.
Teamwork , collaboration and communication skills are important for chemists. A company has many chemists that are skilled in different areas. There are a variety of chemists, including:
o Environmental chemist : Employed by a wide variety of organisations, including mining. Developed expertise in analytical chemistry. They collect, analyse and assess environmental data from the air, water
and soil.o Metallurgical chemist :
They have a high understanding of metals, alloys and ores and their reactions.
They specialise in all aspects of the use and development of metals and alloys in society.
They design and monitor methods of extracting metals from ores.o Biochemists :
They help determine the chemical structure and functions of molecules in living things.
They study organic chemistry and biochemistry They can be employed in areas including pharmaceutical laboratories,
hospitals and in the food and agricultural industries.
MONITORING COMBUSTION REACTIONS The combustion of alkanes obtained from petroleum is a major source of heat and
power. Though they are stable in oxygen, they are combustible when ignited - the products
from this are carbon dioxide and water. In a space where is a plentiful supply of oxygen the reaction is:
2C8H18(g )+25O2 (g)→16CO2(g )+18H2O(l)
However when there is a lack of oxygen - like in a car engine - then incomplete combustion may occur:
2C8H18(g )+20O2 (g)→8CO2 (g )+6CO(g)+2C(s)+18H 2O(l)
CATALYTIC CONVERTERS AND EMISSION CONTROL1 By: Raymond Chen
Topic 3 – Chemical Monitoring and Management
Industrial chemists have developed catalysts that help to reduce carbon monoxide, nitrogen oxide and unburnt hydrocarbon emissions from vehicle exhausts.
Catalytic converters are made from alloys of rhodium and platinum - they speed up reactions that convert pollutant gases to materials which are present in the air naturally.
The aim of this is to convert otherwise dangers NO and CO to N2 and CO2 respectively and also unburnt hydrocarbons into water.
11.2 – THE HABER PROCESS
THE USES AND IMPORTANCE OF AMMONIA Ammonia is a gas that produces an alkaline solution when dissolved in water. Solutions of ammonia in water are used domestically are cleaning agents and also as
refrigerant gas. Ammonia is the feedstock for a large variety of industrial chemicals. Fertilisers account for over 80% of worldwide use of ammonia.
Industrial Product Derived from Ammonia
Use of Product
Urea Ammonium Sulfate Ammonium Nitrate Ammonium Hydrogen Phosphate
Fertilisers
Nitric Acid Production of explosives Nitrate salts Strong laboratory acids
Acrylonitrile Acrylic plastics Diaminoalkanes Nylon plastics Cyanides Extraction of gold from gold veins Hydrazine Rocket propellant Sulfonamides Antibiotic drugs Aniline Derivatives Dyes Alkylammonium Hydrocarbons Cationic detergents
INDUSTRIAL MANUFACTURE OF AMMONIA The production involves balancing the conditions of the reaction so that the products
are produced at a fast rate and the quantity is of the product is maximised. Ammonia is manufactured by a process developed by Fritz Haber in early 20th century.
o It was manufactured from its component gas elements - the reaction is exothermic.
N 2 (g)+3H 2(g )⇌ 2NH 3(g )∆=−92kJ /mol
At the standard conditions of temperature and pressure lies to the left - Haber process changes this.
o Conditions in the Haber process make the manufacturing of ammonia viable.
FEEDSTOCKS FOR THE HABER PROCESS The ammonia industry requires nitrogen and hydrogen.
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Topic 3 – Chemical Monitoring and Management
o They are derived from air, water and natural gas. The ammonia produced is not only used to make solid fertiliser, but is also directly
applied to the soil in anhydrous gaseous form.
NITROGEN Filtered air is the source of nitrogen for the Haber process. Air contains ~78% nitrogen by volume.
o An expensive method for nitrogen extraction is to fractionally distil liquefied air. Nitrogen is more commonly extracted from air using chemical reactions
involving natural gas or methane.
HYDROGEN Hydrogen can be obtained via
the electrolysis of salt water - but it is too expensive.
Hydrogen is derived from the steam reforming of natural gas - CH4.
Extraction of hydrogen is as follows:
o Natural gas is purified to remove sulfur compounds through a cobalt/nickel/alumina catalyst.
o Hydrogen is extracted by reacting natural gas with steam at about 750°C with nickel catalyst - primary steam reforming - 90% of methane is consumed.
o The introduction of air produces steam with nitrogen remaining unreacted - high temperatures ~1000°C ensures combustion of almost all methane.
o Carbon monoxide is removed by passing it over two different catalysts - iron oxide (~400°C) and cooper (~200°C)
CO must be removed as it is poisonous - 0.2% remaining Reaction is exothermic - heat is recovered for further use.
o CO2 is removed by neutralisation with potassium carbonate under pressure - the decomposed potassium hydrogen carbonate is stored for further use.
There must not be any oxygen as it is explosive with hydrogen under high pressure and temperature.
The final gaseous mixture contains nitrogen and hydrogen in a ratio of 1:3 - very small amounts of methane and argon are present.
HABER PROCESS The Haber Process is as follows:
o Reactants pass through the catalytic reactors
o The mixture is cooled to condense out the ammonia formed
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Topic 3 – Chemical Monitoring and Management
o The ammonia is drained out as required with the unreacted gas fed back into the catalyst chamber with incoming reactants
o None of the reactant mixture is wasted .
HABER PROCESS The conditions present in the Haber process is a compromise between temperature
and pressure and also kinetic factors.
N2 (g)+3H2(g )⇌ 2NH 3(g )∆=−92kJ /mol
EQUILIBRIUM FACTORS Temperature:
o The Haber process is an exothermic reaction - high temperature, low yield.
o According to L.C.P. - Lower the temperature the higher the yield.
Pressure:o Stoichiometric equation
shows that there are 4 moles of reactants for 2 moles of products - increased pressure, high yield.
o According to L.C.P. - Higher the pressure, the higher the yield .
KINETIC FACTORS Kinetic factors relate the speed at which reactions occur and how rapidly the
ammonia is formed. High temperatures increase kinetic energies of molecules therefore increased
productivity. High pressures increase frequency of collisions therefore increased productivity. The presence of a catalyst increases the reaction rate.
ECONOMIC FACTORS Constructing strong pipes and maintaining a high-pressure reactor vessel is very
expensive - therefore selected pressure should not be too high. Ammonia plants should be located locally with natural gas. Heat is not wasted as they are recycled in heat exchangers. Carbon dioxide is not wasted as it is used to manufacture urea and sold to brewers
and soft drink manufacturers.
COMPROMISE CONDITIONS The conditions in the manufacturing of ammonia vary but these are some of the typical
ranges:o Reactants : N2 and H2 (ratio 1:3) can be shifted to the left by increased
concentration - stoichiometric ratio must be maintained.o Pressure : 15-35MPa - although it should be as high as possible, but
economic and safety concerns require the pressure to be lower.
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Topic 3 – Chemical Monitoring and Management
o Temperature : 400°C-550°C - equilibrium and kinetic factors are a problem; a compromise has to be struck so that the activation energy level can be reached.
o Catalyst : Magnetite (Fe3O4) - fused with K2O, Al2O3, and CaO - it is then reduced to porous iron. By grinding the iron catalyst to produce maximum surface area - this allows low temperatures and low pressures to be used.
MONITORING AND MANAGEMENT The Haber process must be monitored and managed for productivity maximisation
and safety concerns. Reasons include:
o Feedstock must be pure and free contaminants - they interfere with yield and can damage the catalyst.
Oxygen must not be present as it is explosive .o Ratio of nitrogen and hydrogen must be kept at 1:3 for optimum
production.o Temperature and pressure should be maintained - temperature too high can
damage the catalyst, pressure too high may cause the vessels to rupture.o Overtime, minor gases in the atmosphere such as argon and inert gases
accumulate - they need to be removed when it reaches 5%.o Remove ammonia at regular intervals to ensure no impurity contaminationo Structural integrity of reaction vessel must be maintained.
Monitoring devices are connected to critical parts of the containment vessels. Electronic devices sound alarms when values fall outside acceptable limits.
12 – THE ANALYTICAL CHEMIST
12.1 – IDENTIFICATION OF IONS
ANION ANALYSES Chemists analyse materials
for the presence of specific cations and anions.
Anions can be identified and distinguished using a variety of simple qualitative tests involving the formation of gasses or precipitates.
There is a series of elimination tests conducted in strict order.
Then there are additional confirmation tests.
Solubility rules include:o Nitrate salts are
soluble - no precipitation of cations
o Group 1 salts are soluble - no precipitation
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Topic 3 – Chemical Monitoring and Management
Anion Soluble Slightly Soluble InsolubleCO3
2−¿ ¿ Na+¿ , K+¿, NH4+¿¿¿ ¿ - Most
Cl−¿¿ Most Pb2+¿ ¿ Ag+¿¿
OH−¿¿Na+¿, K+¿, NH4
+¿ , Ba2+¿¿¿¿ ¿ Ca2+¿ ¿ MostPO4
3−¿¿ Na+¿ , K+¿, NH4+¿¿¿ ¿ - Most
SO42−¿ ¿ Most Ca2+¿ , Ag+¿¿¿ Ba2+¿ , Pb2+¿¿ ¿
ANION ELIMINATION TESTS To test for unknown solutions, there is a listed sequence. It is known as the elimination sequence - must be done in order to prevent invalid
conclusions.
Anion
Procedure Observation/Conclusion
CO32−¿ ¿ Add 2mol/L nitric acid Effervescence of colourless gas
(CO2) indicates a carbonateUse limewater to confirm
CO32−¿+2 H +¿→CO2(g)+H 2O( l) ¿¿
Confirmation Test: Test the original solution with pH paper
If solution is alkaline then the results are true.CO3
2−¿ ¿ + H 2O(l)⇌HCO3−¿+OH−¿ ¿¿
SO42−¿ ¿ Acidify the unknown solution with nitric acid
and add drops of dilute barium nitrateA white precipitate of barium sulfate indicates sulfate ions are present
SO42−¿+Ba2+¿→BaSO 4(s )¿ ¿
Confirmation Test: Add drops of lead nitrate solution
A white lead (II) sulfate precipitate forms
SO42−¿+Pb2+¿→PbSO 4(s ) ¿¿
Cl−¿¿ Acidify the unknown solution with nitric acid and add silver nitrate solution
A white precipitate of silver chloride
Cl−¿+Ag+¿→AgCl (s ) ¿¿
Confirmation Test: Add ammonia solution then heat in water bath
White precipitate should dissolvedAgCl(s)+2NH 3→ Ag¿¿
PO43−¿¿ Add drops of ammonia solution then solution
of barium nitrateWhite precipitates forms
2 PO43−¿+3Ba2+¿→Ba3¿ ¿¿¿
Confirmation Tests:Add ammonium molybdate and warm the mixtureAcidify the solution with sulfuric acid, then add ammonium molybdate and ascorbic acid
A yellow precipitate of ammonium phosphomolybdate forms
A blue complex forms
CATION ANALYSISCOLOUR OF SOLUTION
In aqueous solution many cations are colourless - but some are distinctive in colour
Hydrated Cation Solution ColourFe3+¿¿ Yellow-orange to pale yellow
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Topic 3 – Chemical Monitoring and Management
Fe2+¿¿ Pale green to colourlessCu2+¿ ¿ Blue to green-blue
FLAME TESTS Many metal ions produce characteristic colours when their salts are heated - flame
test Some metal ions produce
characteristic flame colours. Chloride salts of various
cations work best. As an atom is heated the
electrons in the atom moves to a higher energy level but it is unstable hence they fall back.
According to the Law of Conservation of Energy the energy in the electron is emitted in the form of a frequency in the electromagnetic spectrum - coloured photons.
There are two ways to perform the flame test:
o Dip a platinum wire into concentrated HCl to clean it.
o Heat the wire to remove impuritieso Dip the wire into acid and then into powdered salt so that the salt stickso Heat using Bunsen burner and colour is displayed in the flame
Dissolve the chloride salt in water and spray the resulting solution into the blue Bunsen flame using an atomiser.
o Sodium may sometimes mask the colour of the unknown metal – it has a strong yellow colour.
Cation Flame ColourCalcium Brick redBarium Yellow-greenCopper GreenSodium YellowStrontium Scarlet-RedCATION ELIMINATION TESTS
Like with anions, a series of elimination tests are carried out. These elimination tests are based on the formation of precipitates in solutions of
varying pH. The cation solutions should have a minimum concentration of 0.1 molar.
Cations Procedure Observation/ConclusionPb2+ Add hydrochloric acid White precipitate indicates lead ions
Pb2+¿+2Cl−¿→PbCl2( s) ¿ ¿
Lead chloride is soluble in hot waterConfirmation Test: Add drops of sodium iodide to original solution
Yellow precipitate formsPb2+¿+2 I−¿→PbI2( s) ¿ ¿
7 By: Raymond Chen
Topic 3 – Chemical Monitoring and Management
Ba2+, Ca2+
Add sulfuric acid White precipitate indicates either barium or calcium ions
Ca2+¿+SO 42−¿→CaSO4(s )¿ ¿
Ba2+¿+SO42−¿→BaSO4( s) ¿¿
Confirmation Test:Add solution of sodium fluorideConduct flame test
White precipitate confirms calcium - no precipitate confirms bariumBrick red - calcium
Ca2+¿+2F−¿→CaF2( s) ¿¿
Yellow-green - bariumCu2+ Add sodium hydroxide then
add ammonia solutionBlue precipitate forms from an original blue-green solution - precipitate dissolves in ammonia to form deep blue solution
Cu2+¿+2OH−¿→Cu(OH )2( s) ¿ ¿
Cu(OH )2 (s)+4NH 3→Cu ¿¿Confirmation Test: Conduct flame test
Green flame
Fe2+, Fe3+
Add same of sodium hydroxide Brown precipitate indicates Fe3+
Fe3+¿+3OH−¿→Fe (OH )3(s )¿ ¿
Greenish precipitate indicates Fe2+ - rapidly turns brown
Fe2+¿+ 2OH−¿→Fe (OH )2(s) ¿ ¿
Confirmation Test: Add HClAdd potassium hexacyanferrate reagentAdd potassium thiocyanate reagent
Dark blue indicates Fe2+
3 Fe2+¿+2 Fe(CN )63−¿→Fe3¿ ¿¿
Deep blood red indicates Fe3+
Fe3+¿+SCN−¿→FeSCN2+ ¿¿¿¿
QUANTITATIVE ANALYSIS There are a variety of techniques to determine the amount or concentration of an
element, ion or compound in the sample. These techniques include:
o Gravimetric Analysis - involves weighing materials and determining the percentage composition of elements
o Volumetric analysis - involves measuring the volume of solutions that react with other solutions
o Instrumental analysis - involves the use of special instruments that can determine the concentration or amount of material by measuring a property of the material.
12.2 – INSTRUMENTAL ANALYSIS
ATOMIC ABSORPTION SPECTROSCOPY (AAS) Atomic vapours selectively absorb and emit various frequencies of light. When a sample of an element is vapourised in a hot flame, electrons are promoted
from the ground state into unstable or excited energy levels. As the electrons fall back to more stable levels they emit light through
characteristic frequencies.8 By: Raymond Chen
Topic 3 – Chemical Monitoring and Management
If white light is passed through an atomic vapour at a suitable low temperature, some wavelengths are selectively absorbed and dark lines appear the in the spectrum produced.
The dark lines correspond to the exact bright line wavelengths in atomic emission spectra.
The AAS was developed by CSIRO scientist Alan Walsh - AAS uses the exact principles as above.
This technique is very sensitive - it can detect concentrations in part per million and parts per billion.
HOLLOW-CATHODE LAMP SELECTION The light source in the AAS is usually a hollow-cathode lamp of the element. Specific wavelengths of light characteristic of the elements being analysed are
generated from this lamp.
STANDARD SOLUTION PREPARATION A standard solution of the metal being analysed is prepared using standard volumetric
techniques.
ASPIRATING THE SOLUTIONS The dilution standards and the unknown solution are sprayed or aspirated into the
flame or graphite furnace. The flame in the AAS is about 1000C to increase absorbance of light. The graphite furnace is about 3000C - it is more efficient.
MEASURING LIGHT ABSORPTION As the light beam passes through the vapourised sample, some of the light is absorbed. A second reference beam passes through a monochromator which contains a
diffraction grating and focussing mirrors. The light then passes through a narrow slight to select only one of the wavelength
bands - the light is now monochromatic. Photomultiplier tubes are used the measure the light intensity and convert it into an
electrical signal.
CALIBRATION Concentration measurements are determined from a calibration curve created with
the standard solutions. A control blank is also run - it should indicate zero.
MONITORING TRACE ELEMENTS AND POLLUTANTS IN THE ENVIRONMENTESSENTIAL TRACE ELEMENTS
There are many elements that are needed in small quantities by plants and animals for the proper function or their physiological processes.
There trace elements include copper, zinc, cobalt and molybdenum. The advent of AAS has allowed for a deeper understanding of trace elements and its
composition in organisms and the environment.
Metal Function9 By: Raymond Chen
Topic 3 – Chemical Monitoring and Management
Copper Haemoglobin formation and enzyme actionZinc Enzyme action, metabolism of amino acids and insulin synthesisSelenium Enzyme actionManganese
Enzyme action, blood clotting, carbohydrate and fat metabolism
Cobalt Red blood cell formationChromium
Required for carbohydrate, fat and nucleic acid metabolism
Iodine Proper functioning of the thyroid glandUSES OF AAS
AAS is capable of detecting the presence of well over sixty metals in minute concentrations.
It can be used for:o To test the purity of metallic samples in the mining industryo Monitor pollution levels in waste waters - especially heavy metalso Detect harmful levels of metals in organismso Monitor dangerous air-borne metallic particleso Quality control of alloyso Detect minute contaminants in food
WHY MONITOR CATIONS AND ANIONS Phosphate occurs in waterways at low concentrations and essential for normal aquatic
plant growth. At high concentrations can lead to:
o Algal bloomo Covers surface of lakeo Prevents penetration of light - hence plants and fish dieo Algae dies when phosphate is used upo Decay uses oxygen in water
Zinc and copper:o Desirable in small concentrations in water bodieso High concentrations are harmful to humans and cause poisoningo Lead is poisonous - intellectually retards young children and causes brain
damage.o Was widely used in petrolo Was a constituent of house paint
CHAPTER 13 – ATMOSPHERIC CHEMISTRY
13.1 – CHEMISTRY OF ATMOSPHERIC POLLUTION AND OZONE DEPLETION
COMPOSITION AND STRUCTURE OF THE ATMOSPHERE The atmosphere is a thin gaseous layer that extends to a distance of about 600 km
above the Earth's surface.
TROPOSPHERE The troposphere is the layer closest to the ground. 75% of mass in concentrated in the troposphere. The air pressure is also the highest - 100kPa on the ground.
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Topic 3 – Chemical Monitoring and Management
At 15km altitude the air pressure drops to 10kPa. Temperature decreases with
increase altitude. 15C at the bottom and -50C to
-60C at the tropopause. The transfer of gases of
pollutants across the tropopause is slow.
Water vapours freezes before reaching the stratosphere as to prevent water loss
The tropopause is at a higher altitude above the equator than at the poles due the expansion of air.
STRATOSPHERE Air pressure continues to decrease with altitude and it drops to about 0.1kPa at the
stratopause. In the first 9km temperature is uniform but increases thereafter. The ozone layer is situated within the stratosphere. The greatest concentration is about 25km altitude The higher layers of the ozone are warmer as they absorb UV-B and some UV-C rays. There is a very little of mixing gases as temperature increase with altitude and
prevents convection currents. Pollutants that enter remain for a long time 99.9% of Earth's atmosphere is present in the troposphere and stratosphere. The average temperature is about -2C to 0C.
MESOSPHERE AND THERMOSPHERE Air pressure continues to decrease from about 0.1kPa to 0.01kPa. Temperature decreased with altitude to about -90C at the mesopause at 85km. Above the mesosphere is the thermosphere. Temperature rises again due to high frequency radiation. The thermosphere is about 600km thick. The ionosphere is within this region. Temperatures can reach 1700C.
COMPOSITION OF THE ATMOSPHERE The concentration of total gas particles drop with increasing altitudes - proportions
remains constant The amount of water vapour in the atmosphere varies between 1-5%
Gas Concentration
Nitrogen 78%Oxygen 21%Argon 0.9%Carbon Dioxide
0.04%
Neon 0.002%Helium 0.0005%
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Topic 3 – Chemical Monitoring and Management
For most gases the concentrations are best expressed in parts per million - 1ppm = 1mL/kL
LOWER ATMOSPHERE POLLUTANTS The atmosphere gets polluted by both natural processes and human activity. Volcanoes release toxic gases and lighting produces nitrogen oxides and ozone.
Pollutant SourcesCarbon Dioxide Combustion of fossil fuels in industries and vehicles
DeforestationDecomposition of organic matter
Carbon Monoxide Incomplete combustionForest fires
Methane Anaerobic decomposition of organic matterNitrogen Oxides (NOx)
Combustion of organic matterInternal combustion enginesFactories
Sulfur Dioxide Internal combustion enginesSulfide ore smeltingSome chemical manufacturingVolcanic gases
Chlorofluorocarbons (CFCs)
Manufactured chemicals used in aerosols, refrigerants, foams and air conditioners
Particulates Dust - from industrial and domestic activitySoot from fires, burn-offs
Ozone Photochemical smog
COORDINATE BONDING Coordinate bonding is a special case of covalent bonding. It is also commonly known as coordinate covalent bonding A coordinate bond forms when one atoms provides both electrons for the shared pair
o Ammonium ions Forms when ammonia reacts with hydrogen ions Non-bonding pair of electrons on the nitrogen atom is shared with the
hydrogen iono Hydronium ions
Oxygen atom in water has two non-bonding electron pairs Hydronium ions form when these non-bonding pairs are shared with a
hydrogen ion.
o Carbon monoxide Both covalent and coordinate
bonds are present in a carbon monoxide molecule
o Ozone Ozone is a bent
molecule
OXYGEN AND OZONE Property Oxygen Ozone
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Topic 3 – Chemical Monitoring and Management
Colour Colourless as a gas, but pale blue when liquid or solid
Pale blue as gas, deep blue when liquid and purple when solid
Odour Odourless Sharp, pungentMolecular Shape
Linear Bent
Melting and Boiling Point
MP = -219BP = -183
MP = -193BP = -111
Density 1.3g/L 2.0g/LWater Solubility
Sparingly soluble, about 4.9mL O2 per 100mL
More soluble than O2, dissolves readily into turpentine, cinnamon oil and many other organic liquids
Effects on Life
Essential for all living things Poisonous and harmful, reactive with chemicals in living tissue
Stability Very stable Easily decomposed into O2Preparation Photosynthesis
Fractional distillation of liquid airDecomposition of H2O2
Effect of UV light on O2Electric charge on O2
Uses Oxyacetylene weldingLiquid O2 as fuel for space shuttlesSteel makingMedical uses for patients
Germicidal actionsBleaching agent in paperPowerful oxidising agent
OZONE IN THE ATMOSPHERE Stratosphere:
Contains 90% of atmospheric ozone
Acts as primary UV radiation shield
Issues: Long-term
downwards trend Antarctic and Artic
ozone holes.o In the stratosphere, the ozone acts like a
UV shield.o UV radiation from the sun reacts with
oxygen gas forming oxygen atoms that bond with oxygen gas to form ozone.
O2( g)UV ,240nm→
2O(g)
O2( g)+O2→O3(g )
o Most ozone is made above the equator as sunlight is most direct.o Ozone can absorb harmful UV-B and UV-C radiation.
O3( g)UV ,200−300nm→
O2 (g)+O(g)
o Ozone can be decomposed using oxygen atoms13 By: Raymond Chen
Topic 3 – Chemical Monitoring and Management
O3( g)+O( g)→2O(2)
Troposphere: Contains 10% of atmospheric ozone Toxic effects on humans and vegetation Effects on humans include:
Irritation to eyes Increased respiratory conditions Compromised lung functions Increase susceptibility to infection
Issues: High surface ozone in urban and rural areas
o Formed during electrical discharge - such as sparks from photocopiers, overhead power lines and lightening
O2( g)→2O(g )
O2( g)+O( g)→O3( g)
o Ozone forms in the lower atmosphere where there are: Sunlight Nitrogen dioxide
o NO and NO2 are produced in high temperatures of internal combustion engines - hence catalytic converters are used.
NO2(g)UV , sunligh t→
NO(g)+O(g)
O2( g)+O(g )→O3(g )
o The oxygen radical is reactive and can readily bond with other molecules.o However, nitric oxide, NO, can destroy ozone.
NO(g )+O3 (g)→NO2( g)+O2(g )
o Unburnt hydrocarbons mixed with ozone forms a toxic photochemical smog.o Ozone is the most harmful, by greenhouse gases readily react with other gases,
so ozone doesn't remain in the atmosphere for that long.
OXYGEN AND THE OXYGEN FREE RADICAL Oxygen atoms have six electrons in their outer shell. When oxygen is passed through electrical discharge or UV radiation, the oxygen
molecule splits, forming oxygen atoms. The atoms have two paired and unpaired electrons.
o This makes the atom unstable and reactiveo They are called oxygen free radicals
The unpaired electrons exist in higher energy states than the ground state. They exist only briefly in the lower layers of the atmosphere. In the thermosphere, oxygen free radicals are formed when far UV photons cause
photodissociation of oxygen molecules Oxygen free radicals are even more reactive than ozone.
HALOALKANES, CHLOROFLUOROCARBONS AND HALONS14 By: Raymond Chen
Topic 3 – Chemical Monitoring and Management
When alkanes react with halogens, they form new compounds that are known as haloalkanes.
Haloalkanes often exist in isomeric forms - the variable location of the halogen within the molecule leads to the formation of isomers
CHLOROFLUOROCARBONS Chlorofluorocarbons (CFCs) and halons are examples of haloalkanes. CFCs are alkanes containing only fluorine and carbon. They are alkanes that only have chlorine and fluorine atoms instead of hydrogen. Halons contain bromine atoms in addition to the chlorine and fluorine atoms in CFCs. CFCs are odourless, non-toxic, non-flammable, inert substances. CFCs were developed in the 1930's to replace ammonia in refrigerators. They were then extensively used as:
o Refrigerantso Solvents in dry cleaningo Propellant in spray canso Blowing agents for expanded plastic products
Halons are dense, non-flammable liquids. Halons are used in extinguishers especially for electrical fires.
STRATOSPHERIC OZONE DEPLETION CFCs are stable and insoluble;
hence they stay in the troposphere and eventually into the stratosphere.
In the stratosphere they come into contact with short wave UV radiation and undergo photodissociation.
o Photodissociation breaks a chlorine atom off the CFC molecule.
CCl3 FUV→Cl∎+∎CCl2 F
CCl2F2UV→Cl∎+∎CClF2
The free chlorine atom can now catalyse the reaction of ozone to oxygen.
Cl∎+O3→ClO∎+O2
ClO∎+O∎→Cl∎+O2
Hence the overall reaction is:
O+O3→2O2
The free chlorine atom is able to continue this process hundreds of times before it gets removed by some other species.
CH 4+Cl∎→CH 3∎+HCl
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Topic 3 – Chemical Monitoring and Management
NO2+ClO∎→ClONO2
This chain reaction is important because one CFC molecule can destroy countless numbers of ozone molecules and can cause significant damage.
The dramatic depletion of stratospheric ozone has been observed only over the Antarctic and then only in spring.
STRATOSPHERIC OZONE DEPLETION In 1976, a British Antarctic Survey noted a 10% drop in ozone levels in the stratosphere
over Antarctica in the southern spring.o It was unusual as levels have remained the same since 1957.o Initially this data was treated as an outliero In 1983, ozone depletion was of mass concern as they observed record losses of
ozone that spring. Measurements:
o In 1985, measurements over Antarctica showed a 50% reduction in ozone concentrations over the past 10 years.
o The results were recorded by the total ozone mapping spectrometer (TOMS) and solar backscatter ultraviolet detector.
o Another technique is through the use of UV lasers - UV laser light is fired into the sky, the level of absorption determines the level of concentration.
THE "OZONE HOLE" The thinning of the ozone layer results in what is
known as an "ozone hole". During 1987, the ozone hole spread over southern
Australia and New Zealand. Further depletions were recorded in the 1990’s; the
worst was in 2003, 2000 and 1998. The ozone hole is not confined to the Antarctic, with
small decreases over the Artic too. By 1996, the thinning of ozone over the Arctic
reached 40%. Decreased concentrations of ozone are a problem
as:o More UV rays would reach the groundo Phytoplankton and zooplankton would be
affectedo The food chain in the end would be affected too.o Skin cancer rates have increased by 66% over 14 years in world's most
southernmost city.o Causes respiratory conditionso Damage and cause mutations of DNA.
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